What advantages does a pure black and white camera have over colour cameras?

Question

Now that Leica has released a pure black and white camera, the Leica M-Monochrome, I started to think about the advantages a pure black and white camera has compared to a colour camera.

I guess you could remove the Bayer filter, but would this give you higher resolution, better contrast?

I assume there is advantages with a pure black and white camera, since Leica has made one, but what are they?

Asked by Håkon K. Olafsen

Answer

The biggest advantage is that you get 3X more light sensitivity.

With a bayer filter, every photosite gets 1/3 of the light that falls on it because the filter blocks 2/3 of incoming light to filter for one primary color. So the sensor becomes more sensitive to light. That means that less amplification of the read-out signal to get the same ISO as with a conventional sensor. The end-result is that you get lower noise at each ISO sensitivity.

There is no need for an Anti-Aliasing filter, so you get better sharpness and micro-contrast. As Nikon proved it though, this is not necessary for Bayer-based cameras but is usually the case. When a camera uses an AA filter, it blurs the light before it reaches the sensor to avoid the occurrence of an artifact called moire. Any time you blur something, you reduce contrast because you spread light over multiple pixels. Without an AA filter, the blurring does not happen and you get better contrast.

B&W sensors also obviously do not need Bayer-interpolation. This means that the readout is the image data and there is no question of softness introduced by interpolation (or the AA filter that is not there) and no need to sharpen at the capture level, although you may sharpen when processing for your output medium (print, screen or other).

Answered by Itai

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What settings should be used for self-cleaning sensors?

Question

This question mentioned a self cleaning sensor. I have a few questions in reference to self cleaning sensors.

1. Is there a reason to clean the sensor at startup, shutdown, (or both) automatically?
2. Can the sensor be damaged in any form from repeated cleaning like this?
3. If you do not have it ran automatically, how often should you clean the sensor using the automatic method?

Asked by Lynda

Answer

1. Is there a reason to clean the sensor at startup, shutdown, (or both) automatically?

   Apparently auto sensor cleaning is effective at reducing sensor dust during normal usage, so it makes sense to use it unless you like dust.

   Startup and shutdown are just good times to do it, the camera is on but not in use and it's just before you take pictures (reduce dust before it becomes visible) or after you use it (after you let some dust in).

   The cleanup takes a little bit of time, so if you want the camera to turn on as quickly as possible you don't want auto cleanup on startup - but apart from that it does seem like an exceptionally good time to prevent dust from appearing in the photos.

2. Can the sensor be damaged in any form from repeated cleaning like this?

   Considering the amount of cameras sold with auto sensor cleanup enabled by default if it caused sensor damage we would have known by now.

3. If you do not have it ran automatically, how often should you clean the sensor using the automatic method?

   Like any cleanup it depends on the camera sealing and your working environment.

Answered by Nir

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Is a self-cleaning sensor necessary when using only one lens?

Question

The Canon 1100d lacks a self-cleaning sensor.

• If I use this camera without ever exchanging the lens, is the absence of a self-cleaning sensor a bad 'feature'?
• Are the bodies of modern DLRS cameras enclosed well enough so that dust does not enter body?

As far as I know the self cleaning sensors are necessary for users that exchange their camera lens. When changing the lens, the dust particles can enter the camera.

Asked by gentmatt

Answer

A self cleaning sensor is never necessary but is always nice.

At the end of this post I've described Checking for Sensor Dust. This is not wholly intuitive and this method may be of value to others.

Dust will "pump" through many zoom lenses as they are zoomed in and out. internal volume increases or decreases and air is drawn in and expelled. Some are worse than others at how much dust this sucks in.

I'll add here a note because of a comment by @nwcs who said that zoom lenses do pump dust, but not into the mirror box. I agree that that would seem to be logical - it is not apparent on inspection that there is a clear air path between the zoom body and past the rear element into the mirror box. However, I have an equally logical experience based argument that suggests they do. As with anything observed, you can create a possible explanation after the event, so I've described what I see and what I think may happen at the end under 'Lens sourced dust?:' at the end. Others can decide for themselves and comment is welcome.

I have a Sony SAL18250 18-250mm zoom and when it is fitted the camera seems to acquire sensor dirt much more readily than when a prior Sigma 18-200mm was used. I find that sensor dirt becomes visible within a month of reasonably heavy use.

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Professional lenses with environmental sealing should be much better and a prime should also be better — focusing of a prime lens can cause some pumping, but much less than for a "superzoom" zooming.

I have heard it suggested that a mechanism will generate substantial debris of its own. I have no good feel for the truth of this, but I know that my SAL18250 pumps in dirt much more readily than my older Sigma 18200. I have various primes and lower zoom range lenses but, as the 18-250 lives on the camera usually, its effects hide what the others may or may not do.

I previously has a Sony A700 (killed by "hunting" dolphins) and now have a Sony A77 — the latter with a fixed mirror. The A77 fixed mirror has a degree of airspace around it but far less air should get by than with the 'waving fan' mirror of an SLR. I had hoped that the sensor may thus be less dust affected. It seems that the particles are as bad or worse as with the A700 BUT that the tiny hairs and similar of the A700 are completely absent. This may point to the dust being internal mechanism generated. TBD.

It occurred to me that the dust MAY be on the part-silvered unmoving mirror. A quick test of this showed one large spot that moved when the mirror was "blown" but the small particles remained unchanged. More playing to do there.

I do my own sensor cleaning but I am not going to recommend that others do. While it seems a safe enough activity when done with a "correct" mix of proper swab, proper cleaning fluid, care, skill and luck, I won't guarantee how much of that is luck or what your result may be. There is much on the web on this and some pages are very encouraging. I use locally sourced "pure" Isopropyl Alcohol and this is recommended by some reputable commentators as acceptable BUT your results may vary and that is NOT a recommendation.

My experience with the 18-250 zoom is that sensor dust will generally not show below about f/16. At f/22 it is getting rather obvious and at f/32 it shows horridly. This will vary with the lens involved.

Checking for Sensor Dust:

I check for sensor dust as follows:

• Turn off any antishake — no need to drive it mad.
• Locate reasonably-even source of illumination — day sky is OK but even a monochrome wall under tungsten lights is OK and a multi-shade target is OK if you use enough integration time (aka waving about).
• Set to maximum zoom if zoom lens used.
  
  • Set to small aperture — f/22 at largest, much smaller is better (larger number).
• Set exposure to 2 to 4 seconds.
• DEFOCUS (not essential but improves result) and open shutter.

• Wave camera to and from across evenly illuminated surface to ensure genuinely even illumination.

• Repeat — you now have two frames.

View frames and swap to-and-from between them.
Zooming in will probably be needed.
Pan across image looking for anything that is not blissfully blue (if sky was used. Dirt will stand out well.
When a candidate is seen, swap to-and-from between the two images. Sensor dirt will be present identically on both images.
Anything that changes between images is from another source.
With true even illumination there should be nothing else that can do this but the inter-frame comparison checks this.
(Dirt on the lens face will be extremely defocused and will not show as a point image.

The above works very well for finding sensor dirt.
If you do your own cleaning, clean and repeat test.
I find that most dirt can be removed in one cycle but that often some may move and relocate in a whole new pattern.
I clean across and up (or down) so that dirt that is left SHOULD all be in one corner. Often works ;-).

The test without cleaning does no harm at all and tells you if cleaning is needed.

Reducing Sensor Dust Effect — Emergency Action

If you are taking photos and notice that sensor dust is visible and are unable to address it immediately,, increasing aperture to larger than f/16 (more light) will make the effect much less obvious or even completely nonobvious. Not an ideal solution but a useful one.

Just did a quick test. Sal18250 at 250 mm.
Kitchen wall — far from monochrome but 4 seconds of waving produced bright grey.
Exposures around 4s.
Adjust ISO and exposure level to keep times around 4 seconds - very roughly.
f/38 - horrid BUT none of these would show in most photos
f/32 - most still visible but much softened.
f/11 - one dot visible and very soft. The one dot is about 1.5% of sensor height = about 60 pixels !!! or 0.3mm - quite a monster. That one removed with a blower! ;-).

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Lens sourced dust?:

I swapped lenses to confirm that dust seen is not lens based. A77 dust does not seem to be mirror sourced.

Examination of he rear element of a typical lens, and of my Sony SAL18250 in particular, do not seem to show a clear air path. The seal around the lens-body interface is not made to be air tight but is such that you'd expect minimal air flow during normal use.

When changing lenses I am reasonably careful re dust entry but far less so than some people. I tend to orient the body orifice down if possible or horizontal as second choice.

I had a Sigma 18-200mm which I used on Minolta 5D & 7D.
I bought a Sony A700 with Sony SAL18250.
I have used the 18-200 occasionally with the A700.
I bought a Sony A77 with a new Sony SAL18250 (insurance replacement or I would have bought a D700 :-) ).

The Sigma 18-200 used to acquire sensor dirt at a moderate rate. This was both small particles of varying size and shape plus some iems more like hairs or short arcs in appearance. So:

5D / 7D + 18200
A700 + 18250
A77 + new 18250

The A700 + 18250 and used to acquire dirt at a noticeably higher rate than the 18-200 + 5D/7D - maybe 2+ times as fast, with more hairs and arcs noticeable.
The A77 + new 18250 acquires small particles at a substantial rate (needs a clean after 4 months of moderately heavy use but all the dirt is particle shaped - there are no hairs or arcs. this is optically very distinctively different from the A700 + 18250

The A77 has a fixed mirror that has noticeable air path around its edges.

SO:

The difference between 18200 and 18250 is pronounced. There is also a change of body but as the new lens involves more hairs and arcs these seem unlikely to originate in the mechanism. It is a possibility.
The difference between the A700 and A77 (same lens type different lenses) is also pronounced, with dirt changing to all particle and no arcs or hairs.

I do not recall checking A77 sensor dirt from new - a sad omission.
Inspection of early photos MAY help.
It is possible that the lack of waving mirror and probable reduction in air movement helps.
It is possible that the mirror blocking almost all of the orifice helps.

It SEEMS that the A700 dirt source was at least partially external.
It SEEMS that the 18250 "causes" more dirt than the 18-200.
It SEEMS that the A77 produces zero arcs and hairs compared to the A700.

One could propose an explanation for almost total internal dust generation plus perhaps admission at lens charge time but Occams Razor suggests that air pressure changes in the lens are conveyed across the rear element "seal" and also that there is a dust path present. This is conceivable and needs more testing.

Having had this issue raised in this detail I will keep a closer watch on it. It may be that the first A77 cleaning will remove dirt from post manufacture (which one would hope was minimal) and that there will be less in future. TBD.

Answered by Russell McMahon

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How can I easily tell if a sensor or lens is damaged?

Question

I am pretty interested in recognizing any damage to digital camera in very low level (without any advanced tests).

For example, how can I identify the damage of a sensor or lens? If there are any ways to identify that the camera was dropped or physically damaged. I am pretty sure we can say it based on photo, or may be camera has any kind of sensor.

Asked by com

Answer

Unless there is obvious or visible damage or dysfunction, minor issues can be quite hard to determine. You can severely compromise the front element of a lens and it will still mostly work. However a scratch on a rear element will be much more noticeable. But you can have dust in a lens or camera body with little or no noticeable effects.

If you have the ability to see images produced by a lens or camera body you can look for some things such as dark spots, which may indicate dust or dirt on the lens, or banding which may indicate some deeper malfunction. For a lens, the most common problems show up as poor focus, or uneven focus across an image.

If you are asking before you buy, just make sure you have some reasonable amount of trust in who you are purchasing from. In general, photography equipment holds up very well, if taken reasoanble care of.

Answered by cadmium

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Do megapixels matter with modern sensor technology?

Question

Are more megapixels good?

Are more megapixels bad?

Do more megapixels increase detail? Do they make my images sharper? On the other side, is there a point that is too much? Do megapixels cause increased noise and other problems? How does print and viewing size come into it?

Widespread Internet-forum wisdom used to be that 6 megapixels was the sweet spot — below that just wasn't enough, but above that, there wasn't much benefit. File sizes got larger, but detail was lost to noise and other problems. The arguments are that cramming too many pixels into a small sensor makes each pixel too small to provide any real benefit, and that higher-megapixel sensors outresolve cheap lenses anyway. (Substitute something like 12 instead of 6 if we're discussing APS-C DSLRs.)

This year, pretty much every camera introduced is in the range of 12-18 megapixels. Does this offer any real improvement over that old "sweet spot"? Has technology improved to the point where the "wisdom" needs to be updated, or are we all suffering for marketing? Or have we gone past the sweet spot in some ways, but it's okay because of previously-un-argued points. (For example: more noise, sure, but more detail as well.)

Within the 12-18 megapixel common-today range, for the same sensor size, is there any actual benefit to the higher end? (We've got a question on that specifically, but it attracted very literal replies — I'm looking for why and how.) How do megapixels directly affect image quality with today's technology? What are the benefits and when do they apply? What are the drawbacks, and when do they apply? How should I adjust my technique (and expectations) based on my camera's megapixel count?

Asked by mattdm

Answer

From a purely theoretical point of view: more megapixels good.

People often talk about how high megapixel sensors were now outresolving most lenses, thus there was no point going higher unless using the very best glass. This is not always true. System resolution is the product of lens resolution and sensor resolution. Thus if you improve one, your system resolution will improve regardless of the other. You do eventually get into diminishing returns, but from a theoretical viewpoint a sensor can't outresolve a lens until diffraction effects take over).

Theoretically for a fixed final output size noise is independent of sensor resolution. Yes smaller pixels capture less light, therefore the per pixel noise level is higher. But if you resize a high megapixel image to match a lower one, you average pixel values thus noise is evened out. People regularly complain about noisy high megapixel compacts when viewing images at 100%. But that's a totally unfair comparison.

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From a practical point of view: more megapixels not bad

From a practical view the noise situation is more complicated, but evidence I've seen suggests that high MP sensors are not much noisier when compared at the same image size (see above). I'll look up some links.

The situation on resolution is complicated by the fact [most] sensors don't see in colour and thus have a bayer grid which requires an anti aliasing filter. Aliasing is worst when the sampling frequency matches your signal (i.e. image detail) frequency. Increasing the megapixel count faster than increases in signal frequency should improve aliasing, to the point where the traditional aliasing filter can be removed.

There are other practical issues which relate to your ability to extract extra detail from your sensor:

• The 1/focal length rule no longer applies as you increase megapixels, you need ever increasing stabilisation, and also increasing shutter speeds as subject motion becomes more apparent.

• Diffraction becomes more of a problem as you increase megapixels as the pixels become smaller than the Airy disk.

• Data processing and storage requirements are higher.

It's worth emphasising that these are not disadvantages of higher megapixel counts, since you can always downsize your images, and you're not lost anything when compared to a lower megapixel count camera. The exception being in camera data processing, since the camera has to read the whole sensor when shooting stills and somehow process this information.

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So how high can you go? I've seen calculations of the diffraction limiting aperture for red light with a 350 megapixel full frame sensor being f/2.8 (green and blue light requiring even larger apertures) so that gives you an idea. Personally I think your returns would get small past a 50 megapixel 35mm sensor, up to a maximum of maybe 75-100. Once you get noticeable diffraction at f/5.6 people are going to become disinterested, and once you have to open up to f/2.8 with a lens that's razor sharp at f/2.8, the megapixel race is over.

Larger formats allow more megapixels before diffraction sets in (at a given f/stop) however depth of field is shallower at the same f/stop, requiring you to stop down more for depth of field, so there appears to be no intrinsic advantage when it comes to diffraction.

The existence of 60 megapixel medium format camera points to the fact it would be possible, diffraction wise, given good enough glass. Though as users of such cameras point out how difficult it is to utilise 60MP this points to it being a good practical limit, if not a theoretical one.

Answered by Matt Grum

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Does a 35mm prime lens show the same bokeh as a 50mm prime lens on a cropped sensor?

Question

I came to know that a 50mm prime lens on a cropped sensor behaves like an 75mm prime lens.

So, does the 35mm prime lens behave like a 50mm prime lens on a cropped sensor?
If yes, then will it show the same bokeh as the 50mm lens on a cropped sensor?

Asked by Anisha Kaul

Answer

The only different between full frame and crop sensors is that the crop sensor is smaller - so the smaller sensor only sees only the center of the full frame image.

Or, another way to put it is that the picture you get from a crop sensor is the same picture you get from a full frame if you crop it and only leave the middle part.

So, does cropping effect bokeh? obviously not, but...

Cropping does have the same effect as zooming in, that's why the field of view of a Nikon APS-C at 35mm (or Canon APC-C at ~30mm) is similar to the field of view of a full frame with a 50mm lens.

And that "extra zoom" means that to fill the frame with the same subject on a crop sensor you will be at a greater distance than with a full frame - and distance to subject does effect DOF.

So, the quality and shape of the bokeh doesn't change in any way but the amount of bokeh does change (simply because distance to subject changes).

Answered by Nir

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When do the differences between APS-C and full frame sensors matter, and why?

Question

I'm trying to decide on a higher-end digital SLR, and I'm down to choosing between an APS-C model and a full-frame model.

I understand that the sensors are of different sizes, and as such have an effect on the perceived magnification of the lens, with the smaller APS-C sensor having an effective focal length greater than what it otherwise would be with a full frame sensor. But why does this matter?

• What things should drive my choice between one or the other?
• In which situations is one better than the other, and why?

Asked by Winston Smith

Answer

• One major difference is that a FF camera produces a depth of field that's around 1.3 stops shallower than an APS-C camera for the same subject & framing. This is most important when you have the aperture as wide as possible, e.g. for portraiture. To replicate the look of a 50 f/1.4 lens you'd have to use something like a 31 f/0.9 lens, which doesn't as far as I know exist!

Quick and dirty comparison image, APS-C Canon 30D left, FF Canon 5D right, same lens (FF image was zoomed in, however to give the same field of view), same composition, both f/2.8

• Another difference is that if you're using a lens designed for a full frame camera (like all Canon EF lenses) you are making full use of the image circle, which is less demanding of the optics and so you can expect a sharper image for the same number of megapixels. It's true that some lenses get softer toward the edge of the image, but you will still get higher average sharpness with most lenses, and telephotos will be sharper right across the frame. The crop factor of APS-C cameras takes the middle out of the lens and blows it up, losing sharpness in the process in a similar manner to a teleconverter.

• Larger formats allow for sharper optics. One of the driving forces for larger formats (other than the relatively constant resolving power per unit area of film) has been that it allows lenses to be produced which resolve a greater number of line pairs per picture height. Going full frame on a DSLR benefits from this to an extend - see: With all other things equal, in a DSLR, will a larger sensor produce a sharper image?

• A bigger sensor means bigger pixels, which in turn means you capture more light usually achieving lower noise levels in the process. Greater dynamic range goes hand in hand with this.

• You get a larger, brighter viewfinder on a full frame camera, which can be helpful composing shots. Having said that, I personally find the 5D viewfinder too large, I've not used a 7D but it has a very high spec 'finder.

• You have more mirror to move on a full frame camera. The larger mirror used to mean shooting speed is limited (the mirror on my 5D moves so slowly I can actually see the world slide sideways/up for an instant) however high speed full frame models are now available.

• Likewise the mirror box, focussing screen and pentaprism are larger, meaning the camera is larger and heavier.

• Lens hoods are designed for FF image circle and are therefore slightly more effective on FF cameras. This mostly applies to prime lenses, as zoom lens hoods are designed cut to accommodate the widest zoom setting, so everything else is already non optimal. If you're using an EF lens on a crop camera you ideally want the hood tighter (since the extra shading will lie outside the smaller sensor a tighter hood wont vignette.

I have nothing against APS-C cameras but for any format it makes sense to use lenses designed for your sensor size. The range of EF-S lenses is smaller than the range of EF lenses. However for some uses (sports etc.) the smaller sensor size is helpful for the extra reach and speed it allows. Also the better noise characteristics of a FF sensor don't quite make up for the higher ISO you need to use get the same exposure when stopping down to match the DOF as a crop. So if you have to maximise DOF crop has a slight edge.

If there are EF-S lenses available for what you want to shoot then it wont be noticeably worse choosing this camera. However I feel full frame gives you more flexibility (speed aside) - as you can get the same deep DOF as a crop, but go narrower if you need to.

Answered by Matt Grum

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Fancy technology cropped vs old technology full frame - Which will give better images?

Question

After selling my 1Dmk2 last year the time has come to get myself a new body, all along I'd decided that I wanted to do more "wow" pictures, big landscapes and do some more portait photography (up until now I had been doing mainly motorsport photography).

My budget is such that I can't afford a 5Dmk2, so the obvious candidate would be a 5Dmk1, its been around for a while and is famous for producing stunning images.

However, the thought has crept into my head that a 7D might be a better option, it's had the benefit of 3 or 4 years extra development and the Dual Digic4 processors, but has this allowed the image quality to catch up with the 5D?

(I'm trying to keep the question generic, but if you want to tailor your answer I use 17-40L, 50mm and 70-200f2.8L and I'm not bothered about shooting video)

Asked by LC1983

Answer

As a former 5D owner I can tell you that using full frame, even on a camera as 'old' as that one, is a joy. The colours and image quality on that 12MP sensor are amazing. You will notice the benefits particularly with your 17-40mm lens.

If I were in your position, I wouldn't hesitate to pick up a second hand 5D.

Answered by Nick Miners

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Why is the sensor size small in D3100 as compared to D3000?

Question

I thought D3100 must be more advanced than D3000 (because of the numbers, and the price).

D3000's sensor size is: 23.6mm x 15.8mm and D3100's sensor size is: 23.1mm x 15.4mm

Well, D3100 has an extra feature of live view, fine, but can there be any special reason that they decided to reduce the sensor size in a more advanced camera?

Is there anything in D3100 which "makes up" for the reduced sensor size or am I simply missing some point?

Asked by Anisha Kaul

Answer

Just to put this into perspective, this is a 4.8% difference in sensor area. Or, linearly, it's 2.3% difference in crop factor.

This is not very much, and generally other measurement tolerances will be less precise. For example, if you measure the actual focal length of, say, a bunch of different models of 50mm lenses, they probably have a greater variation in field of view.

In general, newer sensor technology moves forward, and in this case there's no exception: the D3100's sensor is significantly better, particularly for controlling noise at high ISOs. From dpreview:

The D3100 offers little to complain about in terms of image quality, and its new 14Mp sensor delivers very good results. High ISO performance is substantially improved over the D3000 [...].

Basically, the small difference in sensor size is insignificant compared to the improvements in terms of image quality, and the small difference in framing is likewise a non-issue.

Answered by mattdm

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When is it more suitable to have a full sensor-camera instead of a crop sensor-one?

Question

I've been reading about this a bit, and I get the sense that full sensor-cameras are best suited for shooting landscapes, whilst crop sensor-cameras are preferable for portraits and close-ups. Please correct me if I'm wrong. What I would also like to know is which type of sensor is best suited to these other styles:

• Bokeh
• Action (fast shutter speed)
• Black & White
• Macro
• Low light
• Time-lapse

Asked by the.midget

Answer

I would not say that crop sensor cameras are preferable for portraits or close up. I would say they are preferable for the budget minded individual, especially those who want longer focal lengths for less of an investment, and those who want a higher FPS camera due to the smaller mirror. Full frame almost always has an advantage in quality and technical photographic matters. Some areas where it falls short of crop sensors are in high FPS shooting, size of the body, price, and one may consider the smaller portion of the lens being used to be an advantage as well.

Specifically on your question of which styles of photography are better suited for which sensor - The previous thread already covered the "action" piece here:

You have more mirror to move on a full frame camera. The larger heavier mirror usually means shooting speed is limited. The mirror on my 5D moves so slowly I can actually see the world slide sideways/up for an instant!

for some uses (sports etc.) the smaller sensor size is helpful for the extra reach and speed it allows.(sic)

It also covers in great detail the bokeh piece. For the remaining styles b&w, macro, low light, and time-lapse - the advantages of the full frame sensor in sharper optics, bigger pixels, a better viewfinder, etc are usually all advantages in these styles. Macro is one area that may warrant a separate discussion - as this might get into some more technical discussion. If macro is your main pursuit, I would open up a new question and ask about full frame vs crop for macro use. Generally though I would still prefer full frame for this style.

Answered by dpollitt

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Can the sun damage the camera sensor? Under what conditions?

Question

I want to experiment taking photos where the sun appears. I'm afraid of what might happen if I take one with a narrower angle (where sun would be bigger). Can the lens act as a magnifying glass and burn the CCD or CMOS sensor?

EDIT

Since my query is a little bit obvious, I will reformulate it to:

Under which circumstances (zoom, exposition, aperture, etc...), can the sensor be damaged by the sun?

Answer

Taking direct photos of the sun can destroy your camera, not to mention your eyes. It's exactly as you are afraid, the lens will act as a magnifier and multiply the suns intensity right on your cameras internals. What this effects can vary. Long exposures against the sun can cause permanent damage to your camera's sensor, but besides that, your camera's shutter curtains, and af sensors are also at risk when shooting right at the sun.

Now, taking photos of sunset and sunrises is okay, as is taking photos in direct sunlight (though this does require some finesse to get a good exposure), but pointing your lens right at the sun is not recommended (especially for long exposures).

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What is causing this white spot in every photo with my D90?

Question

Recently I noticed that the photographs from my D90 has an odd white dot in it. The dot is present in every image. The dot becomes clearly visible as I zoom into the image. I have attached a photo taken with dark background and marked the white spot.

What could be causing this white spot? Has any had such issues? Sample image with marked white dot http://imgur.com/trYbG

Answer

This is most likely a Defective Pixel. There's not much you can do about it aside from cloning it out in Photoshop when it is obnoxious. Are you doing a lot of long exposures? Back when I was doing astrophotography I'd see a bunch of dead pixels in my camera, but never saw them in regular use. So it may be related to sensor time.

You can always contact the manufacturer and see if they have a defective pixel policy. You may get it fixed!

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Hmmm, I just fully read the wikipedia article. By their definition, the pixel in the camera sensor would always be off, not always be on! But I tend to doubt that, on or off, they are still defective. This article over at Ken Rockwell's site calls them hot pixels. Same thing in my book. Hot, defective, whatever...

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Why can't digital camera sensors expose each photosite individually?

Question

Why is it impossible for camera sensors to function the way the human eye can? What I mean is, why does a certain portion of the image have to be over/underexposed if we compensate for dark and light areas respectively when taking a photo and deciding on the aperture and shutter speed settings.

I understand that the light getting in depends on aperture and shutter speed but since DSLRs are digital, can't there be a technology that would enable each sensor cell use of its own metering and therefore they wouldn't all be subjected to the same amount of light but depending on the metering, a CPU of the camera would shut off certain cells as not to overexpose them.

I hope I'm not saying nonsense. It sure seems to me like a plausible idea.

Answer

Who decides which pixels get how much gain? Much of what goes on in the human visual system happens in the cortex, not the eye, and depends on what we think is important to see based on a combination of intellectual decision and the (somewhat override-able) instinctual drive for self-preservation. While it's true in one sense that we see what's there, it is equally true in another sense that we see what we want to (or need to) see.

It would be almost trivial to create a relatively low pixel density sensor with large photosites that allow for an enormous dynamic range and (assuming a CCD-type technology, since the current CMOS sensor tech can't work this way) a per-pixel electronic shutter in addition to the mechanical shutter. So what would that get you? A flat image with a lot of bit depth and very low local contrast (if the entire bit depth is converted as-is for display or print) along with a number of pixels that are almost, but not quite, clipped by the sensor saturation (although they are, in fact, clipped by the limiting action of the electronic shutter just before the point of saturation). Let's say for the sake of argument, though, that this sensor and its associated computer could record the clipping data (the reason why it stopped recording at that sensel, which could be as simple as recording the actual exposure duration at that site). That would allow the camera's electronics to reconstruct what the numbers would have been if the photosite could have stayed in the game until the final whistle. So now we have an even flatter image with greater bit depth. And where do you draw the line? 32 bits? 64?

Now comes the hard part -- turning this flat, high-dynamic-range image data into a compelling photograph. The simplest approach is to take the eight bits (or whatever the output bit depth would be) that represent the primary metered image and throw away the rest. It would probably be not much more difficult to fit the data to an S-curve, compressing the extreme shadows and/or highlights -- which is more or less what the extended dynamic range settings on newer cameras already do. But there are only so many output bits available per pixel, and most of the extended highlight values are going to round up to white (or at least a 254 and 255 mix). So you've gained very little by dramatically complicating the system.

But there is still one option open -- selective area mapping. Why not bring the sky, say, or just the clouds in that sky, down in value so it can retain detail, while preserving the desired contrast in the foreground? This is where the hard problem lives. What's important? Should the camera decide for you? If the camera decides, then we have a big advance in machine vision and artificial intelligence to get around to first. If not, then do you really want to make this level of post-capture decision for every picture you shoot? Yes, I know there will be some photo-techno-weinies who really do want to be that hands-on, but can we accept that it's a pathological condition, and that professionals interested in turn-around time and the vast majority of consumers aren't like that?

So you need a new sensor, vastly more complicated electronics around the sensor, an enormous image file for projected raw data (which necessitates larger cards and longer write times/slower frame rates), all to gather data that is going to be thrown away most of the time so that you can occasionally shoot one-shot HDR images that require a lot of human intervention in post (or a huge leap in MV/AI). You could probably sell a few of these, but I'd expect the market to look an awful lot more like the medium format market than the existing 35mm/APS-C market. That is, you'd sell to a select group of well-heeled photographers who either actually need the capabilities for professional reasons or to fulfill their fine art vision, and a few who just get a big enough kick out of post-processing to pay the technology tax.

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Do video cameras use the same image sensors as still cameras?

Question

Do video cameras have the image sensors cameras have?

I really don't know what I'm talking about, so if the answer was explained, I would really appreciate it.

Answer

Yes. Digital video cameras use these sensors, usually CCDs but CMOS too. These are the same designs use for still cameras but with less pixels, since even HD footage only needs 2 MP. For HD cameras, the shape of the sensor is often different to match the 16:9 aspect of widescreen footage.

The major difference you will encounter are cameras labelled as 3 CCD. Again this is the same type of sensor but there are 3 of them, one for each of red, green and blue. Special prisms are used to divide incoming light and reflect it towards each sensor. One a conventional digital camera, colors are almost always divided between adjacent pixels using a Bayer filter. There are some Sigma cameras which use special Foveon sensors which capture different colors in layers instead.

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Where is this dust located on my sensor?

Question

I have a really nasty looking piece of dust on my 550D's sensor, situated in the top right-hand corner of each image I take with anything higher than f/11. I am going to get a decent sensor cleaning kit, but I'd like to know which corner of the sensor should I focus on to get rid of this annoying particle?

In other words, does a lens flip the incoming light from left to right and turn it upside down? I think that's correct, so that means the answer in this diagram will be A, correct?

And before anyone asks - yes, I will clean the entire sensor while I'm in there, but this will help me focus on this particular (pun intended) problem. Thanks in advance!

Answer

(A) is indeed the correct answer (he said, remembering well the reflex finders sans prism that he's used over the years, which didn't flip the image right-to-left and made panning a bitch).

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Can condensation be formed in the camera sensor?

Question

When entering a hot place after leaving a cold place, some condensation can be formed in the lens and manufacturers recommend you to avoid using the equipment in this state.

So, if condensation can be formed in the lens, could it be also formed in the sensor? Is this dangerous? Will this condensation have any impact in the final image?

Answer

Condensation occurs when warm air meets a cold surface and when that air cools down, its ability to carry moisture reduces, so the water will distill and cling to the nearest surface (the same cold one).

As long as you don't detach the lens while the inside of your camera is colder, you should be fine. The little amount of air oozing through between lens and mount won't contain enough moisture to do harm.

You might also want to limit zooming actions to minimum - many lenses act as air pumps during zooming.

Using the camera, especially with live view or for capturing video, will heat up the sensor fairly fast. The rest of camera takes a bit longer to heat up, however; so ventilating its inside might still condensate some other important electronics, or the mirror.

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How can dynamic-range be larger than sensor bit-depth?

Question

Found something that confused me and so I thought the crowd here can probably answer this one since its camera-related and technical at the same time.

How can dynamic-range be larger than sensor bit-depth?

Someone sent me the DXOMark results for the Pentax K-5 which shows 14.1 EV of dynamic-range at its lowest ISO. However, given that the sensor is 14-bits, this does not fit with my intuition... It seems strange that a linear device like a CMOS-sensor can capture more DR than it has bits. Would it have a sparse dynamic-range, skipping EVs in the middle?

Answer

Cambridge in Colour has a very good article on this. If the sensor has a linear A/D converter, the bit depth would cap dynamic range at at 14 EVs as a theoretical limit. However, if it is non-linear, then the bit depth doesn't necessarily correlate. From that, I think we can determine that the sensor in the K-5 doesn't have a linear A/D converter.

I can say, from personal experience, that this sensor definitely has enormous dynamic range. I managed to recover an image that was close to 8 stops underexposed on the K-5.

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What is the Nyquist Limit and what is its significance to photographers?

Question

The Nyquist Limit is frequently mentioned in the context of lens and sensor resolution.
What is it and what is its significance to photographers?

Here is an example of it being used by DPReview.com in their resolution testing.

Answer

Please note that the following is a simplification of how things actually work

Background:

In digital photography, a light pattern is focused by the lens onto the image sensor. The image sensor is made up of millions of tiny light-sensitive sensors whose measurements are combined to form a 2-dimential array of pixels. Each tiny sensor produces a single light intensity measurement. For simplicity, I will look at the 1-dimensional case. (Think of this as a slice that looks at only a single row of pixels).

Sampling:

Our row of tiny sensors, each of which is measuring a single point of light, is performing sampling of a continuous signal (the light coming through the lens) to produce a discrete signal (light intensity values at each evenly spaced pixel).

Sampling Theorem:

The minimum sampling rate (i.e., the number of sensors per inch) that produces a signal that still contains all of the original signal’s information is known as the Nyquist rate, which is twice the maximum frequency in the original signal. The top plot in the figure below shows a 1Hz sine wave sampled at the Nyquist rate, which for this sine wave is 2Hz. The resulting discrete signal, shown in red, contains the same information as the discrete signal plotted beneath it, which was sampled at a frequency of 10Hz. While a slight over simplification, it is essentially true that no information is lost when the original sample rate is known, and the highest frequency in the original signal is less than half the sample rate.

Effects of under sampling:

If the sample frequency were less than 2 times the maximum frequency of the signal, then the signal is said to be under sampled. In that case, it is not possible to reconstruct the original continuous signal from the discrete one. An illustration of why this is the case can be found in the figure below. There, two sine waves of different frequencies sampled at the same rate produce the same set of discrete points. These two sine waves are called aliases of each other.

All discrete and digital signals have an infinite number of aliases, which correspond to all the sine waves that could produce the discrete signals. While the existence of these aliases may seem to present a problem when reconstructing the original signal, the solution is to ignore all signal content above the maximum frequency of the original signal. This is equivalent to assuming that the sampled points were taken from the lowest possible frequency sinusoid. Trouble arises when aliases overlap, which can happen when a signal is under sampled.

But Photographs Don't Look Like Sinusoidal Waves. How is all this Relevant?

The reason all of this matters for images is that through application of the Fourier Series, any signal of finite length can be represented as a sum of sinusoids. This means that even if a picture has no discernable wave pattern, it can still be represented as a sequence of sinusoids of different frequencies. The highest frequency that can be represented in the image is half the Nyquist rate (sampling frequency).

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Meanings of Similar Terms:

Nyquist rate - The lowest possible sampling frequency that can be used while still guaranteeing the possibility of perfect reconstruction of the original continuous signal.

Nyquist frequency - The highest frequency continuous signal that can be represented by a discreet signal (for a given sampling frequency).

These two terms are two sides of the same coin. The first gives you a bound on sampling rate as a function of max frequency. The second gives you the max possible frequency as a function of sampling rate. See Wikipedia: Nyquist frequency for further reading.

Nyquist Limit is another name for Nyquist frequency. See wolfram.com: Nyquist Frequency

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When shooting in mRAW or sRAW, how does the camera generate smaller files?

Question

Most mid- and high-end DLSRs offer two or three sizes for RAW capture. When the camera is generating the medium or small sized RAW files, how does it make them smaller? Does it capture less information onto the sensor? Does it capture the full amount of information and then apply some sort of in-camera compression? Does it do something else that I'm not describing?

Answer

Douglas Kerr gives a masterful and largely non-mathematical summary at The Canon sRaw and mRaw Output Formats . The situation is complicated and not perfectly understood, but much has been deduced by reverse engineering. Evidently sRaw is a 2 x 2 aggregation but with some chrominance subsampling; mRaw is likely a bona fide resampling (involving local interpolation), with heavier chrominance subsampling. One might indeed characterize each as a form of "in-camera compression" performed in a sophisticated way to optimize the appearance of detail to the human eye for a given output file size.

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Why did camera manufacturers create crop sensor cameras?

Question

Why did camera manufacturers create crop sensor cameras? Was there a business reason? Did they figure that crop sensor cameras are cheaper to manufacturer, which would in turn lower prices and make it easier to penetrate the amateur photography market?

I'm under the assumption that full frame cameras were created first and crop sensor cameras have come along only recently (relative the entire life of cameras).

Answer

There were economic reasons, but they were not about getting to amateur market; it was more like getting any market. The main merit seen in early digital photography was speed of delivery (no need to develop films), so news agencies were the first targets.

During the dawn of digital photography, a full frame sensor would have been enormously expensive to produce. The technology was just not ready to produce perfect silicon wafers of that size; even the smaller ones were the price of a really nice car (or a top-notch medium / large format system). The first commercially available dSLR, Kodak DCS (1991, a.k.a. DCS100), managed to sell 987 units (with 1.3MP 1.8 crop factor sensors, some of them monochrome) priced at $20k...$25k (almost a median US household income). Provided that most professional photographers were convinced (and correct) that digital image quality was significantly worse than film, the market would have been too small for full frame cameras at their enormous price.

The first FF dSLR, 6MP Contax N Digital, came 11 years later at $7000, a sixth of average US household income. 11MP Canon 1Ds was announced at $8k the same year.

Smaller sensors were (and still are) several times cheaper, and with the 1.5 or 1.6 crop factor, the quality difference was (and still is) not that big to justify the cost difference for most people. At the same time with 1Ds, APS-C Canon 10D was announced at only $2k.

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