Does a high ISO generally result in bad dynamic range and less accurate color?

Question

I have a Pentax K-x. I've had it set on "auto ISO up to 6400" for a while now, and I'm starting to wonder if this is making my images worse. Honestly I don't trust myself to tell.

Does a high ISO generally result in smaller dynamic range and less accurate color?

Asked by marienbad

Answer

Look at the DxOmark website where they review the K-x: http://www.dxomark.com/index.php/en.../Cameras/Camera-Sensor-Database/Pentax/Kx

In the measurements tab, you can see a graph of how the dynamic range appears limited when using ISO 6400. It appears to be limited to a range of 8.5EV. The color ranges also suffers.

I find the site to be a good tool to compare what different cameras may have at similar settings as technology gets better.

Answered by smigol

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Do ISO values for Film and DSLR cameras have 1 to 1 correspondence?

Question

For example, if we have a film with ISO/ASA speed of 100, is it equivalent to setting ISO = 100 on DSLRs?

Asked by Akram Mellice

Answer

Sort of... because ISO values for film don't even have a 1:1 correspondence. For example, Fujichrome Velvia 50 is rated at ISO 50, but if you set your camera on 50, you will generally underexpose. Most film photographers I knew set ISO 40 instead. However Kodak E-100 typically did expose properly at ISO 100.

A digital camera set at ISO 100 will typically respond "as if" it were film that responded with a proper exposure at ISO 100.

Film photographers grew to understand the characteristics of the particular film they were using and adjusted accordingly. Digital photographers don't typically have that problem.

Answered by Steve Ross

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How did film photographers overcome constant ISO/ASA values in low light situations?

Question

Today in Digital SLR era, when I find myself in low light situation in a place that I can't use my tripod or I don't have it, and still don't want to get blurry images, I use the widest aperture which may not be enough, so I raise my ISO.

I'm curious what did film photographers do with their fixed ISO/ASA per film roll? Specially if they are using a low ISO film like 50 or 100.

Asked by Akram Mellice

Answer

There were several techniques:

• Carry extra film backs (MF) or camera bodies (35mm) with different film loaded
• Push process lower ISO film to sacrifice fine grain in favor of perceived higher ISO
• Rewind the film already in the camera until just the leader is outside the canister, noting what frame you are on, then load a different roll. You can then reload the original roll and expose black frames until you are on the "next" frame
• Use a flash
• Use a tripod for more shots

I've been in many situations where simply walking from a more open area in a city to a denser part where the buildings created shadows made 2 or more stops difference. Carrying multiple bodies or push processing were my technique of choice. Push processing is an all-or nothing thing, so you either push the whole roll or you don't. Push and pull are for carrying one type of film you feel covers the most general case and then adjusting ISO for specific cases outside those boundaries. Carrying a separate body is great because you can switch back and forth quickly. It's not all or nothing, and you don't have to take the time to rewind film, etc.

ISO 50 film, in particular, is dicey because photographers chose it for (among other things) its grain structure. Pushing something like Velvia gives it a different character -- somewhat grainier and even more contrasty. Probably not the effect intended when the film was loaded. Even more so with the slow B&W films like Pan-X.

Answered by Steve Ross

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What is “ISO” on a digital camera?

Question

What is "ISO" in general, and how is the scale defined?

How does the ISO scale for film speed differ from ISO sensitivity as used in digital cameras?

Is lower ISO always better?

Asked by mattdm

Answer

In photography, ISO generally refers to a measure of "Film Speed", which I use including reference to digital sensor sensitivity.

In short, the actual letters ISO are a name for the International Organization for Standardization (not, officially, an acronym -- more information here), and in photography it refers to the ISO 12232:2006 standard and other related standards: ISO 12232:1998, ISO 5800:1987, ISO 6:1993 and ISO 2240:2003. (Links on the Film Speed page.) Film historically has also used the ASA and DIN standards, the former using the same numbering system, and DIN using an entirely different scale.

The standards for film and digital are technically different (in ways I haven't investigated closely enough to report fully on), but generally they're similar enough that for practical purposes, they're essentially the same (notwithstanding Reciprocity Failure, which many films are quite prone to, though digital generally is not). So if you measure an exposure with your digital camera, you could use that exposure with a film of the same rating as the ISO setting you used in the digital camera, and expect to get a similar exposure (unless the shutter speed long or short enough for Reciprocity Failure to kick in for the film) (also assuming similar equipment otherwise -- no differences in filters, etc.).

In both digital and film, a higher number indicates greater sensitivity. A number twice as high is twice as sensitive (e.g. 200 is twice as sensitive as 100, 400 twice as sensitive as 200, etc.). So, when shooting in relatively low light, and wanting a relatively-fast (e.g. fast enough to stop motion) shutter speed, a higher sensitivity rating will be essential (so no, lower is not always better!)

In digital cameras (and similarly but differently in film), higher ISO ratings tend to create noise (the related effect in film is increased graininess). So, while it's not always better (that depends on what you're going for), lower ISO ratings are always (or at least almost always) lower in noise, which may be desirable. (In the case of low light photography where shutter speed is not a concern, long exposures mixed with lower ISO ratings will create a "better" image -- though it's conceivable that some may like the effects of the noise; certainly there's appeal at times to film grain.)

As for how the scale is defined, it is based on measurements of an image produced based on a certain scenario of illumination. The details are complex, so I'll leave the details as an exercise for the reader. A lay summary is that (for digital) it's a measure of how quickly the digital sensor becomes "saturated" with light. (For film, the process is related but different.)

In summary: higher ISO is more sensitive but noisier (but not necessarily worse), digital and film rated for the same ISO (or ASA for the film) will have similar sensitivity, and the scale is based on how quickly an image will become "saturated" given a certain amount of illumination.

---

NOTE: I'm gearing up to do some experimentation related to the controversial answer from Matt Grum. Hopefully, my results will create a nice clear noise-free answer to the important point he brings up: that a high-ISO image with a low amount of light will be less noisy than a low-ISO image with the same amount of light getting to the sensor, which is later amplified in post-processing. More to come, hopefully in... EDIT: Well, I've failed to make this happen for a while now. I may still do it at some point. In the mean time, I'll also point to this article that talks about comparisons of native versus non-native ISO values, and amounts of noise in them, which, while the article doesn't exactly say so, I think is probably directly related to exactly this question.

Answered by lindes

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What are the advantages and disadvantages of using auto ISO on a DSLR?

Question

What are the advantages and disadvantages of using auto ISO on a DSLR?

Using manual ISO obviously gives you more control, but auto ISO may mean that you get "the shot" under quickly changing conditions (such as street photography, varying quickly between direct sunlight, shade, indoor lighting.

Is auto ISO a convenient feature or a bad habit?

Asked by Freed

Answer

Hardware ISO control exists to amplify the signal before readout to maximise the signal to noise ratio. Without read noise you wouldn't need ISO if you had a high precision ADC as you could simply shoot everything at the native sensitivity, and if the image was too dark just apply digital gain (multiply the pixel values).

It's easy to demonstrate this effect. The following two images had the same shutter speed and aperture. The top one was shot at the metered ISO 1600, the bottom one was shot at ISO 100, which resulted in underexposure which was fixed in photoshop:

The ISO 100 image is noisier, on account of read noise being amplified when the exposure is fixed. The ISO 1600 is amplified before readout so read noise is much lower.

As such ISO is not a creative control (like shutter speed and aperture) it is a practicality, required by the need to overcome noise. Contrary to popular opinion using a lower ISO wont give you less noise. Unless you also decrease shutter speed or open the aperture, both of which can result in a different looking image.

I don't consider it "lazy" to use auto-ISO, as the camera is not altering the creative controls, it's just trying to minimise read noise for you.

With auto-ISO in manual the pros and cons are pretty clear:

+ You can adapt to changeable lighting whilst sticking to your preferred aperture and shutter speed.

+ You shouldn't need to

- You're still at the mercy of the camera's light meter, you can get overexposure if the meter gets confused (which wouldn't happen if you fix the ISO to a low value).

- Some cameras don't offer exposure compensation with auto-ISO meaning if the metering is consistently off due to scene content you might not be using the optimal noise reducing ISO.

Using auto ISO in either program, shutter priority or is slightly different as there are two or three values for the camera to choose, and the ISO value is no longer simply dependent on the light reading.

+ You'll be able to sue preferable shutter speeds aperture more of the time than if ISO sensitivity was fixed.

- You're at the mercy of the auto-ISO algorithm, you're still letting the camera make creative decisions.

Answered by Matt Grum

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How important is ISO speed?

Question

My wifes Canon EOS 450D has various speed settings and an auto mode. The quickest one is 1600. She photographs mainly birds, both stationary and in flight and mainly still insects. What are the advantages of the 1600 setting, and when would it and lower speeds be used? If it as simple as a fast speed for moving objects, why have Canon gone to the trouble of providing multiple speed settings?

Asked by Dennis

Answer

For your wife, ISO 800 to ISO 1600 are probably correct, given that she photographs birds. Photographing birds is very difficult, particularly those in flight, with the 450D. It does not have very great AF (autofocus), and bird photography generally requires very long lenses (400mm telephoto is about the shortest focal length one should use when photographing birds, with 500mm or greater more ideal.)

Even with a telephoto lens, one has to get very close to a bird to get a frame-filling shot. The problem is that telephoto lenses are extremely expensive to get in the "fast" variety of f/2.8. Most are f/4 or slower, which makes it difficult to snap motion-freezing shots as the shutter is just too slow (sometimes even WITH image stabilization.)

I have the 450D myself, with the Canon EF 100-400mm f/4.5-5.6 IS lens. This is a great lens, but the maximum aperture (f/5.6 at 400mm) is just not fast enough to freeze birds, even those that are not in flight (a bird never rests, they are always flitting about, preening, or doing something involving motion.) At ISO 800-1600 with IS, I figure I'm about 1-1.5 stops too slow to get decent bird shots in good lighting.

Sadly, the 450D's ISO performance beyond ISO 400 is pretty bad, and produces a lot of noise. For bird photography, a FAR better camera is the 7D. It has one of the most advanced AF systems available these days, and supports ISO up to 6400. I recently had the chance to use another bird photographers 7D at Cherry Creek State Park in Colorado near my home. Using the 7D at ISO 3200 with a Canon EF 400mm f/4 lens, it was a WORLD of difference compared to the 450D at ISO 1600. The better AF snapped right onto birds in flight with ease, and the better ISO performance allowed a high shutter speed even at sunset, without terrible noise. (Noise on the 7D@3200 was better than the 450D@1600.) The 7D also has an edge from a cropping standpoint, with its 18mp sensor. Even if you don't get a frame-filling shot, you have plenty of room to crop down with an 18mp image, 50% more so than with the 12mp sensor of the 450D.

So, to summarize, it is not surprising your wife uses ISO 1600 for her bird photography, given the shortcomings of the 450D's AF system, and the inherent difficulties in bird photography in general. I am also not surprised she uses auto mode for everything else, as when you are out photographing birds, you are almost entirely consumed with creeping up on them without scaring them off, and you want your camera to be adaptable to any situation. By letting the camera pick the shutter and aperture, you don't have to bother with it yourself. I usually use P (program) mode myself when doing bird photography. I have basic exposure compensation control in that mode, which lets me take some control if and when I need it, without having to think too hard.

If your wife were to upgrade (or you were to get her an awesoem gift), the new 550D, or the 7D, would make a much better choice. The 550D goes for some $750 without a lens, while the 7D goes for $1500 without a lens. The extra cost of the 7D is definitely worth it for a bird photographer given its fantastic AF system, if you can afford it. The 60D sits in an odd spot for bird photography. At a price point of $1100 or so, it may be a better buy than the 550D (it might be a toss up), but is a very short cry from the 7D in price. The 7D wins hands down it in every way over the 60D (outside of maybe the 60D's fancy swivel screen.) If you were to upgrade to a better camera body, I would ignore the 60D and decide between the 550D and the 7D.

Answered by jrista

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Why am I getting such high noise when using ISO 320/640?

Question

I'm taking photos at night with a GH1 at ISO 640 and getting a ton of noise in the dark areas.

I've seen a bunch of other peoples night time photos and they never have this much noise. I tried using a really long exposure of 25 seconds to compensate for the low light but there's still a ton of noise. Is this unavoidable or are there other settings I can use to get rid of the noise? Or am I supposed to de-noise all the images in post?

The first shot is right before the moon came out and the second shot is when the moon was fully out (its was around a 65% full moon that night). I also shot these in Aperture Priority mode if that makes any difference. Thanks for the help!!

EDIT: I was googling and I just came across Exposure to the right(ETTR). Could this be what I'm experiencing here? The images were underexposed and therefore lose a lot of values?

20mm, F11, ISO640, 25sec exposure

20mm, F6.3, ISO320, 15sec exposure

Answer

This question is probably best answered in two parts.

Part1:

You may need to increase the ISO to combat noise. This sounds counter intuitive due to common misconceptions regarding noise.

Noise is principally caused by lack of light. Lightsources emit photons randomly, the more photons you collect, the more the randomness averages out, leaving a smooth image. Fewer photons -> more random -> lots of grain.

All increasing the ISO does, is amplify the signal (and noise) you get from the sensor prior to digitization. It doesn't create noise, and as it amplifies the signal and noise by the same amount the signal to noise ratio remains the same. Needing to amplify the signal implies you have a weak (and hence noisy) signal to begin with, hence the association with high ISO and noise.

Noise is also introduced when trying to read the analogue signal. By amplifying before reading you add a small amount of read noise to a big signal, with little effect. By shooting at a lower ISO (keeping the other settings the same) you amplify less, to you add a small amount of read noise to a small signal, giving you a worse result by lowering ISO.

Part 2:

Reducing read noise by upping the ISO will help, but the primary problem you have is lack of light. You need to get more light down the lens, by either opening the aperture, or increasing the exposure time.

If increasing the exposure time/aperture causes other parts of the scene to be too bright (to the point where they show up pure white) then you have an additional problem that the dynamic range (the difference between the brightest and darkest parts) of the scene is too high, this can be fixed by using graduated filters, or by combining multiple exposures.

If you have a static(ish) scene I would recommend shooting several identical 25s exposures and then averaging them in software to reduce the noise. This is analogous to having one very long exposure, except without problems due to overexposure of certain areas. This is how most good DSLR astrophotography is done.

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What settings should I use to photograph aurora borealis with a WIDE ANGLE F4 lens?

Question

I've tried my first night photography of the stars last week. Most articles I've seen were recommending a fast lens, fully opened, @ ISO 800 and about 20 - 30s exposure, etc. Unfortunately, my only "fast" lens is a 50mm f1.4 which was fine but not as wide as I would have liked.

There is a strong possibility of aurora borealis tonight and I would like to use my wide angle lens (Canon EF 17-40mm f/4.0 L USM) With maximum possible aperture of f4, what settings should I use to produce reasonably noise free photo. I would like to omit star trailing if possible.

I will be using Canon 60D

Answer

I could make an educated guess..

Consider this picture, its EXIF data says exposed for 5 sec @ f2.8 at ISO 1600, ignoring the focal length of 14mm. and considering that you said most articles recommend shooting @ ISO 800 using max aperture (in your case f4) you should set your exposure for 20 seconds.
@ISO 400, shutter speed needs to be 40sec
@ISO 200, shutter speed needs to be 80sec
@ISO 100, shutter speed needs to be 160 seconds
but longer shutter speed might cause star trails.
Then again you can set the ISO to 3200 to reduce the shutter speed to 5 seconds.

Assuming the brightness of the Aurora Borealis is the same as in this picture.

I also suggest turning on the custom fuction on the 60D for high ISO noise reduction and Long Exposure noise reduction.

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Why does the camera default to ISO 400 in auto ISO mode with flash?

Question

I've observed this behaviour on my Canon 550D, and I was wondering why it didn't use a lower or even variable ISO. I'm not sure if this is a camera/brand specific setting.

Is there any particular reason for this behaviour? Are there any scenarios where a different ISO is chosen, as I have not encountered it so far?

Answer

While the specific value chosen will be brand-specific, you're right that this is common behavior. My Fujifilm point & shoot favored ISO 800.

Increasing the ISO from 100 to 400 doubles the effective range of your flash, which is important with a relatively-anemic built-in flash. It also means that half of the flash power can be used for a subject within the normal range, saving battery life, decreasing the time it takes to be ready to flash again, and shortening the duration of the flash pulse to better freeze motion.

Another approach would be to increase aperture (more open; smaller numbers), which would have a similar effect, but would reduce depth of field. That means focus has to be more dead-on, and is generally not what auto-exposure modes are programmed to go for unless they have no other option.

I don't know the specifics of the auto-ISO logic in your camera model, but I wouldn't be surprised if it favors keeping ISO at around 400 and only increasing it only when there's not enough flash power.

When I'm using off-camera flash, I usually set ISO manually, but I generally default to around 400, just like your auto mode. This gives me a lot of flexibility in aperture, and even though my camera isn't the latest sensor generation, that ISO is clean enough that I don't really worry about noise.

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What megapixel value is equivalent to which ISO film?

Question

Is there some kind of equivalency table or formula that expresses what kind of pixels you need in a digital camera to have roughly the same quality as a particular ISO graded film? What other variables would influence this (focal-length, exposure time, etc)?

Answer

I remember seeing a figure of 22MP was "as good" as 35mm resolution (of course, with film it isn't just the ISO, but the manufacturer and age of the film, skill of developer etc.)

Higher ISO film tended to have more grain; and higher ISO digital shots exhibit more noise - a similar cause, but the visual appearance is different.

Digital ISO noise is related to the size of each pixel, as the noise is per-pixel (so the more pixels you have, the less obvious noise is when viewed the same size). One analogy I've used in the past to demonstrate this is to ask several people to time with a stopwatch how long it takes a car to drive around a car park, and then to time how long a person takes to do the same journey - because the person is slower, the margin of error is a smaller in proportion to the overall figure, even though different people will give timings to within a few seconds of each other.

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What is the quantative relation between flash guide number and ISO?

Question

I found that formula on the Internet:

Guide Number = (Shooting Distance * Aperture) ÷ ISO Sensitivity

Is it correct? If it is could someone please explain why ISO is related to guide number in this way. The formula in wikipedia article about guide number does not have ISO in it so I wanted to know if the one I found is the right one and why.

UPDATE: this formula should look like this

Guide Number = (Shooting Distance * f-number) ÷ ISO factor

See the answer for the details!

Answer

The formula you've given is incorrect, at least for "straight" values of ISO numbers. ISO is related to sensitivity in that each stop in increased ISO is the same as a single stop of increased aperture. That means that to get ISO 200 guide numbers from ISO 100 numbers, you multiply by sqrt(2), just as increasing aperture by that factor is one stop. Quadrupling the ISO doubles the guide number, and so on. Or, expressed the other way around in the equation, as in your formula: the guide number required for a given aperture and distance goes down by a factor of about 1.4 for every stop of increased ISO.

So, it works if you replace "ISO sensitivity" in your formula with something like "ISO factor", where:

ISO  100 = 1
ISO  200 = 1.4
ISO  400 = 2
ISO  800 = 2.8
ISO 1600 = 4
...

Note the familiar sequence of numbers — that's no coincidence.

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What makes a good sample image for ISO comparison?

Question

To be more specific this image is always used by Digital Photography School for ISO comparison. What are those features that make this image suitable for ISO comparison.

Answer

In ISO comparisons you need to have a bit of everything, shadow, good range of mid tones, and some highlights plus a range of colours. This shot has all of that, which is why it's an ideal shot to analyse how well a camera deals with various ISO settings. Basically for any performance analysis in a single shot there needs to be as wide a range of colour, tone and detail as possible to judge how a camera deals with each of these factors so thats what makes images such as this very good for the job.

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What are the pros/cons of the high-ISO noise reduction feature on D90?

Question

So my D90 offers "High-ISO Noise Reduction". Sounds like a good thing, but obviously there must be a cost of some kind. The settings are "Off", "Low", "Normal", and "High", and the default is "Normal".

What are the pros and cons of using this feature? Why not always set it to "High"? Or, should I be turning it off?

Answer

The pro's are self-evident:

• Lower noise on high-iso

The biggest con:

• Loss of detail

The high-iso noise reduction might remove detail mistakenly. While newer camera algorithms have gotten better at it, it's still not fool proof.

The settings between Off, Low, Normal and High dictate the amount of tolerance used for the setting, which affects the amount of detail that might be lost.

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Does shooting at lower RAW resolution using crop sensor camera mimics qualities of full frame cameras?

Question

I'm not talking about changes to the focal length.

I've read many post that say in full frame camera the pixel density is lower compared to crop sensor camera's and so it captures more light and has thus has better ISO performance and greater dynamic range. So if I change by crop sensor camera to shoot at a lower resolution, will that equate to a better pixel density and mimic the performance of a full frame (or medium format) or will it always shoot at maximum resolution and the reduce the size?

--EDIT: 1--
I've a Canon 60D and I've 3 options for RAW image sizes (RAW, M-RAW amd S-RAW). If RAW is just a dump from the Camera sensors, How can their be 3 different sizes? Does the camera also scale down RAW images as well?

Answer

Given that you have a Canon, the lower RAW modes, mRAW and sRAW, DO INDEED UTILIZE ALL of the available sensor pixels to produce a richer result without the need for bayer interpolation. The actual output format, while it is still contained within a .cr2 Canon RAW image file, is encoded in a Y'CbCr format, similar to many video pulldown formats. It stores luminance information for each FULL pixel (2x2 quad of 1 red, 1 blue, and 2 green pixels), and each chrominance channel is derived from half pixel data (1x2 pair of 1 red+1 green or 1 blue+1 green).

I am not exactly certain what the specific low-level hardware read and encoding differences between mRAW and sRAW are, however generally speaking the smaller the output format, the more sensor pixel input information you can use for each output pixel. The small amount of interpolation present in m/sRAW is moot, as both formats interpolate far less than native RAW. It should also be noted that neither mRAW nor sRAW are actual "RAW" formats in the normal sense...sensor data IS processed and converted into something else before it is saved to a .cr2 file.

For more details about YUV derived formats and Canon sRAW, see my answer here: Why isn't the xvYCC color space seeing uptake for still photography?

From "Understanding What is stored in a Canon RAW .CR2 file":

The sRaw format (for "small RAW") was introduced with the 1D Mark III in 2007. It is a smaller version of the RAW picture.

For the 1D Mark III, then the 1Ds Mark III and the 40D (all with the Digic III), the sRaw size is exactly 1/4 (one fourth) of the RAW size. We can thus suppose than each group of 4 "sensor pixels" is summarized into 1 "pixel" for the sRaw.

With the 50D and the 5D Mark II (with the Digic IV chip), the 1/4th size RAW is still there (sRaw2), and a half size RAW is also appearing : sRaw1. With the 7D, the half size raw is called mraw (same encoding as sraw1), 1/4th raw is called sraw (like the sraw2).

the sRaw lossless Jpeg is always encoded with 3 colors component (nb_comp) and 15 bits.

Jpeg code of Dcraw was first modified (8.79) to handle sRaw because of the h=2 value of the first component (grey background in the table). Normal RAW have always h=1. Starting with the 50D, we have v=2 instead of v=1 (orange in the table). Dcraw 8.89 is the first version to handle this and the sraw1 from 50d and 5D Mark II.

"h" is the horizontal sampling factor and "v" the vertical sampling factor. It specifies how many horizontal/vertical data unit are encoded in each MCU (minimum coded unit). See T-81, page 36.

3.2.1 sRaw and sRaw2 format

h=2 means that the decompressed data will contain 2 values for the first component, 1 for column n and 1 for column n+1. With the 2 other components, decompressed sraw and sraw2 (which all have h=2 & v=1), always have 4 elementary values

[ y1 y2 x z ] [ y1 y2 x z ] [ y1 y2 x z ] ...
(y1 and y2 for first component)

Every "pixel" in sRAW and mRAW images contain four components...a split Y' component (y1 and y2), as well as an x (Chrominance Blue) and z (Chrominance Red). All four components (from a 1/2 image perspective, sRAW1/mRAW) have a column height of 2 (h) and a width of 1 (v). This indicates that the Luminance value (Y') is comprised of a FULL 2x2 pixel quad...or two 2x1 pixel columns stored in y1 and y2.

The references below do not seem to specifically state this, so I am speculating a bit here, however with the sRAW2 (1/4 raw) I believe Luminance information would be derived from a 4x4 pixel block where h=4 and v=2. Encoding chrominance would get more complex at a 1/4 size image, as the bayer color filter array on the sensor is not arranged in neat red and blue columns. I am unsure whether alternating 2x1 height columns are processed for each Cr and Cb component, or if some other form of interpolation is performed. One thing is certain...the interpolation of source data is always larger than the output data, and no overlapping (as in normal bayer interpolation) occurs as far as I can tell.

Finally, sRAW1/mRAW and sRAW/sRAW2 are compressed using a lossless compression algorithm. This is a critical distinction between these formats and JPEG, which also uses a ycc type encoding. JPEG performs lossy compression, making it impossible to restore pixels back to their exact original representation. Canon's s/mRAW formats are indeed able to be restored back to original full precision 15-bit image data.

References:

• Understanding What is stored in a Canon RAW .CR2 file, How and Why
  • For pulldown details, see text just before section 3.2.1
• DIGITAL COMPRESSION AND CODING OF CONTINUOUS-TONE STILL IMAGES – REQUIREMENTS AND GUIDELINES

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Is an artificial external source of light required even with a low-noise high ISO range?

Question

Is an artificial external source of light required even if you have a camera with a high range of effectively noiseless ISO?

Are there some special cases which "need" external lights, at all costs?

Answer

Lighting is about a lot more than simply making a subject visible to the camera, it can change a shot dramatically so always relying on naturally occurring light severely limits creative outcome. You have to bear in mind that high ISO shots rely on the available light and will simply make the scene uniformly brighter the higher you go. The available light may be perfectly adequate to get a correct exposure at high ISO but may be so even for instance that the resulting image is flat and boring. In the other extreme if available light is very low boosting ISO will just give a uniformly bright image with no tonal range. Introducing external lights will add contrast and depth to the shot if done carefully.

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What does “expanded ISO” mean?

Question

The ISO specification for the Canon EOS 7D reads as follows:

High ISO For handheld shooting in low light, the EOS 7D offers ISO speeds of up to 6400. Expandable to ISO 12800, for low light scenes where using flash is undesirable.

Why is it phrased this way? Is there something extra that is needed in order to "expand" ISO to 12800?

If not, then presumably the camera is capable of 12800 out of the box — so why not just list that as the max ISO speed?

Similarly, the Nikon D5100 uses Hi1 and Hi2 instead of numeric ISO settings above 6400. If these are "real" ISO settings, why not just call them ISO 6400 and ISO 12800?

What about cameras which have an expanded ISO range on the low side? For example, an expanded ISO setting may allow a choice of 50 rather than 100. Generally,the standard high ISO is very noisy, with the expanded ISO even more so. Are these lower ISOs less noisy than the "base"?

How do these expanded ISOs affect image quality on either side? Is it better to avoid them and do the equivalent processing with RAW files later, or is there an advantage to using these settings in-camera?

Answer

There are two reasons why an ISO is not made part of the 'normal' range:

1. It is considered a non-trivial drop in quality and you do not want users complaining about its performance. In other words, if the quality difference between ISO 12800 and 6400 is stronger than the one from 3200 to 6400. Note that there may be more changes than simply more noise, colors can be affected as well.

2. The camera meters and exposes for the said ISO, say 12800, but the results do not strictly comply with the ISO standard. When that happens, you will notice that the ISO is NOT stored in the EXIF of the image. This usually happens because of a drop in dynamic-range at the expanded setting.

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Does shooting at lower RAW resolution using crop sensor camera mimics qualities of full frame cameras?

Question

I'm not talking about changes to the focal length.

I've read many post that say in full frame camera the pixel density is lower compared to crop sensor camera's and so it captures more light and has thus has better ISO performance and greater dynamic range. So if I change by crop sensor camera to shoot at a lower resolution, will that equate to a better pixel density and mimic the performance of a full frame (or medium format) or will it always shoot at maximum resolution and the reduce the size?

--EDIT: 1--
I've a Canon 60D and I've 3 options for RAW image sizes (RAW, M-RAW amd S-RAW). If RAW is just a dump from the Camera sensors, How can their be 3 different sizes? Does the camera also scale down RAW images as well?

Answer

Given that you have a Canon, the lower RAW modes, mRAW and sRAW, DO INDEED UTILIZE ALL of the available sensor pixels to produce a richer result without the need for bayer interpolation. The actual output format, while it is still contained within a .cr2 Canon RAW image file, is encoded in a Y'CbCr format, similar to many video pulldown formats. It stores luminance information for each FULL pixel (2x2 quad of 1 red, 1 blue, and 2 green pixels), and each chrominance channel is derived from half pixel data (1x2 pair of 1 red+1 green or 1 blue+1 green).

I am not exactly certain what the specific low-level hardware read and encoding differences between mRAW and sRAW are, however generally speaking the smaller the output format, the more sensor pixel input information you can use for each output pixel. The small amount of interpolation present in m/sRAW is moot, as both formats interpolate far less than native RAW. It should also be noted that neither mRAW nor sRAW are actual "RAW" formats in the normal sense...sensor data IS processed and converted into something else before it is saved to a .cr2 file.

For more details about YUV derived formats and Canon sRAW, see my answer here: Why isn't the xvYCC color space seeing uptake for still photography?

From "Understanding What is stored in a Canon RAW .CR2 file":

The sRaw format (for "small RAW") was introduced with the 1D Mark III in 2007. It is a smaller version of the RAW picture.

For the 1D Mark III, then the 1Ds Mark III and the 40D (all with the Digic III), the sRaw size is exactly 1/4 (one fourth) of the RAW size. We can thus suppose than each group of 4 "sensor pixels" is summarized into 1 "pixel" for the sRaw.

With the 50D and the 5D Mark II (with the Digic IV chip), the 1/4th size RAW is still there (sRaw2), and a half size RAW is also appearing : sRaw1. With the 7D, the half size raw is called mraw (same encoding as sraw1), 1/4th raw is called sraw (like the sraw2).

the sRaw lossless Jpeg is always encoded with 3 colors component (nb_comp) and 15 bits.

Jpeg code of Dcraw was first modified (8.79) to handle sRaw because of the h=2 value of the first component (grey background in the table). Normal RAW have always h=1. Starting with the 50D, we have v=2 instead of v=1 (orange in the table). Dcraw 8.89 is the first version to handle this and the sraw1 from 50d and 5D Mark II.

"h" is the horizontal sampling factor and "v" the vertical sampling factor. It specifies how many horizontal/vertical data unit are encoded in each MCU (minimum coded unit). See T-81, page 36.

3.2.1 sRaw and sRaw2 format

h=2 means that the decompressed data will contain 2 values for the first component, 1 for column n and 1 for column n+1. With the 2 other components, decompressed sraw and sraw2 (which all have h=2 & v=1), always have 4 elementary values

[ y1 y2 x z ] [ y1 y2 x z ] [ y1 y2 x z ] ...
(y1 and y2 for first component)

Every "pixel" in sRAW and mRAW images contain four components...a split Y' component (y1 and y2), as well as an x (Chrominance Blue) and z (Chrominance Red). All four components (from a 1/2 image perspective, sRAW1/mRAW) have a column height of 2 (h) and a width of 1 (v). This indicates that the Luminance value (Y') is comprised of a FULL 2x2 pixel quad...or two 2x1 pixel columns stored in y1 and y2.

The references below do not seem to specifically state this, so I am speculating a bit here, however with the sRAW2 (1/4 raw) I believe Luminance information would be derived from a 4x4 pixel block where h=4 and v=2. Encoding chrominance would get more complex at a 1/4 size image, as the bayer color filter array on the sensor is not arranged in neat red and blue columns. I am unsure whether alternating 2x1 height columns are processed for each Cr and Cb component, or if some other form of interpolation is performed. One thing is certain...the interpolation of source data is always larger than the output data, and no overlapping (as in normal bayer interpolation) occurs as far as I can tell.

Finally, sRAW1/mRAW and sRAW/sRAW2 are compressed using a lossless compression algorithm. This is a critical distinction between these formats and JPEG, which also uses a ycc type encoding. JPEG performs lossy compression, making it impossible to restore pixels back to their exact original representation. Canon's s/mRAW formats are indeed able to be restored back to original full precision 15-bit image data.

References:

• Understanding What is stored in a Canon RAW .CR2 file, How and Why
  • For pulldown details, see text just before section 3.2.1
• DIGITAL COMPRESSION AND CODING OF CONTINUOUS-TONE STILL IMAGES – REQUIREMENTS AND GUIDELINES

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Is an artificial external source of light required even with a low-noise high ISO range?

Question

Is an artificial external source of light required even if you have a camera with a high range of effectively noiseless ISO?

Are there some special cases which "need" external lights, at all costs?

Answer

Lighting is about a lot more than simply making a subject visible to the camera, it can change a shot dramatically so always relying on naturally occurring light severely limits creative outcome. You have to bear in mind that high ISO shots rely on the available light and will simply make the scene uniformly brighter the higher you go. The available light may be perfectly adequate to get a correct exposure at high ISO but may be so even for instance that the resulting image is flat and boring. In the other extreme if available light is very low boosting ISO will just give a uniformly bright image with no tonal range. Introducing external lights will add contrast and depth to the shot if done carefully.

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How do I enable auto-ISO in non-scene modes in a Nikon D5100?

Question

I'm trying to enable auto-ISO on my dad's Nikon D5100. There is an "Auto" option for "ISO sensitivity" in the menu, logically-enough under "ISO sensitivity settings"). This works fine in the Scene modes, but with P, A, S, or M, I get:

This option is not available at current settings or in the camera's current state.

Surely auto-ISO is not only available in the preprogrammed modes. How do I enable it?

Update: there is a separate section to "ISO sensitivity settings" below the main choice. That is only available in the P, S, A, and M modes, and has "Auto ISO sensitivity control". I've set this to ON, but empirically it's the hard-set value above that actually gets used. What is this separate setting, why is it separate, and (most of all) why doesn't it work?

Answer

Auto ISO isn't available in the non-scene modes per se.

The ISO Sensitivity Auto control option you mention in the latter part of your question is essentially 'Auto Pro'. You'll note in the menu that you can set a max ISO and a minimum shutter speed.

When ISO Sensitivity Auto control is enabled, the camera will endeavour to use the ISO you set, but will increase the ISO automatically (up to your defined max) if it's not achieving the shutter speed set in the menu. If you actually take a shot then check the settings used on the LCD, you may well see that the ISO will be a different value, and shown in red, to indicate that the camera took over and adjusted it automatically.

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What is exposure compensation?

Question

What does exposure compensation do?

If I take a photo with a given shutter speed, aperature, and ISO, and then take the same shot with +1EV or -1EV, what is actually happening?

Is this just a gain control on the sensor?

Can you achieve the same thing by changing ISO?

Answer

Exposure compensation changes the target exposure. Normally the camera is trying to work the settings to get about 18% grey (reflectance), but with exposure compensation of +1EV you are basically just saying to the camera "I want to expose this scene 1 stop lighter than the normal average."

Changing ISO vs Exposure Compensation

• Manual shooting

  • Changing the ISO would have the same effect on exposure that Exposure Compensation would have in AUTO mode. However, so would changing your shutter speed or aperture.

• Automatic modes

  • The camera will change either ISO, aperture, or shutter speed as needed to achieve the correct exposure, so adjusting your ISO will only change the shutter/aperture that the camera sets. Changing the exposure compensation will change the target exposure.

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