Max aperture on zoom lenses - stepwise or linear?

Question

When zooming with lenses without a fixed max aperture, the change in aperture* that the camera displays is stepwise, usually in steps of thirds. But, is the max aperture really the same all the way as long as the camera says it's the same, or is there a differense within the same steps? I.e, will the aperture actually be a little bigger at the widest end of such an interval?

And, are there any charts etc available that shows zoom lenses' intermediate max apertures, between longest and shortest focal length? I'm mostly interested in Canon, but other brands as well.

*) edit: I guess F-number is more acurate.

Asked by Daniero

Answer

The selected aperture is rarely the actual exact physical aperture of the lens. Usually there is a certain amount of discrepancy between the relative aperture the camera reports, such as f/3.5, and the actual physical area of the current aperture. As such, exposure is rarely exact, and can vary to a measurable (and often visible) degree between samples of a given lens and between a given lens and samples of a given camera. There are also often variations by as much as 1/3rd of a stop or more between camera brands for what would otherwise be exactly the same exposure settings with otherwise equivalent lenses.

The best real-world demonstrations of the difference between a reported aperture and the specification of aperture of a lens required to achieve its design are usually lens patents. Some recent Canon lens patents as reported by Canon Rumors are a great example of some real-world "relative aperture" values for pending lens designs:

Example 1
- Zoom ratio 4.01
- 135.50 – - 290.90mm 72.50 focal length
- Fno 4.66 -. 4.97 – 5.87
- 9.07 – - 4.25deg 16.62 a half angle of view.
- Image height 21.64mm
- 171.47 – - 204.08mm 144.08 full-length lens
- BF 40.08mm
- 18 sheets 12 group lens configuration
- 3 UD glass sheet
- A plane diffraction
- Seven-group zoom positive and negative positive positive and negative polarity
- Inner Focus (Group 6)
- Shake correction (group 2)

Example 2
- Zoom ratio 2.84
- 200.00 – - 292.50mm 103.00 focal length
- Fno 4.67 -. 5.44 – 5.77
- 6.17 – - 4.23deg 11.86 a half angle of view.
- Image height 21.64mm
- 189.12 – - 210.66mm 162.16 full-length lens
- BF 45.16 – 58.25 – 70.16mm
- 13 pieces in 11 groups Lens Construction
- 2 UD glass sheet
- A plane diffraction - Five-group zoom of positive and negative positive positive and negative
- Rear focus

You'll notice the Fno specifications for these two examples of new DO or diffractive optics lens designs. The first example lists the F-Number range as 4.66 to 5.87. Neither of those are standard F#'s, such as f/4.5, f/5 or f/5.6, however they are the specified engineering limits of the lens. You can't actually dial in an exact f/4.5 aperture on Example Lens #1...when you do, you are actually getting an actual f/4.66 aperture. Same deal if you dial in f/5.6, which in reality would mean your getting an actual aperture of f/5.87. (The middle aperture number, if I understand the rather oddball nomenclature of these patents, would be what you get around the middle of the lens, which seems to be the first focal length number, which in the case of Lens #1 is 135mm.)

As you change the focal length of a lens at maximum aperture for a variable aperture lens, the physical size of the aperture DOES NOT change. The diaphragm remains at its widest-possible ("relaxed") setting. The real F-Number's will change smoothly and not in a non-stepped manner. The lens will report the nearest "well-known" 1/3rd stop aperture at one of the specified points (i.e. f/4.5 for f/4.66 @ 72.5mm, f/5 for f/4.97 @ 135.5mm, f/5.6 for f/5.87 @ 290.9mm), and that is what will show up in EXIF as the selected aperture, however the actual apertures (i.e. f/5.87) will often show up in EXIF as "Max Aperture Value" or something similar.

You can usually observe this smooth change in aperture if you point a lens up towards your face with a bright light above your head so it illuminates the inner barrel of the lens, and adjust the focal length. You will see that the camera does not do any kind of micro-adjustment of aperture for you as you zoom. This is always the case at maximum aperture, and usually the case at all other apertures, although sometimes there are slight differences between instances of stopping the lens down to a smaller aperture simply due to the nature of a diaphragms operation. (Use a DOF Preview button to see the behavior at any aperture, including max aperture...otherwise the camera will always remain in a "relaxed" state.)

The same precision differences are present in other aspects of the lens as well. Example Lens #1 is actually a lens with a zoom ratio of 4. Funky nomenclature aside, the lens is actually a 72.5mm to 290mm lens...or a replacement for the 70-300mm f/4.5-f/5.6 DO lens. Similarly, Example Lens #2 is actually a 100-300mm f/4.5-5.6 DO lens.

While these imprecise specifications, at least relative to the idealistic numbers we photographers usually think about, are actually very exact and very necessary to the successful manufacture of a given lens at a given price point. Lens manufacture for DSLR lenses is very complex, and particularly when you get into larger lenses or extremely wide angle lenses, can be very costly due to the physical size of many of the necessary lens elements. Diffractive Optics (DO) lenses have the added complexity of diffraction-grating elements that, while they allow lenses to be made physically smaller, require an extra set of complex manufacturing procedures.

Precise specifications like this allow manufacturers to create a 100-300mm DO lens they can actually sell and make a bit of a profit on, without the discrepancies with their "sales specifications" (i.e. 103mm - 292.5mm rather than a lens that is exactly 100mm - 300mm) really having much of a difference in real-world user. It should be noted that these discrepancies really do not matter in the real-world, and the discrepancies can be larger at longer focal lengths. Some ten millimeters or so difference at supertelephoto lengths, a few millimeters of difference at normal to short telephoto lengths, or a fraction of a millimeter at wide-angle lengths, as well as small discrepancies in F-Number are all indistinguishable by photographers in the real world, so don't let them bother you.

Answered by jrista

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Do rounded edges on aperture blades improve image sharpness, and how?

Question

In the description for the XF lenses for its new X-Pro system, Fujifilm crows:

The [XF lenses] all offer precise control over depth-of-field and deliver excellent out-of-focus bokeh thanks to the design of the molded aperture diaphragm blades. The blades are curved to create a circular image at all aperture settings, while the very edges of each blade are meticulously rounded off rather than simply cut off, which delivers a sharper image. [Emphasis added]

I understand that curved blades — often described as rounded blades — help out-of-focus areas appear more attractive when stopped down, by producing a circular bokeh pattern instead of a polygonal one. But this is bragging about something else: apparently the edges are smooth in the other dimension.

Does this really matter?

• Will it really improve sharpness? Does it affect diffraction in some way?

• What about subjective qualities — might the bokeh be affected in some way?

• Is this construction used on other modern, premium primes with rounded aperture blades, or is it unique?

Asked by mattdm

Answer

When they talk about increased sharpness, they're talking about the blades' edges, not their plan shape. The actual edge of the blade is shaped to be somewhat like a dullish single-bevel knife blade:

That does two things: it means that there is only one edge in play (rather than two, as would be the case with an iris blade that is stamped or otherwise cut with its edges perpendicular to its surface); and that there is virtually no reflective surface in the optical path.

I'm not entirely sure I buy the reflective surfaces part, since that can be overcome with a surface treatment, but the single-diffraction-edge part would be significant, especially at smaller apertures. Now, there's "significance" as a theoretical matter, and there's "significance" in practice, and I'm not equipped to actually test the lens independently of the camera or with differently-shaped iris blades. It may simply be a technical flourish, but Fuji do seem to be going out of their way (between the lack of AA filter, different colour mosaic, etc.) to make a bit of a sharpness and detail statement in the APS-C world, and it just may be the case that the cumulative effect of paying attention to a lot of little details (like the iris blades, the geometry of the exit cone, and so forth) are what they think they have to do to make an appreciable dent in the market.

Answered by Stan Rogers

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Why do compact digital cameras not have the aperture range of DSLRs?

Question

Why is it that on compact digital cameras the aperture never seems to go any smaller than about F8 ? Even on high-end compacts such as the Cannon G10 or Panasonic LX5. Is there some practical or physical limitation due to the size of the camera or the CCD which prevents this from being possible?

Asked by mdresser

Answer

Although the relative aperture numbers — the ƒ stops — are the same regardless of format, the actual focal lengths of the lenses on small cameras are quite low: 5mm or 6mm at the wide end. That in turn means that the real aperture is small, which is why the diffraction limit kicks in sooner.

The smaller format also means that depth of field is hugely increased — even wide open at f/2.8, a camera like the Canon G10 has an infinite depth of field if you're focusing farther away than a few feet. So, there's not much difference in that aspect of changing aperture, so from that point of view there's no point even bothering. And that's presumably why there's usually not many choices besides wide open and one closed-down stop like f/8. (Because everything is small, and competitive price pressure significant, adding the mechanics for more intermediate stops is easily deemed not worth it.)

The other aspect of a smaller aperture is, of course, controlling exposure in bright light, without artificially dropping ISO beyond the sensor base or using very high shutter speeds. Some compact cameras actually use a dark neutral-density filter instead of closing down the aperture, specifically to avoid issues with diffraction.

Answered by mattdm

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Why are the area of aperture, focal length, and amount of light specified in these terms?

Question

So I'm looking at the wikipedia article on Aperture, and I'm a bit confused. I understand aperture to be the diameter of the hole that lets light in. In the article, someone states that

The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Area = pi ( focal length /(2 * f number) )^2

But in the f number article, they define f number as

f = focal length / aperture

It then seems trivial to substitute in:

Area = pi ( focal length / (2 * focal length / aperture) )^2

Area = pi ( focal length * aperture / (2 * focal length) )^2

Area = pi ( aperture / 2)^2

Area = pi ( radius )^2

So, my question is: Why did they bother putting something so blatantly obvious into the article? Didn't we already know that the area of the aperture was pi * r^2? Is this just me reading too much into a wikipedia article?

Asked by Chaosphere2112

Answer

The focal length and f-number are photographically relevant quantities, so the formula is expressed in terms of those convenient variables. Photographers don't generally know the radius of their lens aperture for every f-stop.

Answered by coneslayer

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How to test what aperture is actually used?

Question

It seems odd to me that Canon EF 100-400mm f/4.5-5.6L does away with a front element of only about 63 mm, as reported by @jrista - which would be enough for only f/6.3 at 400mm, missing the spec by third of a stop.

It makes me wonder if it's possible to measure what aperture is actually used during taking a photo. It'd be useful both in the described case and exploring how exact stopping down to a smaller aperture actually is.

So my question is - how to measure what aperture is actually used to take a photo? It's okay if the scene has to be specially constructed/measured for performing the test.

Asked by Imre

Answer

You can probably calculate this by rearranging the DOF formula to solve for c, or circleOfConfusion, as @MattGrum stated. I haven't tried to rearrange a formula as complex as DOF for a while, so I hope my math is correct here:

DOF = (2NcF^2s^2)/(f^4 - N^2c^2s^2)

The terms of that equation are as so:

DOF = depth of field
N = f-number
f = focal length
s = subject distance
c = circle of confusion

For simplicity sake, I'm going to reduce the DOF term to just D.

Now, the term for c appears twice in this equation, one of them to the power of two, so were probably looking at a polynomial of some sort in the end. To rearrange:

D = (2Ncf^2s^2)/(f^4 - N^2c^2s^2)
D * (f^4 - N^2c^2s^2) = (2Ncf^2s^2)
Df^4 - DN^2c^2s^2 = 2Ncf^2s^2
0 = 2Ncf^2s^2 + DN^2c^2s^2 - Df^4
DN^2c^2s^2 + 2Ncf^2s^2 - Df^4 = 0 <-- QUADRATIC!

As Indicated, rearranging terms produces a quadratic polynomial. That makes it pretty strait forward to solve, since quadratics are a common type of polynomial. We can simplify for a moment by substituting some more general terms:

X = DN^2s^2
Y = 2Nf^2s^2
Z = -Df^4

That gives us:

Xc^2 + Yc + Z = 0

Now we can use the quadratic equation to solve for c:

c = (-Y ± sqrt(Y^2 - 4XZ)) / (2X)

Replacing the X, Y, and Z terms with their originals and reducing:

c = (-2Nf^2s^2 ± sqrt(4N^2f^4s^4 + 4D^2f^4N^2s^2)) / (2DN^2s^2)

(Whew, thats pretty nasty, and I hope I got all the right terms replaced and typed in correctly. Apologies for discrepancies.)

My brain is a bit too fried right now to figure out exactly what it means for the circleOfConfusion to be quadratic (i.e. having both a positive and negative result.) My first guess would have to be that c grows both when you move towards the camera from the focal plane (negative?), as well as away from the camera and focal plane (positive?), and since quadratic equations grow to infinity pretty quickly, that would indicate the limit on how large or small the circle of confusion could actually become. But again, take that analysis with a grain of salt...I scratched out the solution to the formula and that took the last bit of brainpower I had left today. ;)

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If that is the case, then you should be able to determine a maximum CoC for a given aperture and focal length, which would, hopefully, be (or allow deriving) the diameter of the aperture (entrance pupil.) I am willing to bet, however, that this is not actually necessary. My analysis on the linked answer of @Imre's question was rather rough...I don't quite have the ability to observe my 400mm lens' aperture at "infinity", so I am probably seeing the entrance pupil incorrectly. I would be willing to bet that at a sufficient distance that you could call "infinity", the 100-400mm lenses f/5.6 aperture at 400mm would indeed appear to be the same diameter as the front lens element, so at least 63mm in diameter. My measurement of the diameter of that lens was a bit rough too, and it could be off by ±3mm as well. If Canon's patent for a 100-400mm f/4-5.6 lens is telling, the actual focal length of the lens is 390mm, and the actual maximum aperture at "f/5.6" is really f/5.9. That would mean the entrance pupil would only need to appear 66mm in diameter "at infinity", which is within margin of error for my measurements. As such:

I believe the EF 100-400mm f/4.5-5.6 L IS USM lens from Canon is probably spot-on as far as aperture goes, with a 390mm actual focal length and a 66mm entrance pupil diameter, all of which would jive with my own actual measurements of this lens.

Answered by jrista

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What is T-number / T-stop?

Question

Usually, when discussing aperture of a lens, F-stop and F-number are used for quantifying. But some photographers, and especially videographers, also mention T-stop. The concept and numbering used (e.g. T/3.4) seem to be similar to F-stops.

What is a T-stop, how is it related to F-stop, and what are the differences?

Asked by Imre

Answer

F-stops are purely geometrical, the ratio of aperture to focal length, regardless of actual light transmitted. But all lenses absorb a part of the light passing through them, and the amount being absorbed varies lens to lens. So, in situations where even the slightest change of lights being transmitted affect the output, i.e cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable, T-Stop is used as an standard. Since all lenses absorb some light, the T-number of any given aperture on a lens will always be greater (less light transmission) than the f-number. For example, a lens with f-stop 2.8 can have a t-stop 3.2, meaning a small portion (0.4) of the transmitted light have been absorbed by the lens glass elements.

A real lens set to a particular T-stop will, by definition, transmit the same amount of light as an ideal lens with 100% transmission at the corresponding f-stop. A f/2.8 lens can have t/3.2 and another f/2.8 lens can have t/3.4, so the actual lights being transmitted are not the same though they both have the same f-stop.

Answered by ShutterBug

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Why does Canon and Nikon limit or disable autofocus beyond certain f-numbers?

Question

One thing that I've noticed with Canon DSLR camera is they will disable or limit autofocus when the lens is below certain aperture values, which are specified in my answer to What are the aperture limits for autofocus on Canon DSLR cameras? Note that Nikon cameras are also typically limited to f/5.6 except for models featuring the latest revision of the Multi-CAM 3500 AF system as in the D4 and D800.

As far as I am aware, Olympus and Pentax do not have these limitations, even if autofocus may be slow or unreliable beyond f/5.6. (Sony/Minolta requires at least f/6.3, with one exception for a 500mm f/8 autofocus mirror lens, and the Alpha 850 and 900 have a center AF point with high precision at f/2.8.) For example, on Pentax, 9 of the 11 points are cross-type and remain so at all apertures. The Pentax AF system has been reported to work at f/8 and beyond, though extremely slowly. Olympus Four Thirds cameras will continue to attempt to focus below f/5.6 as well. Why do Canon and Nikon choose to limit AF functionality instead of attempting to focus with reduced performance beyond the above cutoffs?

Asked by DragonLord

Answer

The auto focus systems are not capable of using the highest precision cross type focusing points if you do not use a large aperture lens. They aren't artificially limiting the precision, they are simply working within the constraints of the maximum aperture.

Canon puts these limits in to ensure reliable AF. If you don't believe that theory, add a piece of tape between your teleconverter and lens, on the top three left pins. This will trick your body into thinking that you don't have a teleconverter attached, and your lens is simply a 300/4.0 or whatever you have.

Furthermore, only the Canon 1-series cameras are capable of AF with a f/8 lens, albeit at the center point only. Usually this setup comes from combining a long lens with a extender.

This really comes down to a issue of user experience. Sure, they could include ISO 204800 or 409600, but the quality would not make anyone happy. They are trying to keep the majority of users satisfied with what the camera can do, and most consumers are satisfied if AF doesn't hunt endlessly.

Answered by dpollitt

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How does aperture affect exposure?

Question

I've heard that aperture can change the exposure, but I would like to know if this is true, and how it does it.

P.S. I hope this question isn't dumb like my last two.

Asked by J. Walker

Answer

It depends on your definition of exposure. Exposure is the overall amount of light that falls on the film or sensor. So by that definition, if you have a fixed shutter speed, then if you use a wider aperture, you'll let in more light, so you will increase the exposure. A narrower aperture opening will let in less light, so less exposure.

Keep in mind that a camera on Auto mode will adjust, so if you narrow the aperture, the camera will slow the shutter to compensate, and you'll end up with roughly the same exposure. So for aperture to affect exposure, you have to keep other parameters constant.

People also use the term exposure as a substitute for shutter speed, as in "I took a long exposure of the stars". If you are thinking exposure = shutter speed, then I can understand your confusion. But the real definition of exposure is the overall amount of light. This is affected by the exposure triangle of ISO, aperture and shutter speed.

Answered by MikeW

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How can I use aperture to make my pictures better? [closed]

Question

Possible Duplicate:
What is aperture, and how does it affect my photographs?

I've recently started trying to change the aperture level on my camera to get cool effects, and I would like to know how I can use aperture to my advantage?

Asked by J. Walker

Answer

As you are fairly new and just starting, so I think it's better to go with simple things first. One important thing you should know first is the value you increase or decrease in your camera to control the aperture is called F value and this is inversely proportional to aperture. The less the F value = More the aperture, More the F value, less the aperture.

By playing with aperture

1. You can make your subject look more popped and separate from the subject. More the aperture (decreased the f value), more the background is blurred.

2. You can make your subject sharp by decreasing the aperture (increasing the f value), most lenses have a sweet point but for now just keep in mind that keeping your f value 5.6-9 should do that.

3. Increasing the aperture (decreasing the f value) will help you take photos easily where the light is low.

That's mainly it

Answered by Hasin Hayder

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Why isn't my variable-aperture photo more interesting?

Question

I took a photo where I changed the aperture of the lens during the exposure. I was hoping to get a cool or unusual effect, but I didn't. Why not, and is there any really interesting effect that I can get by changing the aperture mid-exposure?

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I know that changing settings within an exposure can yield interesting (artistic) results. For example, changing the framing lets you blur the background while retaining a sharp image of a fast-moving subject; zooming in or out surrounds the in-focus subject with a neat effect; you can do several things by changing the lighting, from light painting and light trails to long exposures and using flash with first- or second-curtain sync.

I was wondering what else I could change, and since I don't know how to change ISO or what it means to change shutter speed mid-exposure, I wanted to try changing the aperture during the exposure by rotating the aperture ring on a lens during a very-long (long enough for me to change settings with some accuracy) exposure.

I took a photograph in a dark room that was exposed correctly at f/2.8 and 4 seconds. I decided to use 4 aperture settings, each accounting for 1/4th of the overall exposure. Since my lens has an aperture range from f/2.8 to f/22 (in half stops, so I had to click through two "settings" for each stop), I decided to use f/2.8, f/5.6, f/11, and f/22. That gave me this table of the amount of time to spend at each aperture:

• 1" @ f/2.8
• 4" @f/5.6
• 16" @ f/11
• 64" @ f/22

which I think should result in the same exposure (Ev=Tv+Av) as 4 seconds at f/2.8.

I hypothesize that this combined exposure will give me a final image very similar (both exposure and depth of field) to a a 32-second exposure at f/8, although I'm hoping something unique or cool will happen.

I focus on some foreground objects on a table, set up a timer so I can count seconds, start holding down the button on the remote shutter release, wait one second, rotate the aperture ring 4 clicks as quickly as possible, wait another four seconds, rotate through 4 more clicks, and then another 4 more clicks sixteen seconds later, and then wait sixty-four seconds and then release the button on the remote shutter release. I look at the result, and... I get a pretty normal-looking picture.

I set up my camera for a 32-second exposure (by setting the shutter speed to 30 seconds), set the lens to f/8, and take the picture, and get a somewhat-similar picture.

I look closer at the two pictures, ping-ponging between them in my image viewer trying to spot differences, and I see:

• the variable-aperture exposure seems to have a very slight bit of camera blur, which I assume is from me moving the camera slightly during the exposure while rotating the aperture ring.
• the variable-aperture exposure has slightly less contrast. Perhaps that's just an artifact of the camera blur? Or maybe it's because the exposure setting isn't exactly the same as the constant-aperture exposure, since it takes a little bit of time for me to change the aperture, and the lens spends some small amount of time at several intermediate apertures, and because I've introduced some human error in the timing by not changing settings at the exact seconds specified in my table.
• the foreground objects (glass of water and camera) appear to be pretty much identical between the two exposures, with the exception of some of the reflections in the water glass, which I assume are due to me and a cat moving around during the exposure.
• the background books are much more in-focus for the variable-aperture photo. Did I just reinvent focus stacking, or is there something else going on here?

So, back to my original question: is there anything I can do with this technique to get some interesting or unique effect? Can I compose the scene differently or change the focus point to get better results? Would a different selection of apertures yield a more interesting effect?

Asked by drewbenn

Answer

The only idea I have for something mildly interesting is to create a photo with sharp subject that has out of focus "halo" around it.

• use a light color subject and a dark background

• set aperture to wide open

• focus so the subject is completely blurred (the more out of focus the better)

• start the exposure, mid exposure stop down all the way to make the subject sharp

• experiment with different time ratios between wide open and stopped down until you like the effect

• you can try to add some low power flash while stopped down to make the sharp version even sharper, just make sure the light does not hit the background

I didn't test this because I don't have a lens with an aperture ring but I tried to do something similar by changing the focus - not the most impressive picture I've ever shot but at least it gave a unique special effect.

Answered by Nir

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Do bigger focal lengths capture more light?

Question

I never understood what the f really stands for in the f-stop values, like f/1.8. Wikipedia explains it in various articles, but I still find it very confusing. What I understand, though, is that it has something to do with focal length.

Should I assume, therefore, that bigger focal lengths capture more light? For example, should 85 mm f/1.8 lens capture a lot more light than for example 24 mm f/1.8?

References to back up my confusion:

The article about aperture goes on to claim:

The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Where f is focal length and N is the f-number.

The "f-number" is mentioned here. But the article about F-number claims:

In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter.

This seems very recursive. Why does the aperture article refers to both the focal length and the f-number, when the latter article claims that f-number already carries the property of focal length?

What's going on here?

Asked by RiMMER

Answer

The f-number is in use to express how much light a lens can capture, so the 85mm f/1.8 and 24mm f/1.8 can capture the same amount. Here, f is the focal length, and f/1.8 means that maximum aperture diameter is 47.2mm in first example and 13.3mm in second.

What you have to consider here is that the 85mm lens has a much narrower field of view, therefore it has to gather the same amount of light from a much smaller area - to compensate the narrow view, aperture has to be bigger.

How much the aperture has to be bigger is linearly correlated to focal length. We could say that a 24mm lens with 13.3mm aperture can gather as much light as a 85mm lens with 47.2mm aperture, but talking about the F-number makes this much easier to notice.

Answered by Imre

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How can I take hi-speed photos of water rockets?

Question

How can I take a picture of water rockets as they fly through the air? What settings would be best under normal daylight conditions? What is the slowest shutter speed I can use that the lighting will still look natural, but it will also capture the water trail and droplets? And what would be the best aperture so that the background is not in focus, and the main thing in focus is the rocket?

Asked by J. Walker

Answer

"Real" water rockets accelerate EXTREMELY fast. While some with restricted orifices may stagger off the launcher, anything that aims at performance accelerates faster than almost anything else you are likely to photograph. A top rocket 9by calculation and experience) accelerates at up to about 100 g !!!.

There are various shots you can take. Looking up at the rocket well above you is a fairly general shot and panning can work. Exposures and shutter speeds can easily enough be arrived at by experiment.

Getting a good shot as the rocket leaves the launcher up to say 10 to 15 feet in the air, takes care, planning and luck. Smaller aperture for better depth of field helps. Fast shutter speed can help BUT there is another way. Set camera to manual, adjust for a relatively low shutter speed - below max flash sync speed, and then add flash. Rear curtain flash is even better if you have it. Adjust flash to ambient lighting so rocket stands out when the flash operates. Good shots may be had by framing from launcher to about 15 feet pointing upwardsish from near the ground so the rear curtain flash catches the rocket sharply. You get a water trail with some blur (depending on shutter speed) and a sharp rocket IF you are lucky. Just using flash plus ambient can also be good.

An interesting point to take photographic advantage of is that a rocket which has the majority of its mass as propellant and which is not very restricted in its jet will accelerate to faster than its own exit velocity. (The rocket equation makes this clear when you look at it and plug in high Full/empty mass ratios). Using an eg 1.5 litre bottle with about 100 to 120 psi and bottle mouth as orifice and 1/3 fill (about optimum) the water will be all exhausted by about 15 feet after takeoff BUT the rocket will exceed its exhaust velocity after about 5 to 10 feet from the ground. This means that water below this point is being ejected with a downwards velocity BUT above this point the ejected water actually has a net upwards velocity. The end result is that the water plume in the air changes shape as the water exiting slows and then reverses. Above the reversal point it starts to fan into a cloud with increasing spread as the upwards velocity of the water increases with height and is spread more by th air as it hits it faster. When I first saw my photos of this I did not understand what I was seeing but it makes sense once you realise. The effect is amazing - quite unlike what you expect to see and fast enough that you don't really notice with the eye alone unless you are looking for it. So the photos are especially surprising.

I can find some photos of this effect if of interest. Quite some years ago now :-)

Also fun is a two or more photo sequence of the ascent with the photos manually stuck together. That they don't quite match due to change in perspective and angle doesn't matter. People see immediately what you've done and appreciate it for the fun effect. (I have one such pair where the rocket went to about 6,200 feet ASL - BUT it was launched from 6,000 feet ASL ;-) - highest altitude water rocket shot in NZ (I think). Have the photographer stand back some way to do this. First shot shows water plume standing maybe 20 feet in air plus some sky above and second shot overlaps and has lots of sky and cloud and rocket high above. Chances of getting rocket in the first shot is about zero but water plume is easier. Rocket in second shot is at apogee so easy.

NB - be prepared to get you and your camera deluged.

---

Added:

You could try "cheating a little" :-):

• Use a very low pressure so that the rocket comes off the off the pad more slowly, relatively. Adjust pressure to speed.

• Adjust fill %. Almost full and almost empty will produce less speed and less altitude.

• If you can adjust the orifice, use a smaller opening. Some launchers do not allow this directly - eg those that use a "launch tube". This can be overcome by inserting a movable orifice plate in th bottle that sits in the bottle during ascent up the rod and then moves into the neck once it clears the launch rod. Some place this on top of the launch rod so that the rocket collects it as it ascends. You need a restrictor that can be pressed through the rocket nozzle but which then expands / pops out / ... so it is trapped on ascent.

• Combinations of the above.

Answered by Russell McMahon

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What is the effective aperture of two stacked lenses for macro?

Question

Is there a way to calculate the aperture of two lenses stacked together for macro? In my specific case, I have 90mm f/2.8 and 18mm 1/3.5 lenses.

Asked by Paolo

Answer

If you want to precalculate the value, I imagine it's possible, but I don't know how.

If you have the two lenses and want to determine through experimentation what the effective f/stop is, that's doable.

First mount the one of the lenses. Put camera in manual mode, and take a shot of something, preferably a neutral gray card. Something that is a solid color would be best. Set the shutter speed so that you get a good exposure.

Now add the reversed lens. With the same fixed ISO, take shots of the same subject (card) and with a range of shutter speeds. Because this will now be a macro shot of a small area of your original subject, that's why I suggested a solid color. Be careful that you are not blocking your light source with the lens.

Then compare those shots and find one that matches the tones of that first shot.

The difference in shutter speeds will tell you the additional stops of light beyond f/2.8 that you've lost by adding the second lens.

So if the original shot was at 1/60, and you have a shot with the two lenses at 1/15 that roughly matches the brightness of the original, then that's two full stops (1/60 --> 1/30 --> 1/15), so your effective aperture is f/5.6 ( f/2.8 --> f/4 --> f/5.6 )

Answered by MikeW

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Why does my camera show “f00” when using extension tubes for shooting close ups?

Question

As a beginner in close up photography, I am using extension tubes with my Canon 550D and standard 18-55mm lens. I noticed that when I use this, the f-stop meter in the camera does not work and always shows f00. I am using the Fotodiox extension tubes that I bought from Amazon. Is this normal or am I connecting these extension tubes wrong?

Asked by kabir

Answer

These extension tubes do NOT have any electrical contacts to send lens information to your camera, therefore, the camera assumes there is no lens attached and shows f00. These extension tubes just increase the distance between your camera sensor and the lens which enables you to focus closer and achieve greater magnification. Needless to say, you lose any control over your lens while using these extension tubes the same way you do in reverse-lens macro technique.

However, there are some extension tubes available which has electrical contacts and allow you to communicate with your lens while being used i.e change aperture, auto-focus but they are way more expensive.

Ideally, you should set the aperture prior to detaching the lens off the body (by pressing and holding the DOF preview button while taking the lens off) and mount the extension tubes in between. Lack of AF shouldn't bother you much as the DOF is too shallow, you'd have to focus manually in most of the case.

Answered by ShutterBug

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Why are my photos taken at f/11 less sharp than those taken at a wider aperture?

Question

I am new to photography. I have Fujifilm FinePix HS10 camera. Lately I was trying my hands on landscape photography.

I learnt from different tutorials that setting high aperture value gets most of the scene in focus. Hence proceeding with this notion I set my aperture value to f-11 which is the highest on my camera.

But what I observed is that, f-11 images were very soft and I don't see that background elements well focused.

On the contrary, images taken in range f-4 to f-5.6 were better looking in terms of sharpness of background elements. I have attached sample images for your reference.

Can you please shed some light on this?

Image taken with f-4

Image taken with f-11

Asked by Vaman Kulkarni

Answer

Images with your camera will start showing signs of diffraction around f/4 to f/5.6 due to the size of the sensor. Shooting at a significantly smaller aperture (like f/11) will only increase the diffraction problems. You'll lose resolution.

Here's a good tutorial on diffraction:

http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

Answered by Eric

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In what kind of scenes does it make sense to use f/1.4 aperture?

Question

I have been told that it is difficult to focus with an aperture of f/1.4.

So, in which kind of scenes does it actually make sense to use the f/1.4?

In portraits they say that it is important to keep focus on both eyes (ruling off f/1.4).
In moving subjects (street photography) it s simply not possible to focus with f/1.4.
In food photos it will then focus only a particle of food.

What are the PRACTICAL uses of f/1.4?

Asked by Anisha Kaul

Answer

There's several practical cases here. Your depth of field isn't only a function of the aperture so if other factors suite, then its still fine.

If you're sufficiently far away from your subject, then using f/1.4 would result the majority of your subject being in focus.

If you have a high performance AF system (something like the 7D perhaps), then you're more likely to keep the point of focus exactly where you expect.

If the scene is so dark, you may need to shoot at a wide open 1.4 in order to get enough light - you're willing to trade extreme subject isolation and possibly not get the exact point of focus for noise and proper exposure.

You do see it in portraits some. Its a very trendy thing to do to. The eyes should be roughly on the same focal plane, so you should be able to get them in focus and the ears will be blurred out.

Frankly though, I don't - nor do most people I know - use f/1.4 all that often.

Answered by rfusca

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Why does the max aperture change when use Nikon 60mm AF-D Micro on D90 camera?

Question

I have found that my Nikon 60mm Micro AF-D changes aperture from 2.8 to 3.2 when used with a D90. I have tried both A and M mode. Is it by design? Why?

I intend to buy Nikon 50mm 1.4 AF-D, but it also has a switch on the lens and it will lock the maximum aperture at 2.0 (my guess) instead 2.8. So should I buy it or not, or would it be better to buy the newer 50mm 1.4 AF-S version?

Asked by Ngan Pham

Answer

I think you must have noticed something slightly different. The max aperture is a function of the lens and should be unaffected by camera.

The Nikon 60mm Micro AF-D will change its maximum aperture as a function of the focus distance. As it focuses closer, the reported max aperture drops. By life size 1:1, its down to f/5.

There is apparently a button on the 50mm AF-D that will lock it to its minimum aperture. This is a property of manual aperture lenses. Modern cameras control the aperture automatically from the camera body. Setting this switch locks it at the minimum aperture so that the camera can automatically control it. Since your camera has an in-body focus motor, you're able to use the AF-D or AF-S, you just need to decide if there's a difference for the price that you care about. They'll both take great pictures, the AF-S will just focus faster and quieter mainly.

Answered by rfusca

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Does maximum aperture change with focus distance with the Canon 60mm f/2.8 macro lens?

Question

This site says

The Canon EF-S 60mm f/2.8 Macro USM Lens loses 1/2 stop at 1:5, 1 stop at 1:3, 1.5 stops at 1:1.5 and 2 stops at 1:1 (lifesize).

Can anyone confirm that at 1:5 magnification, the Canon EF-S 60mm f/2.8 USM lens has a maximum aperture of f/3.4, at 1:3 magnification the maximum aperture is f/4.0, at 1:1.5 it is f/4.8 and anything below that is f/5.6?

Asked by Nick

Answer

My Nikon 105mm drops from f/2.8 to f/4.5 at closest focus, so that sounds right.

A post at betterfamilyphotos has a post where they say (emphasis mine):

You would imagine that using a macro lens is the same as using a normal lens, and you would be right except that with a macro lens when you get close to 1x magnification, you start losing light. My 60mm for example starts losing light at close ranges until it reaches 2 stops of light loss at 1x magnification, this means that the effective aperture is f/5.6 instead of f/2.8 (regarding light quantity entering, not DoF). If you are using auto modes on the camera like aperture priority or using flash in TTL mode then the camera will auto compensate for the light loss, but if you're metering light manually you need to take it into account, Canon has included a table in the user manual with the light loss values at each magnification level.

So if you have access to the manual for this lens, or request one from Canon, it should verify the information. It's expected for a close focusing macro to lose 1-2 stops.

Edit: in response to the comment, the above post is incorrect in saying that the reduction in aperture doesn't affect DOF. It does. Two references for those interested in the physics of it:

Cambridge in Colour has a good explantion here (scroll down to LENS EXTENSION & EFFECTIVE F-STOP.

Also see explanation here: Do normal macro lenses suffer the same light reduction as tubes?

Answered by MikeW

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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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Is overall light gathering of a lens only dependant on aperture?

Question

My impression is that the aperture value of a lens determines its light gathering ability, but I'm not sure I understand how it works...

When considering light gathering in telescopes, it is dependant on the diameter of the objective lens (or mirror). This makes perfect sense to me, since light is radiated in all directions, so a larger area means you gather more light. It seems to me it should be the same in camera lenses also - a larger lens would pick up more of the cone of light from the subject, and focus it onto the sensor.

What got me thinking about it was I've seen an F/0.95 lens, but it doesn't look hugely larger than F/2.8 lenses, so I don't understand the physics of how that would work.

Answer

Essentially yes, light gathering ability of a lens is determined by its maximum aperture. Transmission rates of the materials used also has an effect but it is very small.

You intuition is correct in that you would expect a large aperture lens to have a large barrel, however the aperture is specified as a ratio of the apparent* size of lens opening divided by the focal length. So a 200mm f/2.0 lens must have a front element large enough to see a 200/2.0 = 100mm aperture, so the barrel must be at least 10cm. However a 20mm f/2.0 only appears to have a 10mm aperture, which is small is comparison to most lens sizes.

To complicate matters wide angle lenses need larger front elements than dictated by their aperture to prevent vignetting across the frame. For focal lengths shorter than about 50mm lens sizes increase as focal length decreases despite apertures, and thus light gathering ability, also decreasing.

Here's nice example, this Nikon lens is only f/2.8:

but is absolutely huge, due to its extreme wide angle nature.

* note that 100mm f/2.0 doesn't mean the physical opening in the middle of the lens is actually 50mm diameter, only that the image of said opening when viewed through the front of the lens appears to be 50mm in diameter. The actually opening is often smaller, but the lens front element has to be large enough to accommodate its theoretical size.

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