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I've read for the second time this morning that whenever we don't know which aperture would fit best, we should take something between f/8 and f/11.

Now my logic would dictate me to set an aperture of minimum value to ensure having the most part of my picture in focus (example : f/22 to f/32). This way, we could still create blur with a software where creating sharpness wouldn't be possible...

Of course, going that high would create a light problem but I don't get why this aperture get so much success... Is there an optical best ratio to use that aperture or is it just the best compromise ?

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I don't think blurring with software gives acceptable results in most cases, at least not unless you also somehow record the depth information of the scene and feed it to a very fancy blurring algorithm. –  JohannesD Jul 22 '13 at 9:18
    
@JohannesD or use a Lytro :-) –  Philip Kendall Jul 22 '13 at 9:31
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@PhilipKendall: Yeah, but I'd say using a light field camera counts as "recording the depth information and feeding it to a very fancy algorithm" :) –  JohannesD Jul 22 '13 at 9:35
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There's an old expression: F/8 and be there that many, I think, still adhere to. –  John Cavan Jul 22 '13 at 10:23
    
If you want to see some examples of what diffraction means in terms of lost sharpness have a look at this blog post over at lensrentals.com. –  Håkon K. Olafsen Jul 22 '13 at 11:13

3 Answers 3

up vote 17 down vote accepted

If you care about image quality then there isn't really a "don't care" aperture any more. With 35mm film the effects of diffraction at f/11 or f/16 weren't readily apparent, however with a high megapixel DSLR diffraction has a measurable effect at apertures as wide as f/5.6!

It's worth stating for the record: more megapixels does not make diffraction worse. But they do make it more apparent when quality is lower than it could be. If using a FF DSLR your images will be no worse than those taken at f/16 with a film SLR, but peak sharpness could be higher. That's why it makes a difference whether you really care about image quality or not.

If you don't care about image quality then use whatever aperture you feel like. The idea that if you have tons of light then you should stop down as far as possible to maximise DOF and cover focus errors and thus obtain the sharpest image is a misconception as you could potentially lose more sharpness over the critical parts of the image due to diffraction.

The only thing to do if you care about image quality is to care about the aperture setting and chose the appropriate value for each scene, taking into account your focal length, the distance the the subject, it's depth and the type of image you want to achieve. Good focusing technique is necessary too.

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Could you provide references for diffraction being visible as low as f5.6 on high MP DSLRs? I ask because the commonly held theory 'when in doubt shoot at f8 for maximum sharpness' wouldn't be the case anymore and I'd wanna know! :) –  Shizam Jul 22 '13 at 19:22
    
Thank you very much for your answer! –  Andy M Jul 22 '13 at 19:35
    
@Shizam I should have said "detect" rather than "see" but if you look at lens tests of any really good lenses and the peak sharpness tends to be at f/4, meaning diffraction is causing the f/5.6 image to be softer. –  Matt Grum Jul 22 '13 at 20:48
    
"if you look at lens tests of any really good lenses and the peak sharpness tends to be at f/4" - The amount of diffraction primarily depends on the physical size of the aperture, but the physical size of "f/4" depends on the focal length. f/4 at 15mm will actually be physically smaller than f/5.6 at 100mm, and thus should exhibit more diffraction, despite being a smaller f-number. So I think the aperture at which "diffraction has a measurable effect with a high megapixel DSLR" must be dependent on both the f-number and the focal length. –  BlueRaja - Danny Pflughoeft Jul 22 '13 at 22:47
    
@Shizam: The best aperture, and what aperture is really diffraction-limited, really depends on the lens. The concept of DLA, or Diffraction-limited Aperture, as it pertains to a sensor has to do with the aperture of a theoretically perfect lens at which diffraction would begin to affect IQ. If the lens is aberration limited at f/5.6, then it doesn't matter if the sensor DLA is f/5.6. If the lens doesn't become diffraction limited until f/8, then it is quite likely, even with a high resolution sensor, that you could get better results by stopping down. –  jrista Jul 23 '13 at 4:16

It's a compromise (everything is, isn't it?)

There are two factors:

  1. f/8 is a middle-of-the-road aperture where you have a deep DOF but still not everything is in focus, this is usually better than absolutely everything in focus.

  2. In small apertures diffraction makes the image softer, the diffraction starts to be a factor between f/5.6 and f/8 (depending on camera) and starts to be noticeable (obviously "noticeable" is a personal preference thing) at around f/11 and smaller.

Those two factors together make f/8 pictures sharper and more pleasing for most cases where you don't care about DOF

Now, both of those factors are dependent on your personal taste and style so feel free to ignore them

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I think "f/5.6 to f/8" is true only for APS-C sensors. If I've got a 1/2.3" sensor, the limit's a lot higher than that. If I've got a full-frame sensor, I can probably go to f/11 or so. If I've got a medium-format sensor, errr... I don't actually know! –  Philip Kendall Jul 22 '13 at 9:15
    
If you've got a 1/2.3" sensor, chances are very slim that you're actually able to select f/8 or f/11. In fact the odds of your camera having an adjustable aperture at all aren't that good... –  Matt Grum Jul 22 '13 at 9:18
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@PhilipKendall The sensor size doesn't matter at all, it's the pixel size that matters. –  Håkon K. Olafsen Jul 22 '13 at 10:53
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@PhilipKendall - f/5.6 to f/8 is true for APS-C and high megapixel 35mm FF cameras (and probably also MFT, didn't really do the math) - I sort of assume this covers most of the readers of this site who ask about aperture (especially when the question talk about f/32, f/32 is a very nice mid-range aperture for medium format but I don't expect it to even be an option on 1/2.3") –  Nir Jul 22 '13 at 11:07
    
Thank you very much for your answer Nir! –  Andy M Jul 22 '13 at 19:35

AFAIK, the "Who Cares" part refers to something like "Who cares what the DOF is, because everything in the picture is more or less in the same plane". I.e. when taking an image of something flat, or relatively flat compared to the distance at which it is taken.

In that case, to get the sharpest image, you would select the aperture where the lens is at its sharpest, this is usually around f/8 to f/11, but it depends on the lens.

The lens is usually sharper stopped down a bit because you block some of the light that takes the 'edge' paths in the lens. This light has been subjected to the highest amount of refraction, which softens the image. But stop it down too much, the diffraction in the aperture will soften the image.

So the best thing is to know your lens, and know at which aperture it is at its sharpest.

And don't forget that stopping down the lens forces longer exposure times, which could lead to softer images if shot hand held.

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Thank you very much for your answer! –  Andy M Jul 22 '13 at 19:36

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