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08-12-2013, 01:36 AM   #1
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lens testers say quality drops above f11

Can anyone explain why most online lens tests on DSLRs say the image quality drops with apertures above f11 due to diffraction but I see the EXIF info on many published photos both pro and amateur show pics taken at f16, f22.

Do these folk ignore image quality in order to get greater DOF or is the lens testing not able to properly evaluate the higher apertures on APS-C sensors?
Do I have to compromise on quality to get DOF or is there a trick to it?

Thanks.

08-12-2013, 02:13 AM   #2
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You're right on the issue, it's all about balancing things to get the result you want. If maximum resolution was the only factor all shots would be taken at for example F8 1/1000 with a tripod and a remote but that isn't the case as we got artistic freedom involved. Photographers often put resolution on second priority to get the results they want, whether it be long shutter speeds, large apertures or small apertures.

Note that often it's impossible to see the difference in max resolution between for example F8 and F11 without zooming to 100%.
08-12-2013, 02:31 AM   #3
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"Image Quality" is not really a measurable quantity. Even "sharpness" is rather an individual conception of combined properties such as resolution and contrast. The latter two can be measured and here it may well be true for a given lens that these two parameters start to decline around f/8-f/11. But that doesn't necessarily mean that Image Quality - as you see it - will have to start declining at the very same spot.

Furthermore, while diffraction (in principle - for ideal lenses) depends only on f-stop, proper sampling depends on pixel-pitch/pixel size as well.

So, the only "trick" I can point at is to use the settings that your experience tells you will produce the best Image Quality (again, as you see it) in a given situation.
08-12-2013, 04:15 AM   #4
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Agree with the others but would like to add that professionals sometimes use full frame cameras which often have a bigger pixel size and therefore not effected as much as a APS-C camera. This is generalising a bit much but on some FF cameras f16 quality can be similar to f11 on a typical APS-C camera.

08-12-2013, 04:42 AM   #5
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The answer is that sharpness isn't everything and that even if photos are softened a little due to diffraction, it isn't that big a deal, if you need the depth of field or slower shutter speed you can get by stopping down more. That said, I don't usually go above f8 if I can help it.
08-12-2013, 07:30 AM   #6
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Diffraction effet is also function of pixel density; on past full frames (ie Nikon D3), pixel density was much less (12 Mpix on FF) on K5 APS-C (16 Mpix), thus f16 should be usable without loss of sharpness.
Furthermore on medium formats (645 D) you need to stop down more than on APS-C to achieve wanted depth of field, and the number of pixels is so high that a small lack of sharpness should not be noticeable.
08-12-2013, 09:32 AM   #7
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It's always been my understanding that diffraction has more to do with the physical diameter of the aperture and the amount of light transmitted rather than the format although they are related simply because larger formats = larger lenses. In theory anyhow, FF lenses on APS-C should perform a little better than an APS-C lens at smaller aperture. With many of my film lenses and especially my telephotos, my sharpest shots come at f/11. The reason landscape photographers love larger formats is that diffraction is essentially a non issue. It's also another part of the FF vs APS debate. The physical size of f/11 is not the same on every lens. Regardless, with a good lens, you can get very good shots at f/16 and f/22. Ansel Adams shot his landscapes at f/64 with a large format, an aperture that is nearly impossible on our cameras.
08-12-2013, 10:43 AM   #8
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It is sometimes better to have part of your image in focus and slightly less sharp, than it is to have that part fuzzy due to lost focus.

08-12-2013, 05:54 PM   #9
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I photography there are always compromises involved, there are a myriad of factors that degrade image quality - the only way to get good photographs is to eliminate these variables or minimize their impact.

With some photographers getting the shot - even if it is degraded by diffraction, is more important than not getting it.






QuoteOriginally posted by reeftool Quote
Ansel Adams shot his landscapes at f/64 with a large format, an aperture that is nearly impossible on our camera
f/64 on a 300mm 8X10 format lens is roughly the DOF equivalent to f/16 on a 50mm lens on a 35mm SLR camera. Ansel Adams wasn't the only photographer who used this aperture - being a key part of the anti-pictorialist movement drove him away from the use of shallow DOF connonly used for pictorial effect on large format, also the longer exposures the use of such a small aperture afforded him wasn't lost on him either.

Last edited by Digitalis; 08-14-2013 at 03:55 AM.
08-14-2013, 02:08 AM   #10
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This may help in understanding and evaluating the effects of diffraction:

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

When you want to use the calculator at the bottom of the tutorial, do click on "show advanced" and tick the button marked "Set circle of confusion* based on pixels". The calculations will then indicate whether of not there is diffraction limiting, and show circle of confusion versus pixel size.
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