Diffraction Limited?


posted on 20th of november, 2008

I've been looking into a new body and have decided not to go with the latest Canon 50D. I'm turned off by the high file size, but also because we seem to be getting into territory where more megapixels are making some things worse. While I haven't tried either the 50D or 40D for any length of time, and am therefore not qualified at all to talk about ultimate image quality, I did encounter a few discussions on diffraction, and the impact of smaller pixels.

As someone who shot film pre-digital, I've always been under the understanding that a smaller aperture meant more depth of field, but also that stopping down meant you got more sharpness, too. This doesn't necessarily seem to be the case, and newer digital cameras seem to have more of an issue. I've seen examples in various places, but one that make the difference plain to see is at The Digital Picture. In this example, a high quality lens is compared at f/5.6 and f/16 on the Canon 50D.

This has implications in how you shoot, as when doing macro you want maximum depth of field, but stopping down to f/32 may be limiting the ultimate sharpness. It is a trade off to be sure.

Personally I don't know that this would affect my shots when printed very much, but for now I'm going to stick to f/8 when I want maximum sharpness, but don't need the depth of field. The interesting part of the 50D review on the same site is the table that shows the minimum aperture before diffraction starts to reduce sharpness - it is f/7.6 for the 50D, f/10.3 on my 20D and f/13.2 on the 5D. Again, I don't know how these numbers are calculated - but if they are accurate it is quite a difference between bodies!

If anyone can comment on why a high resolution sensor is more troubled by diffraction than a low resolution sensor of the same size, please advise! Another example is on a Canon G10 Comparison it talks about how the aperture in program mode stays pretty wide (f/4) to help keep diffraction to a minimum.

Comments (5)

Posted by Holgs on March 13, 2009
A smaller aperture does give a larger depth of field, with digital sensors, which are generally smaller than 35mm film, the same depth of field is achieved with a much larger aperture.
For example, the G10 you mention at its widest angel and aperture (f2.8) will have an equivalent depth of field and angle of view as 28mm at f14 on film. On the camera I use, 25mm at f5.6 already gives me an equivalent of f11 in 35mm format. For most situations I find that this is sufficient depth of field. I find that diffraction doesn't start limiting sharpness at about f11, but then there aren't really that many situations where I'd want to shoot at such a small aperture, with the exception of macro images. A good article on the subject can be found at:

I think this issue is more of a problem in regards to lens choice than body choice - if a particular lens doesn't reach its sweet spot until past the point of diffraction, then its a...(More)
Posted by Bradcalkins on March 13, 2009
Each lens will definitely have its own sweet spot, independent of the sensor. The trouble arises when the best aperture for lens sharpness is smaller than the aperature where diffraction starts to limit sharpness. From what I've seen, this shouldn't be an issue with any lenses on the old 5D, but newer cameras, the 50D especially, are starting to hit this point...
Posted by Tipareth on December 15, 2008
Interesting...

I got a 5D and work with f. 8 a lot, maybe I should try f11 as well... However I thing that the lenses used are also important, and maybe those numbers are different depending of lenses as well...
Posted by Bradcalkins on December 14, 2008
Thanks for the comment, Creativei!
Posted by Creativei on December 13, 2008
Thanks for this blog. very informative.



Comments (5)

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Photo credits: Alptraum, Brad Calkins, Steve Mann.

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