Picture 1. I tested two objectives: Canon EF 70-200mm f/4L USM (upper) and Canon EF-S 10-22mm f/3.5-4.5 USM (lower).
19.07.2007 Category: Photography
Unprocessed photos taken with small apertures tend to sometimes look a little smooth compared to photos taken with larger apertures. The smoothness in photos taken with small apertures can be caused by diffraction.
Diffraction means bending of wave as it passes an edge. In this article I observe the effect of diffraction in digital photography. In digital photography diffraction happens when light waves go through the aperture of the camera. I won't go deeply into the physics of diffraction but I show some photographic tests and explain what the effect of diffraction means in practice.
I think the effect of diffraction is pretty subtle. Therefore, if you want to test the effect of diffraction yourself you should use an adequate tripod, and maybe even a mirror lock-up to avoid camera shake. Also notice that when testing large apertures, using long lens and photographing close by subjects the depth of field is very shallow so you have to be very careful what part of the photograph you evaluate.
I used the following equipment with my tests:
I also made my lens tests indoors to avoid effect of wind. With Canon 70-200mm I used focal length 140mm and with Canon 10-22mm I used focal length 16mm. I want to emphasize that diffraction is not a feature of these high quality lenses but of every lens.
Picture 2. There is a considerable difference in sharpness between f/8 and f/32 with Canon EF 70-200mm f/4L USM. The picture is scaled down but the difference in the sharpness is as noticeably as it is in 100% photo.
Picture 3. There is a considerable difference in sharpness between f/8 and f/25 with Canon EF-S 10-22mm f/3.5-4.5 USM.
The results of both lenses are pretty similar. Both are very sharp in certain aperture range. Sharpness of both lenses decrease noticeably when using smaller apertures than f/13.
Canon 70-200mm is very sharp between f/4 and f/13. There is a considerable difference in sharpness between f/8 and F32 as one can see in picture 2.
Canon 10-22mm is very sharp between f/5 and f/13. There is a considerable difference in sharpness between f/8 and F25 as one can see in picture 3.
Picture 4. This landscape photo was taken with Canon EF-S 10-22mm f/3.5-4.5 USM. I shot two versions, one with f/13 and one with f/22 to see the effect of diffraction in a real life situation.
Picture 5. A cropped detail of the foreground of the original photo (scaled down 50%). There is a considerable difference in sharpness between f/22 and f/13.
Picture 6. A cropped detail of the background of the original photo (scaled down 50%). There is considerable difference in sharpness between f/22 and f/13.
Finally, I made some real life testing while photographing a landscape on the island of Lesbos in Greece. I shot the same photo with both f/13 and f/22 just to see the difference in sharpness. I used tripod, mirror lock-up and self timer to minimize camera shake. Picture 4 is the photo I took.
Pictures 5 and 6 show comparison between apertures f/13 and f/22. The photo taken with larger aperture is considerably sharper all around.
The difference in sharpness with different apertures can easily be seen in my tests. It seems that both of the lenses tested experience lack in sharpness when the aperture gets smaller than f/13. Keeping that in mind I try to avoid using smaller apertures unless there is a need for them. Because of diffraction and camera shake I recommend always using the largest aperture that provides sufficient depth of field. Of course it's sometimes necessary to use very small apertures to get long exposure times for example when photographing a sunset or flowing water.
So, the effect of diffraction can easily be seen in photography but does it really matter? Can the smoothness of the photos be corrected easily in post processing? I think that it can if the only problem with the photo is the smoothness caused by diffraction. This is a small issue but if you want to get the maximum sharpness into your photos, diffraction is something to consider.