Small Apertures and Diffraction

Like everything in photography, every benefit comes at a cost.

We all know about "stopping down" to increase depth of field. But at a certain point, every lens reaches a diffraction limit - a point at which resolution begins to degrade. If the situation calls for the DOF to be more important than absolute resolution, then by all means stop down. But realize that stopping all the way down should not always be your ultimate goal.

A good explanation to this can be found here.


Two examples here. Both scenarios shot with the Canon 180mm f/3.5L macro lens showing the uncropped and cropped sections. The 2nd example shows the other byproduct of stopping down to tiny apertures - every little speck of the smallest dust particles on the sensor is revealed to its full ugly glory.

A picture is worth a thousand words - thanks for taking the time to do this. Some lenses are better than others and although these are pretty typical from what I've seen, I have seen worse. I had a Nikon 300 f4.5 ED, which was probably the best manual focus 300mm they had (some of the f2.8's were just as good). This lens was incredibly sharp wide open and at f5.6 but quickly fell off to almost unuseable at f32. Maybe there was something wrong with mine, but since I normally would shoot a lens like that wide open anyway I didn't have anyone look at it. I wish I had examples from this lens at both ends of the aperture scale...

This is something I've learned since being on these forums..it was mentioned somewhere not too long ago.
These examples really drive it home!
I always figured, I should always step down as far as I could for maxomum dof. Now I seldom go past f/16.
This should maybe be a sticky.

another view - glad to do it and hope it helps others here. I wonder if diffraction is largely uncorrectable so that, for any given focal length lens, the diffraction effect would progress at the same rate to the same extent?

Frog - no need for a separate sticky here. But I will include it to the one already in the nature/wildlife forum in the thread titled "the 4 basics".



Something just came to mind - the degree of the diffraction effect should be a factor of simply the physical size of the aperture and not the aperture value (f-number). Sure an f/32 on a 28mm lens lets in the same "amount" of light as an f/32 on the 500mm lens; but the physical opening size on the 28mm will be so much smaller. The aperture value is simply the diameter of the opening divided by the focal length of the lens.

So intuition tells me that the diffraction will be more noticeable sooner on a wide angle verses a telephoto lens. I'll have to think of tests than can give comparative examples over a range of focal lengths.