We test the diffraction effects when using two lenses on an infrared converted Canon 350D digital camera.
When we got our recently converted for infrared photography Canon 350D back from www.MaxMax.com it came with a convenient set of comments. One of these waas that, because of diffraction effects, you could expect to get the sharpest images with the lens either wide open or at the first one or two f-stops down from wide open.
To test this, we took two of our single focal length Canon lenses, a 50mm F1.8 and a 100mm f2.8 macro out for some shooting in the sun. These are 100% crops from the center of the RAW image with no sharpening sone and the results presented as minimal compression JPEGs.
Test 1: Canon 50mm f1.8 (the first two images are over exposed compared to the others because in the bright sun, even at 100ISO, the 350D could not set a fast enough shutter speed.
Here the sharpest image seems to be that taken at f5.6
Test 2: Canon 100mm f2.8 Macro
Here the sharpest image seems to occur at f8.
This seems to suggest, at least for these lenses, that 3-4 stops down from wide open is
a better bet. As soon as I can I will shoot the same lenses at the same location on a conventional digital body to see which are the
sharpest apertures in visible light.