Is it really diffraction that lowers the MTF performance at small apertures u2013 or something else?

It’s always good to challenge assumptions. And I’ve made the same assumption as everybody else that it is diffraction that drops the MTF performance at small apertures. But is this necessarily true?

Demonstrating diffraction at small apertures is easy – I did a ‘blog on diffraction and included some example pictures a couple of months ago – but that doesn’t mean diffraction is the only cause of a lower MTF figure. The obvious alternative is blur due to camera-and-lens movement during the prolonged exposures required at smaller aperture.

One way to avoid blur would be to illuminate the test target using flash but the illumination brightness range needed to test a wide variety of lenses, with apertures from f/1.4 to f/45, is awkwardly wide for flash lighting. That said, I’m still working on this alternative.

In the meantime, one way to eliminate the effect of camera-and-lens movement would be to fix the aperture setting (at, say, f/22) and then to vary the exposure sensitivity (ISO) to give a range of different exposure times. If the MTF figure remains effectively constant then it will look very much as if the fixed aperture (diffraction) is the dominant factor rather than the varying exposure time (and sensitivity). Of course it is just possible that there is an effect from the exposure time and sensitivity but these two variables cancel each other out to make it look as if they are each having no effect when there is only no effect overall.

To investigate this theory, I used Tamron’s 180mm Macro (a genuine 1:1 macro that performed very well in earlier testing). As the accompanying graph shows, there is no significant variation in the MTF performance at different exposure times (and sensitivities) when the overall effect is to keep the aperture constant. So it looks very much as if the common assumption about MTF being limited by diffraction is valid.

Graph caption: MTF results for Tamron 180mm f/3.5 LD (IF) Macro at constant aperture (f/22) varying the exposure sensitivity (ISO) to give a range of exposure times from 1/250s at ISO 3200, to 1/15s at ISO 200. The MTF figure is effectively constant, supporting the common assumption that (provided the camera is firmly mounted) it is the F-stop that determines the MTF performance, not the exposure time. MTF testing carried out using Imatest image analysis software (