Reading different articles on macro photography, I see different opinions expressed on what the minimum aperture is that you can safely shoot at before you start losing out to diffraction. In many cases you’d like to shoot at the smallest possible aperture so as to eke out another few microns of field depth. The popular wisdom is that these two requirements meet somewhere between f/11 and f/22 (depending on who you’re reading).
For this crude test, I bolted the D90 to the desk, stuck on the Nikkor 105mm f/2.8 macro lens, put a ruler and a circuit board a short way away, focused manually, set up strobes (commanded via Nikon TTL) to light it, and then went through various apertures.
The Depth of Field range turned out as follows:
Nikon computes the effective aperture for the lens, so even though I had the lens wide open (and at f/2.8 according to the lens), the camera reports the corrected f/4.
So how about detail? Below are a collection of details at 100% crop from the same scene.
Just for kicks, I did a similar test with an image that was sort of kind of close to parallel to the focal plane.
1. Pixel-peeping at 100% shows me that my focus isn’t great at any aperture.
2. While there’s a definite degradation at f/45, f/32 is barely worse than f/22. For absolute sharpest, it looks like f/11 is indeed the best.
3. Wide open, there are other aberrations at work.
4. Color and contrast shifts continue beyond the wide-open, although it gets more subtle.