I've written up a 4-way comparison of macro lenses on the KP at 1:1 and a discussion of the useful aperture range for the LAOWA 100mm at 2:1
on the German Pentaxians forum. The later may be of interest in this thread, so here's a brief version in English.
I wanted to know how far I can close down the aperture at 2:1 before diffraction takes away any resolution advantage over a crop from a 1:1 picture. I took a series of pictures at different apertures both at 2:1 and 1:1 of flowers of a willow, that somebody had cut down. Much easier to keep still than a bee
, but with similar detail and contrast. The Full scene taken with a KP at 2:1 and f/8, developed in darktable with minimal settings (just exposure adjustment for same median, rather soft filmicRGB curve, no sharpening, no noise reduction) looks like this:
The following shows crops around the focal plane, at 100% for the 2:1 exposures and 200% for the 1:1 exposures. I arranged them so that pictures with the closest match in terms of depth of filed are displayed side by side. Normally it's not a full stop that you need to close the lens down when going from 1:1 to 2:1, but that the closest match in my full-stop spaced series.
F (2:1) | LAOWA 100mm 2:1 @100% | LAOWA 100mm 1:1 @200% | F (1:1) |
2.8 | | | |
4.0 | | | 2.8 |
5.6 | | | 4.0 |
8.0 | | | 5.6 |
11 | | | 8.0 |
16 | | | 11 |
What you can see is that the LAOWA 100mm is sharp at 2:1 wide open already. If any, improvements by stopping down to f/4.0 are minor. Chromatic aberrations are very well corrected, transition into both front and back unsharp regions is pleasant, some contouring is going on in the highlight disks a little further from the focal plane. Already at f/5.6, details are very slightly reduced. At f/8.0 (2:1), everything one gains in terms of detail from the increased magnification over the 1:1 shot at similar DoF, is eaten up by diffraction. There are some advantages in terms of noise/color, but I may have underexposed a bit, exaggerating the effect.
So in conclusion, to really reap the benefit of high magnification, you have to shoot at large to medium apertures. Close down to f/8 at 2:1 and you could as well use f/5.6 at 1:1 and crop instead.