I have tested 4 lenses at a wider end with some unexpected results.
Method:
I photographed a banknote in the center of the frame with different lenses at different focal distances. In each shot the banknote has the same size on the sensor (i.e. I moved the camera relative to the subject to keep the image same size on the sensor). The lenses were mounted on a KP, on a tripod, shot with a 2-sec. timer, in DNG. Images were looked at 200% magnification in Lightroom and compared side by side for sharpness, resolution, and contrast. I did no image processing (like sharpening), other than occasional minor exposure correction to compensate for changing lighting conditions. Then they were ranked relative to each other for each FL and each f-stop, i.e. for each stop there were four positions: 1, 2, 3, 4 (higher number = better IQ). Corollary: IQ at different f-stops cannot be compared in terms of absolute quality. At times, it was very hard to tell the difference in IQ and I admit that sometimes difference between adjacent ranks is very subjective. When one looks at extremes end in IQ, the difference is rather obvious.
Lenses tested:
Sigma 10-20mm F4-5.6 EX
Pentax DA 16-85
Pentax FAJ 18-35mm F4-5.6 (the only full-frame lens here)
Pentax DA 18-55mm F3.5-5.6 AL
The lenses were tested at 16mm, 18mm, and 20mm, at f-stop of 4.0 (or whatever the largest f-top is available for that lens), 5.6, and 8.0. Sometimes, a lens had f/3.5, but I plotted it as 4.0.
Results:
---------- Post added 04-26-21 at 05:45 PM ----------
Discussion: 16mm: I was surprised to see Sigma outperform the DA 16-85. Sigma is a budget lens, while DA16-85 is a rather respected lens on this forum.
18mm: Again, Sigma is above all at any f-stop, but an even more surprising result is that wide-open DA16-85 looses to FAJ18-35 and DA18-55, both known as very weak lenses! How come? Any ideas?
20mm: DA16-85 is still struggling a lot. FAJ18-35 is the best at f/8! This is not normal. I must have messed up my test.
Last edited by pentax_amateur; 04-26-2021 at 10:03 PM.