[mlag]

some more idea's:

I looked for your focus method, you stated you refocus each image in live view. But to compare pictures you should be really sure the focus is on the same object (the lamp for example). Exactly. So focussing on one specific focus point (manual set) and not on the wide area of the picture (don't let the camera decide where to focus in your frame). The foliage of the scene might result in different focus results in between pictures....

Sometimes on tripod any SR system should be off. Maybe use mirror up, remote control or 10sec delay to fire in order to reduce camera body vibrations , i gues the shutter time gets significant larger at F8 and picture registration more sensitive to any vibration (motion blur by camera house vibrations).....

Increasing motion blur impact is consistent with (higher aperture and hence) longer shutter times (at same iso).

Or try a flash image at F8 (more light?).

[RICHARD L.]

I always test my lenses at all aperture openings. Last week, I set up my Pentax K3 III + DA* 60-250 mm f/4 ED on my Manfrotto 028 heavy tripod, selected ISO 100, JPEG, Av mode and shot the same exact scene @ 60 mm Focal Length within 1 minute at f/8, f/11, f/16, f/22 and f/32. All pictures were later post-processed in exactly the same way. The "Optimum Aperture" was f/16 on my Mac monitor (middle picture).

I then tried another picture, a close-up of Aspen leaves @ 140 mm Focal Length. f/8 proved to be the sharpest Aperture for this image. So I think the "Optimal Aperture" can be different depending on the optical characteristics of the lens being tested, the Focal Length being used as well as the Aperture. One should not "agonize" on which Aperture to use, a good lens will usually retain good sharpness between f/5.6 and f/16.

[GUB]

I always test my lenses at all aperture openings. Last week, I set up my Pentax K3 III + DA* 60-250 mm f/4 ED on my Manfrotto 028 heavy tripod, selected ISO 100, JPEG, Av mode and shot the same exact scene @ 60 mm Focal Length within 1 minute at f/8, f/11, f/16, f/22 and f/32. All pictures were later post-processed in exactly the same way. The "Optimum Aperture" was f/16 on my Mac monitor (middle picture).

I then tried another picture, a close-up of Aspen leaves @ 140 mm Focal Length. f/8 proved to be the sharpest Aperture for this image. So I think the "Optimal Aperture" can be different depending on the optical characteristics of the lens being tested, the Focal Length being used as well as the Aperture. One should not "agonize" on which Aperture to use, a good lens will usually retain good sharpness between f/5.6 and f/16.

Unfortunately the jpg sharpening process has likely masked if not outright altered the outcome here. (best using raw with sharpening off in the developer.)
Plus what actual subject did you focus on here?
The image has a range of subject distances making it a poor test subject.
Might be stating the obvious but a lens only has "sharpness" at the plane of focus.

[pschlute]

The other thing to consider when testing camera/lens combos on a tripod is to ensure shutter shock does not play a part. Avoid speeds in the 1/60-1/200 range or use electronic shutter if your camera has it.

[GUB]

Avoid speeds in the 1/60-1/200 range or use electronic shutter if your camera has i

True - and the scene seems to have shadows in it suggesting (sunny16 rule) that f8 might have a shutter speed in that area.

[RICHARD L.]

I use various 645 and P67 lenses on a 645Z and I know these lenses better get used @ f/8 rather than wide-open ...

[RICHARD L.]

[normhead]

Higher resolution sensor will resolve more and thus the effects of diffraction to be visible earlier: 24Mp full frame Vs 16Mp APS-C. There are also the aforementioned issues in the posts above to consider as well.

This is it......^ (plus 1)
The better the lens, the wider the sharpest aperture.
Look at the FA 31 ltd.
Pentax SMC FA 31mm f/1.8 AL Limited - Lab Test / Review - Analysis



Being a generally excellent lens, it's sharpest at ƒ2.8.

The SMC 35 2.4 on the other hand is sharpest at ƒ4


The DA 21 ltd. is sharpest at ƒ5.6.


So if you see where I'm going, as the quality of the optics goes down, the sharpest ƒ-stop is higher. With lenses 300mm and over you also run into lenses that are sharpest at ƒ8. SO there are a number of factors, the quality of the optics, and focal length probably being the big 2.

Personally, I wouldn't spend too much time on this. Test the lenses you have, utilize your test results. Apart from what I've contributed, which only shows you what to expect, you have to evaluate your own lenses individually. Sample variation is enough to over-ride assumptions made from single lens measurements done in a lab.

For example, I've noticed the DA 21 ltd. is very good for pastoral almost pastel images. It's not the sharpest, but it renders nicely especially in low contrast transitions. (Where as I'm guessing the DFA 21 2.4 probably renders well for pretty much everything.) The DA 21 is not good for contrasty sunsets. You figure out what a cheaper lens is good for, then use it to its strength. This is more important for cheaper lenses. They are almost all good for something, you just have to figure out what.