Kevin_A Sigma 40mm as it is better than their 35mm
That's too close to the XP50, in case I can make that XP50 f/1.2 Samyang work. The Samyang wasn't good when I last tested it, but that was before the retaining ring trick. Without the retaining ring, you either get the 18 spikes from the 9 bladed iris, or random (albeit smaller) spikes from the step down thread.
If the XP50/1.2 remains a piece of crap,, then the 40mm is actually a good compromise, given the trees I have here :-).
I just received (thank heavens for Amazon Prime deliveries :-) a 28mm Tiffen UV filter to use as an aperture mask (thrown everything but the retining ring already :-). Perhaps tonight if the clouds don't come in again, like last night. The retaining ring measures to 24.3mm clear aperture, or f/2.06 for a 50mm lens. That would be a factor of 1.7 (1.5 f-stops, thereabouts) from the wide open f/1.2, and may be enough to tame it. We'll see.
Now, the problem with smaller and smaller diameters is that manufacturing tolerance in percentage starts growing, and at some point, the retaining rings are no longer smooth and circular enough. It is easier to play the game at 135mm than at 35mm.
Now, some people may actually have been using the Sigma 35/1.4 at full aperture, and that is a disaster, if you believe the apertures shown at lenstips.
https://www.lenstip.com/359.7-Lens_review-Sigma_A_35_mm_f_1.4_DG_HSM_Coma__astigmatism_and_bokeh.html
At f/1.4, the aperture shows very strong "almond" shape at the APS-C corners. It appears much better by f/2, although from the full frame corner shape, even f/2 may not be circular enough. So, it might take stopping down between f/2 and f/2.8 before that is tamed. Once the diffraction spikes are tamed, we are down to MTF, and the Sigma really shines there.
I still remember when I first read a Zeiss white paper on MTF (this was before taking Fourier Optics classes), and it really impressed on me how it determines the perceived quality of an image (around late 1960s, Zeiss was then fighting with Leitz (Leica) on who is better. Leica designed for sharpness. Zeiss based things on MTF.
I still have my Leica 50mm dual-range Summicron (f/2). Unfortunately, virtually impossible to couple it to CMOS cameras since the rangefinders have very short flange focal distances. But I read that the lens by designed by 3 mathematicians using just mechanical calculators and slide rules. The lens came out at about the same time as the M3 (which is one of the Leicas I still have, but no film for it :-), so this is late 1940s, early 1950s (and you can imagine Germany in those years). One reviewer back in the '60s dubbed it "best lens honed by man." I would love to try that lens out with CMOS sensors; I had only taken some Ektachrome slides of constellations with it (50mm full 24x36mm frame).
Chen
