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I've ran a lot of tests with lots of lenses, modern and vintage:
* Sharpness in stills mode
* Sharpness in video mode
* Sharpness in video mode with line skipping
Here I will present a summary of my findings relating to image quality. For comments about build quality, alternatives, price, etc., head over to my equipment recommendations.
The lenses I have tested are the following:
Very sharp in the center at any focal length andaperture. In the corners, it's better to close to f/3.5 or f/5.6 (specially at 11mm). Very curved focus plane at anything except 16mm. Bokeh is not nice, but this shouldn't be an important issue for such a wide lens.
Very sharp in the center at the wide end, relatively soft on the long end (better close to f/3.5 at 35mm, and f/5.6 or narrower at 50mm). Exactly the same in the corners. Reasonably flat focus plane at all focal lengths. Bokeh is not nice, but not horrible either (no green halo or bright center spot, but it can show some weird internal rings; some cinema glass shows that too, so, go figure). Closing aperture down helps bokeh quality a bit, but bokeh quantity will of course go down.
Sharp in the center and corners at all focal length and aperture combinations. And with very flat focus plane too. Bokeh is very nice, but there's too little of it: it's a pity that this lens is so slow (and badly constructed).
Sharp in the center at all apertures. In the corners it goes from very soft wide open to just about sharp enough at f/9 (acutance is not as bad as resolution: this corner softness will be less of a problem at 1080p than at 5K, specially when closing down the iris). The focus plane seems reasonably flat, but that's kind of irrelevant because the corners are soft no matter what. The 18-55 kit lens, which can also do 20mm f/3.5, beats this one handily at any settings. Bokeh is extremely nice, but again that's not very relevant for such a wide lens.
In terms of sharpness, very similar to the MIR-20M, but even softer in the corners at anything but f/9. Bokeh is not nice, again not an issue for a 20mm lens.
A decent wide angle prime, but at least on APS-C it doesn't make a lot of sense, the Tamron 17-50 f/2.8 non-VC is a better option (similarly priced, but sharper, specially in the corners wide open). Bokeh is nearly perfect in this Canon, and not nice on the Tamron, but this doesn't matter too much on such a wide lens.
Sharper than most vintage ultra-wide-angle lenses, but not as sharp as a sharp modern 24mm. Big plus: bokeh is great (and yes, you can get shallow DoF on very wide lenses).
Fairly sharp in the center, and with very nice bokeh. It's a pity that it can't keep its sharpness in the corners too. It could be an option for video if it had a lower price.
Sharp in the center, but not in the corners, unless you stop it down quite a bit (to f/9 or so). Weird bokeh (I would say pentagonal, but at times it seems even star-like!).
The best lens I've tested at around 35mm. Perfect at all apertures: very sharp in center and in corners, flat focus plane, amazing bokeh. As good as it gets.
This one is nearly as good as the Leitz: at f/2.8 corners are somewhat soft and bokeh is not great, but it's still sharp in the center; at f/3.5 and beyond, it's sharp across the image, and has very nice bokeh. Keep in mind that this is a specialty lens: it's a shift lens, with lots of rings and confusing manual controls, it takes some time to learn how to use it, and it may never feel confortable to use. But it shifts.
One of the sharpest 50mm lenses I've tested. Extremely sharp in the center at all apertures, and sharp in the corners too at f/2.8 or narrower (f/3.5 or narrower if shooting 5K). Bokeh is not stellar at f/1.8, but it gets very good as soon as you close the iris just a little bit. A little treasure.
One of the sharpest 50mm lenses I've tested. Not as sharp as the Rollei in the center, but very sharp there anyway, and sharper than any other in the corners at f/2.8 and onwards. Bokeh is ugly at f/1.8 (very marked outlines in highlights), but gets quite nice as soon as you close the iris to f/2.8 or narrower (and, unlike the Rollei, this one keeps a nearly circular shape when closed down). Great lens for the money. (Mine had a marked yellow tint, not sure about others).
The nifty fifty is a surprisingly sharp lens: very sharp in the center at all apertures, and in the corners too at f/2.8 or narrower (f/3.5 or narrower if shooting 5K). Very close to the Rollei or the Pancolar, and this one has autofocus too. The problem here is that bokeh is not nice: hard, greenish outlines in highlights wide open, and marked pentagonal shape when closing aperture down.
Not the sharpest 50mm in the center, but not bad either, and quite good in the corners. Not the best bokeh at f/2, but very nice at f/2.8 or narrower. It dies in the middle ground: not as good as the more expensive Summilux, and not better than the cheaper Rollei (which is arguably the best surprise in this list).
Very sharp in the center at all apertures, but softer than others in the corners: this one has to be closed down to f/3.5 or narrower if you want very sharp detail in the corners too (the newer aspherical version might be a lot sharper than this, but it costs a small fortune so I may never know). What sets it apart from the rest is the absolutely lovely bokeh at any aperture. Great lens overall, specially for portraits on APS-C (actually, this is my favorite 50mm: I'll often take sharp-enough with very nice bokeh over extremely sharp but with mediocre bokeh).
Lots of nice bokeh, as expected; maybe only second to the Summilux among the fast 50s tested, maybe just on par. What was a bit of a surprise is that it really shines in terms of sharpness: at f/1.8, it's the sharpest 50mm in the corner (specially if you compensate for the not-perfectly-flat focus plane). The copy I tested has a marked magenta tint that is more pronounced at wide apertures (and it doesn't seem to be longitudinal CA thing: it's all across the frame).
Extremely sharp in the center at all apertures. Just somewhat soft in the corners at maximum aperture, sharp if slightly closed down. Very nice bokeh too. The problem with this one is that it's a relatively slow lens: there's not a lot of that beautiful bokeh, and at these apertures, the other lenses are sharp too anyway.
There's a reason this lens is so incredibly cheap: sharp in the center, but very soft in the corners unless closed down to f/9 or so (maybe f/5.6 if shooting 1080p). Ugly bokeh at maximum aperture, but it gets nicer at f/2.8 or narrower.
Not great, but also not as bad as you'd expect from a modern long zoom in this price range. In the 70-150mm range (if shooting 5K) or the 70-180mm range (if shooting 1080p), it's sharp in the center at all apertures, and in the corners too if closed to f/5.6 or narrower. Beyond 200mm, it's very soft at all apertures. Bokeh is ugly at all focal lengths, horribly so on the long end.
Sharp in the center and corners, specially if closed down just a bit. And with nice bokeh too. In fact it can trade blows with the Leitz Elmarit-R 90mm f/2.8, which is not a small feat. Great lens. And very small too!
Sharp in the center except at maximum aperture. For sharp corners, it has to be closed down to f/3.5 or narrower (f/2.8 if you're shooting 1080p). Bokeh is ugly at f/1.8 (green halo and center spot in highlights); it gets smoother when the iris is closed down a bit, but then it gets a very sharp hexagonal shape.
Sharp in the center at all apertures. In the corners, acutance is also perfect at all apertures (so no need to stop it down if shooting 1080p), but resolution is not stellar (better to close down a bit if shooting 5K, and even so it won't be extremely detailed). Bokeh ranges from reasonably nice wide open to very nice when stopped down a bit. In summary: very similar to the Nikkor 85mm f/1.8, except that one is faster and this one has nicer bokeh.
Sharp in the center and corners at all apertures (maybe in the corners it's better when closed down a bit than when wide open). Bokeh is not nice at f/4, but gets better by f/5.6. I think this lens dies in the middle ground: the Nikkor 85mm is faster and sharper, the Leitz 90mm has nicer bokeh. But it's a macro lens, which is nice at times too.
Sharp in the center and corners at all apertures. And it has very nice bokeh too, just very little of it: the problem with this lens is that it is relatively slow (f/4 when focusing at infinity, f/5.6 at minimum focusing distance; yes, some old lenses do this).
Very sharp in the center and corners at all apertures. Bokeh is not as good as that of the Leitz 135mm, but it's fairly good in any case. Very nice lens.
Moderately soft in the center and corners at all apertures. And bokeh is not great wide open, but it gets better from f/4 onwards. While costing much more, not a clear improvement over the CZJ 135, apart from being faster.
Moderately soft in the center and corners at all apertures. And with very nasty bokeh too, which leads me to think I got a damaged copy (this lens is supposed to have great bokeh, and I've seen some tests around the web that confirm that).
Sharp in the center at all apertures. In the corners, acutance is good (sharp at all apertures if shooting 1080p) but resolution not so much (better to close down to f/3.5 or narrower if you want a lot of detail and are shooting 5K; then, even closed down a bit, it will be just about adequate). Bokeh is mediocre wide open, but gets much better at f/4 and narrower.
Moderately sharp in the center and corners at all apertures. Pretty nasty bokeh at f/4 (green halo and magenta center in highlights), and still not particularly nice at narrower apertures.
Moderately soft in the center, and soft in the corners, at all apertures. It seems to get softer at narrower apertures, but, being so big and heavy, this can just be a random result, since it's so difficult to focus in any case. Bokeh is not nice at f/4, but gets better by f/5.6.