I have a couple of M42 lenses, a 50 1.8 and a 135 2.8. both are awful wide open, but stopped down a bit you can get very nice results (or at least, you can get fun 
here’s one with the 135 that I posted a while ago:
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The old takumar are not bad either including the SMC 50mm 1.4 and the 105mm 2.8 (a little too sensitive to flares for the 105)
What do you mean by that? The only resources that I have seen talking about that seemed too esoteric for me.
It really comes down to looking at photographs made with modern digital lenses versus photographs made with antique lenses for film cameras. Do your own experiments: ideally, put the camera on a tripod, and do someone’s portrait with a modern lens and again with an antique lens with the same focal length at the same aperture. Then look at the pictures and consider which lens is giving the better rendering of the three-dimensionality of the face.
What you may first be aware of is the impression or feeling of the rendering: one looks somehow more ‘real’ or ‘natural’ than the other — closer to how you see the face with your eyes.
As I understand it, there are technical reasons for this difference, to do with the optics, with the behavious of types of glass, with microcontrast and so on. But what really matters is how the pictures look — draw your own conclusions from that.
But if you do want more technical detail, have a look at Yannick Khong’s articles:
The problem with modern optics
The flattening of modern lenses or the death of 3D pop
and he has a whole series of articles on his website about lenses.
HTH
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I don’t own M42 lenses, but being a Nikon D700 user I enjoy using some old manual lenses (Ais 2.8/24, Ais 1.2/50, Ais 1.4/85, Ais 1.8/105 and Ai 4.0/200).
Knowing the characteristic of each of the lenses is important (Ais 2.8/24 has strong CA, which fortunately can almost completely be corrected by RT raw-ca-correction, Ais 1.2/50 is only usable at F >= 2, but then it’s among the sharpest lenses in this focal-range, Ais 1.4/85 is just a gem, usable at every aperture, Ais 1.8/105 is really good for close distance even at 1.8, but not so good for landscape at infinity, Ai 4.0/200 is good for close disctance, but very low contrasts when used at infinity)
M42 users: please also have a look at this issue and provide raw files if possible
Thanks for the links…some really interesting reads!
No m42 for me but I have a collection of ten Contax Zeiss (C/Y mount) lenses.
They’re mostly sharp enough to compete with modern lenses, but also very full of character.
A great compromise, IMO.
Those articles don’t mention nothing technically sound (they don’t even define microcontrast that seems to be snake oil, there was a reddit thread a bit ago and no one could find a good resource explaining it). The compared pictures look too similar to say which is better (maybe the color rendition may tip the scale).
I understand that for portraits you don’t always want a technically perfect lens, as the brain process faces differently anyways, and the look of older lenses may be preferable to people (and they may be cheaper), but I still think that is personal preference being hidden by some abstract concept that is even purposefully not well defined.
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I suppose it only matters if you see a difference between photographs with digital and antique lenses. If you don’t, there’s no point wasting your time.
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Yes, I have a 45/2.8 Tessar. Lovely character-ish lens that gets quite sharp stopped down.
Fine. I have two Tessar M42 (Carl Zeiss Jena), plus a couple of Soviet Industars – would there be an easy/simple method to spot/show what quality difference there would be between your Contax version and mine?
I don’t know if there’s any way to do it remotely. It’s hard enough for me to put a finger on why I prefer its output to my Tamron 45/1.8 that is technically superior in every way save for size. I doubt it would be easy with another Tessar.
Is this really a 24? I only know about the 35mm.
Thanks, you are right, it’s a 35mm
Actually, I was hoping 
I was like damn that’s some bokeh for a 24mm… I have a modern Nikon 24mm f1.8 and it doesn’t produce bokeh even remotely close to that!