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Particularly, I am interested in the performance on film for Canon leica thread mount (LTM) 50mm lenses versus their FL/FD SLR lenses, in aperture size 1.2/1.4/1.8. Their performance on full frame digital bodies like the A7 may also be valid with respect to things like sharpness, contrast and bokeh.

The main motivation for my question is the difference in prices these lenses commandeer on the used market, you can get a 50mm 1.8 SLR lens for less than $50 but the LTM one is usually three times as much.

Given that most of these lenses are probably six element double gauss designs, is there any intrinsic property that makes the LTM worth more/better than SLR version? For example, from this link, I quote this about the 1.4 LTM:

Impressive image quality for a basic design (I have found this lens better at f/1.4 than my copies of FD SCC, Takumar 7 and 8 elements, Minolta MD 50's)

What can be said about the comparison between the Leica Summicron f2 vs Summicron-R? Or the Zeiss planar 50mm f2 versions made today for DSLRs vs Leica M mount.

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    Not an answer to the question but a related side-note: the shorter flange focal distance of rangefinders allows some lens designs which are not possible for SLRs. For example, to the best of my knowledge, it's not possible to have a 50mm Zeiss Sonnar for SLRs. I'm not sure whether this is true for any of the Leica designs. – godfatherofpolka Jul 18 '16 at 17:35
  • Are you talking about using these on a dSLR? (in which case, all the rangefinder lenses are utterly useless). Or adapted on a mirrorless mount? Or just theoretically on their native camera mounts? This question, to me, is not unlike, "What tastes better, an apple or an orange?" where the answer can be highly subjective. Better in what way? Also, you're asking for LTM vs. FD; LTM or M vs. R, and Zeiss F/EOS/K/M42 vs. M. That's a LOT of lens experience you think we can afford. – inkista Jul 19 '16 at 3:01
  • i am asking about these with respect to performance on 35mm film with appropriate camera bodies. I have edited the question to reflect this and readded the film tag. As I said in my first sentence, I am mainly interested in comparing the Canon lenses. However, I note in the rest that this comparison is not just limited to Canon lenses, it applies to Leica lenses from a certain era, + in modern times similarly branded lenses for different digital lens mounts. – Alex Jul 19 '16 at 3:27
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    @Alex, IMO, lens quality is not top of the list in deciding factors to go with a rangefinder or an SLR for film. It's a lot like whether to go for mirrorless or dSLR for digital. It's more about handling and the viewfinder than image quality. And that $50 LTM lens might require a $100 CLA, which then prices it the same as a dSLR 50/1.8. Just a thought. – inkista Jul 19 '16 at 3:33
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TL;DR I am afraid that you need to evaluate the lenses individually.

Rangefinder an DSLR lenses are often different in construction because both have different design goals and restrictions. RF lenses are often made to be smaller. RF lenses cannot be used from very close proximity, so no corrections or compromises for very close range are necessary. RF lenses, especially those intended for use with film, can have the rear element placed very close to the film/sensor. So these are often different constructions and therefore there will be differences. Since you ask about performance on a mirrorless digital camera - wider angle lenses (usually under 50mm) designed for RF can have reduced performance on digital cameras due to proximity of the rear element to the sensor and the oblique angle under which the rays reach it. Notable exception here is Leica M with sensor design that can handle that for all but very extreme cases.

The price differences between used RF and SLR are most likely commanded by the demand, but there is one technical difference that could support higher cost of RF lenses - rangefinder coupling. When you turn rangefinder lens, it's exact focusing position needs to be mechanically transferred to the camera rangefinder. This needs to be very precise and requires precise manufacturing and occasional corrections.

Summicron M and R - there are many types designed by different designers across several decades with different technology available at that time. where construction is identical between R and M (1976R and 1979M) so is the performance and character.

Modern Zeiss Planar - RF version is an f/2 and SLR version is an f/1.4. Different designs, different performance, different character.

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