SLR lens adaptability

Which commonly available modern 35mm SLR bodies, with commonly available adapter if necessary, will mount M42 screw mount lenses? Will any of them do aperture priority auto with a manual screw mount lens? Is there a practical discernible difference in image quality between lens adapters which are simply a ring with air in the middle and those which have an additional lens element?

I have some answers and opinions based on my own research, and I have a table of register sizes, but I'd like to know what others have found from their own practical experience.

And another related question: what is the minimum distance from the camera at which a lens set on infinity would be expected to be in focus (making no allowance for depth of field)? Does it vary by focal length?

The following bodies can accept 42x1 lenses with a proper adapter ring, i.e. an adapter without lens: Alpa (not the latest, Japanese production branded Alpa), Bell & Howell, Canon, Contax/Yashica, Fujica-X, Konica, Leica M, Leica thread mount, Minolta, Miranda, Mamiya-Z and NC1000, Pentax K, Petri if the diameter of the bottom lens is <= 49mm, Praktica B200, Rollei (24x36), Rectaflex (source: Annuario Fotografico 1984, Editrice Progresso, Italy).

Most of them I suppose will do aperture priority auto exposure in stop-down mode. I don't know about auto exposure with diaphragm simulation.

Ring adapters without lens are the proper "ring adapters" and are the good stuff. Ring adapters with a lens are more of a work-around, the quality (relata refero) suffers, I deem it would normally not make a lot of sense unless to satisfy particular needs.

Register size tables are not necessarily reliable as there is also to consider the thickness of the ring itself and the different diameter of the mounts. Not every combination which appear to be feasible in theory looking only at register size table is also possible in practice, without additional lens that is.

Regarding the last question, I suppose you mean the distance of the last lens from the focal plane. That depends on the optic scheme, besides depending on the focal length. In general shorter focal length make physically shorter lenses but how short depends from the scheme.

Wide-angle lenses for SLRs use an "inverted tele" scheme to allow more distance between rear lens and focal plane (so as to provide space for the mirror box).
Wide-angle lenses for rangefinder don't need to use an "inverted tele" scheme (somebody call them "true wide-angle") which make the scheme simpler, the lens lighter and smaller, the quality better.

Some ultra-wide angle lenses are mountable on SLRs only by manually raising the mirror. The viewfinder is in the case an external viewfinder typically mounted over the flash slot.

Some ultra-wide angle lenses for rangefinder also may not be mount on certain cameras theoretically accepting them. E.g. some Voigtlaender Bessa bodies cannot accept certain Leica wide-angle lenses, you "can" mount them but seriously risk to damage something.

Thanks, Diapositivo, for your comprehensive response.

What I meant with the last question was: if I am standing in front of a scene with my camera, and the lens focusing ring is turned right around to infinity, how far away would an object need to be to be in focus? 50 metres, 100 metres, 200 metres?

Ah. Interesting question! Made a bit of research over the internet. A lot of mental masturbation about infinity being over the horizon, or at the Moon, or beyond Mars (well, infinity is by definition "nowhere to be said where it is", if that's for that).

There certainly is some formula somewhere that defines distance of "practical" infinity as a function of f/value and acceptable circle of confusion. It's certainly something not very practical on the field (unless you bring a scientific calculator with you, but it's probably tough in any case).

I would reason by observing the progression of marks on lens barrels. Probably somewhere around 4x the last figure mentioned on the barrell (before infinity) would be my practical, totally unscientific but somehow sensible answer.

That would give around 40m for a 50mm, 20m for a wide-angle such as a 24mm, and around 120m for my Tamron 500/8 (catadioptric) which is the longest lens I own and has 30m as the last mark.

My Voigtlaender Vito CLR has the last mark at 20m (50mm), all my other 50mm have the last mark somewhere around 10m, so you can easily see how approximate is this rule of thumb. But then, not everybody has a thumb of the same length.

Maybe it could be derivated as a function of hyperfocal distance, as in "3x the hyperfocal distance". That's because it is commonly said that DoF extends 1/3 behind and 2/3 beyond the focus plane. If focus is set at hyperfocal distance, given a certain CoC, infinity is "in focus" at 3 times hyperfocal distance. (That is, given a certain Circle of Confusion, I repeat). Lens makers tend to be a bit forgiving in placing DoF marks (they tend to choose too big an "acceptable" CoC).

Thanks for that comprehensive reply Fabrizio.
I always went canon for adapting, this shows some other options.

Not commonly available and not even reccomendable without a few caveats, but Rolleiflex 35mm SLRs will maintain the auto-diaphragm function of M42 lenses.

Aperture priority AE (on the cameras which have it) requires pressing the DOF preview button before shooting (forgetting to do so makes the camera defaut to 16 seconds - Grrrrr! At least you know for sure you've missed the shot!).
Otherwise, pretty traditional stop-down metering.

Rol_Lei Nut said:

Not commonly available and not even reccomendable without a few caveats, but Rolleiflex 35mm SLRs will maintain the auto-diaphragm function of M42 lenses.

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What are the caveats? I picked up one of the M42 auto-adapters for my SL35M, but haven't tried it yet.

wotalegend said:

Which commonly available modern 35mm SLR bodies, with commonly available adapter if necessary, will mount M42 screw mount lenses? Will any of them do aperture priority auto with a manual screw mount lens? Is there a practical discernible difference in image quality between lens adapters which are simply a ring with air in the middle and those which have an additional lens element?

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I recommend against using an adapter with glass in it; the quality is generally very bad (like, visibly horrible) and the adapter acts as a teleconverter so you end up with a longer, slower lens than you thought you were using. You got a nice big list of mounts above, but the ones I know about are:
- Minolta/Sony AF: glassless
- Canon EOS: glassless
- Nikon: with glass if you want to achieve infinity
- Pentax-K: glassless IIRC

The main issue for adapting glasslessly: is the registration distance (from lens flange to film) short enough that the adapter can have non-negative thickness? If the camera is "thinner" than an M42 body, you just make the thickness up with the adapter and achieve correct focus. If the body is too thick, you have unavoidable additional extension (causes the lens to focus closer), so you're forced to either have glass in there or not achieve infinity.

Some people have been known to modify old cheap lenses (e.g. Rokkor 55mm f/1.2) to make them fit on newer (e.g. Alpha) bodies by machining down the lens mount and replacing the bayonet to achieve correct focus on a more modern (often digital) camera. The Rokkors in particular are cheap because it's difficult to adapt them to a modern camera without modification, at least compared to M42 lenses, the reason being that Minolta MC/MD (manual) cameras have an unusually short registration distance.

wotalegend said:

And another related question: what is the minimum distance from the camera at which a lens set on infinity would be expected to be in focus (making no allowance for depth of field)? Does it vary by focal length?

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If the lens is calibrated properly, it will be focused at infinity, i.e. with parallel rays. However, even the fastest (f/1.4ish) lenses have non-zero depth of field because film is not infinitely sharp - there comes a point where something is so far away therefore so small on the film that it is smaller than the natural softness of the lens, i.e. it is inside the DOF whether you would like to disregard that or not. For example, 50mm f/1.4 has a hyperfocal distance of 90m even with a very tight sharpness criterion of 20um, which means that everything past about 90m will be focused when you're at infinity. In that case, the blur due to lens aberrations wide open will likely be greater than the softness due to DOF for anything more than about 10m away.

What the manufacturer marks the lens with (i.e. the largest non-infinity point on the distance scale) is completely irrelevant, that's just paint on the barrel. You need to google up a DOF and/or hyperfocus calculator and see what's going on with the focal length and aperture of your particular lens.

Towermax said:

What are the caveats? I picked up one of the M42 auto-adapters for my SL35M, but haven't tried it yet.

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Basically that, for all models, they are not that reliable. Though, one still working now has a good chance of keeping on working....

I especially use the SL35-E (& VSL-3) and the 3003. Lovely cameras to use, very good viewfinders & handling & Zeiss lenses!!!. Just can't give a blanket reccomendation....

As I mentioned previously, the M42 adaptors work brilliantly.

The Mamiya Auto XTL and Auto X1000 cameras will accept M42 lenses with the P adapter. There are two different versions of the P adapter but they work the same way. They also provide auto diaphragm operation. I came close to getting one from an eBay seller last year and the item was mysteriously withdrawn. There are depth of field tables you can get to calculate infinity focus with various focal lengths. Between a 200mm f/4 lens and a 200/2.8 lens [35mm format] there is a big difference in infinity focus wide open.