Digital original lenses versus analogic adapted lenses

Started Jan 19, 2011 | Discussions thread
dopravopat Senior Member • Posts: 1,177
M42 lenses on Canon EOS

There are basically four types of M42 to Canon EOS adapters. They have either some electronics to confirm focus - which is practical, or a flange that presses the pin on each M42 lens when it is being screwed on, so the aperture closes automatically to whatever value you set it on the lens. You can combide these two features and get an adapter with focus confirm and with the flange, then one with focus confirm but without the flange, the next would be just the flange without focus confirm and the last would be the simplest of all - as I have - a mechanical M42 to Canon EOS coupling without anything that affects either the lens or camera body, so the camera basically shoots "dumb", believeing no lens is attached and no EXIF is recorded (btw. same with the "only flange" version).

The oldest M42 lenses do not even have a pin and need to be closed manually by a ring on the front, which usually stops at a pre-set value, so you do not need to look at the lens to know that you closed it to f8 for example. More recent M42 systems had a pin that closed the aperture automatically just when taking the picture. Some lenses for this system werre depended on the mechanism inside the camera to actually close the aperture, the lens aperture could not be closed manually when mounted on the camera. The flanged adapters are intended for such lenses, with the exception of metering, which again was a camera-dependet action and the metering button not only started camera metering, but also pushed the pin on the lens. I cannot remember any lens of this type to write an example now.

Most M42 lenses that came through my hands havd not only the pin, but a switch that can close the aperture and keep it closed. It was usually on the side of the barrel, close to the lens mount. Like mine old Carl Zeiss Jena ones do. I pre-set the aperture and can focus and compose with a bright viewfinder, which makes focusing easier, as you have the shallowest DOF possible. I close it to meter the scene, then open again to have a bright viewfinder. I move the aperture operation switch on the lens from postition "A" (automatic) to "M" (manual) by my left thumb for Carl Zeiss Jena lenses or left index finger for Pentacon lenses (the switch is on a different part of the lens barrel). And I do this just a fracton of a second before I press the shutter button on the camera. So operation is incovinient, but possible and fast, once you get used to basically "take the picture by moving two fingers instead of one".

Of course, having a mechanism inside the adapter that would automatically push the button is unlikely, as it would need not only mechanical feedback, but some wiring to process the information from the camera body and obvious some sort of motor (maybe linear?) that actually makes the movement and presses the pin. On the other hand, such an adapter could be made to allow open aperture metering with special "electric" M42 lenses (which I have). Every "electric" lens has three contacts at the back end which do connect to plates on the camera body (of a Praktica VLC, PLC or EE) and close the circuit. The aperture ring of the lens is connected to a potentiometer inside it, enabling open aperture. This open metering is pretty accurate and easy to calibrate if you find it be off. I was never off by more than 1/2 EV when I comparted to stop-down metering, which is fine, giving the tolerances of aperture openings (f8 does look a tiny bit different on both Pancolars 50 f1.8. which I still have). That is pretty precise for me.

Such an adapter could report the focal length and aperture to the camera body. Well, but I would need a different adapter for each lens to get correct EXIF, I mean correct lens focal length. I think it might be simpler to modify the old M42 lenses to Canon EF mount. Putting some electronics inside foc focus confirm should not be a problem, it already works with regular adapters. There is enough space insede. The bigger problem is the need for some sort of motor, maybe a simple tiny contactor, that would drive the aperture mechanism. The good part is that, at least on the Zeiss Jena lenses, the aperture is released by a clever spring mechanism that needs only a small and constant movement independet of actual aperture selected and a relatively small force (this does not apply to the Carl Zeiss Jena Sonnar 2.8/200, althouth I never owned this lens to be 100% sure, I saw and heared it has a different aperture mechanism that only closes to a pre-set value automatically when taking the picture, you have to open the aperture after taking the shot manually) . The Pentacons (and their derivates which are basically the same but with a slightly different desing and brand) are different built, they need a longer movement to close the aperture and that movement is different for each aperture setting.

Conclusion: I guess M42 Carl Zeiss lenses could be adapted do fully communicate with a Canon EOS body and with automatic aperture closing. The only thing that would not work is autofocus. Fair enoguh for me. Maybe a heavily modified Canon EOS 5D with an M42 mount from a scrapped old Praktica and some mechanism to press the pins on lenses would do the same. But I find the lens modifying way to be better. But both are sci-fi. I would pay not more than 25€ to convert each lens to such operation. Clearly the costs, if it were possible, are too high.

 dopravopat's gear list:dopravopat's gear list
Canon EOS 30D Canon EOS 60D Canon EF-S 18-135mm F3.5-5.6 IS Sigma 50mm F1.4 EX DG HSM Sigma 8-16mm F4.5-5.6 DC HSM +9 more
Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow