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M42 T-adapter vs. T-mount for Astrophotography

Bob McCready , Aug 05, 2003; 12:42 a.m.

Hi everybody,

I was thinking about buying a 400mm lens with an M42 screw mount and then getting a T-adapter to put it on a Nikon F3. However, from what I have read in the forums, with the simple metal ring type of adapter I won't be able to focus at infinity; instead I would have to get the type with a corrector lens (which is more expensive and probably reduces image quality). But that got me thinking: in astrophotography they use something that is also called a T-ring or T-adapter to routinely attach Nikons to telescopes. These adapters don't have a corrector lens that I'm aware of. And they must obviously be able to focus on infinity when taking pictures of things 2 million light years away. So are these two completely different things that just happen to share the same jargon? Or is it the same thing but because a telescope can be focused past infinity (no stop like in a lens) it isn't a problem? And if the latter is the case, how hard would it be to modify a lens to either get rid of or change the infinity stop?



Mark Wahlster , Aug 05, 2003; 09:39 a.m.

The differance is the scope has the ability to move the focal point closer to the film them the lens does by way of design. The two adapters may look very very simular and in fact be interchangeable in use but the use is what makes the differeance. In the case of the lens the adaption of an adapter on the back of a lens acts as an extension tube that effects the infinity focus of the lens.

Douglas Stemke , Aug 05, 2003; 09:45 a.m.

T-adaptors are designed for T-mount lenses (and Spotting scopes, telescopes and I think for mounting with certain microscope eyepieces). These lenses are threaded to accept various screw-on adaptors that matches the camera mount (Nikon, EOS, FD, K, Universal 42, MD, Maximum, etc)and are positioned so the lens focuses at infinity. I think "T" stands for Tamron which first produced it, although this is not to be confused with Tamrons new Adaptol lenses.

Regarding the M42 to Nikon. While I am no expert on Nikon lenses and adaptors the Pentax K to M42 does give one the ability to focus at infinity (I use it all the time on a 500 f4.5 lens). Since Nikon mounts are wider than Pentax K (I think) I would imagine that the M42 to Nikon adaptor without glass *should* focus to infinity. This is just an educated guess though. What you could do, if you already have the adaptor is before you pick up a 400mm lens pick up a $5 35mm M42 lens (You can even get them cheaper than that-nobody wants them) and test it out for yourself.

Douglas Stemke , Aug 05, 2003; 09:46 a.m.

I should also note the Pentax K to M42 has no optical elements.

Bob McCready , Aug 05, 2003; 11:28 a.m.

Response To M42 T-adapter vs. T-mount for Astrophotography

Thanks for the info. My understanding is that the M42 adapters focus to infinity with all camera models EXCEPT Nikon. I believe this is because the distance from the lens mounting flange to the film plane on Nikons is a millimeter or two more than other brands for some reason.

richard oleson , Aug 05, 2003; 11:59 a.m.

They are two completely different things. A T-adapter will not adapt an M42 lens to fit another camera. A T-adapter is an interchangeable lens mount made to fit onto a T-mount lens to enable it to be used on a camera. The T-mount interface is a 42mm thread, but it is not the same as the M42 camera mount and (significantly) the lens barrel is shortened so that the adapter permits infinity focus on any 35mm camera make.

Glassless M42 adapters area available primarily for camera brands that began with the M42 mount and then developed bayonets: Pentax and Yashica come to mind. There are also some cameras with a shallower body such as Miranda, and, I believe, Canon, which can permit a glassless adapter. Nikon is not the only camera that doesn't allow it, though. For most other cameras, adapters simply aren't available at all, but there are some focus-correcting adapters available for Nikon.

If you don't already have a lens that you want to adapt, you would be much better off buying either a T-mount or a Nikon mount lens to go on your F3.

rick :)=


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