Are there cons of buying medium-format lens for 35mm film SLR?

Question

Right now I have Pentax ME Super SLR camera and I want to buy some telephoto lens. Additionally, I plan to buy medium-format camera in some (near) future. In consequence I wonder: Is it reasonable to buy right now the medium-format lens and use it with full frame SLR, so I will already have right lens for future camera? I would also like to find the cons (and pros) of such a configuration, especially:

1. Can the greater field of view be a source of some internal reflections in the camera body?
2. How does the focus range change with such a mount (especially ability to focus at infinity)? Are the lens adapters designed in such a way that this focus range can remain unchanged?
3. Does greater field imply that the 35mm image is less prone to lens aberrations?
4. Apart from stop down metering, is there any other shortcoming of such a mounting?

Answer 1

Of course there are some cons. If there weren't, everybody would do it all the time. :-)

For some obvious cons, consider that a medium format lens will almost always be:

• larger
• heavier
• more expensive, and
• lower resolution

...than a lens built specifically for 35mm.

As for the specific questions you asked:

• Can the greater field of view be a source of some internal reflections in the camera body?

At least in theory it undoubtedly can. I've never seen a substantial loss of contrast though. I don't remember with certainty, but I'd more or less expect an adapter to include some baffling to minimize the amount of light transmitted outside the film/sensor rectangle.

• How does the focus range change with such a mount (especially ability to focus at infinity)? Are the lens adapters designed in such a way that this focus range can remain unchanged?

At least in the cases I've seen, infinity focus wasn't a problem at all. Every medium format camera of which I'm aware has a considerably longer registration distance than an 35mm camera, so getting the lens close enough to the film plane is no problem at all.

• Does greater field imply that the 35mm image is less prone to lens aberrations?

In theory it might. In reality, I've seen little or no real difference.

I think I've already covered a number of the other obvious disadvantages above, so we probably don't need to repeat them specifically as answers to your last question.

Answer 2

Can the greater field of view be a source of some internal reflections in the camera body?

Not to my knowledge.

How does the focus range change with such a mount (especially ability to focus at infinity)? Are the lens adapters designed in such a way that this focus range can remain unchanged?

A medium format lens is designed for a camera that has a larger flange length than your 35mm camera. What this means is that any adapter acts essentially like an extension tube - except it will attach the lens at the distance from the film plane that it was originally designed for. All of your focusing, including to infinite, will work as originally designed.

Does greater field imply that the 35mm image is less prone to lens aberrations?

The projected image circle is the same size - you'll just be using less of it. The effect is similar to the crop factor experienced when going from full-frame to APS-C in digital. This means that you'll be using only the center of the image circle, which is generally the "sweet spot" of the lens.

Apart from stop down metering, is there any other shortcoming of such a mounting?

The larger you go in format, the less resolution a lens is tasked with. Think about how many times you have to blow up a 35mm negative to get to an 8x10. How many times do you have to blow up a large format 4x5 to get to 8x10? What this means is that lenses designed for 35mm (generally speaking) will out-resolve their large format cousins, simply because the resolution isn't as necessary.

Now, what this means in practice for a medium format lens to 35mm...? I'd guess that you will still have plenty of resolution to work with, regardless of your lens chosen.