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I'm looking for a very cheap (~$50), fast (large aperture), full manual (non-CPU) lens for my Nikon D7000. After searching on eBay, I found that the same lenses (e.g. Vivitar 135mm f/2.8) are cheaper for older mounts (M42, Canon FD etc.) than the Nikon F-mount.
Is there a difference in using a full manual F-mount lens with or the same lens for another mount with an adapter?
Sadly for Nikon users, the F mount has one of the longest registers ever.
(Mechanically) adapting a lens designed for a certain system to one with a shorter register is easy: just manufacture an extension tube of the correct length. The ability of controlling the lens will be mostly lost but this is less of an issue with lenses with mechanical aperture rings.
Doing the opposite is impossible: there are thus two possible solutions to the problem.
Design an adapter adding the least possible distance and let it be: the lens will be limited to close-up work and will become unsuitable for general photography.
Add a corrective lens: The original range of available focusing distances will remain available, but quality will degrade (how significant the change will be depends on the specific implementation, with most adapters only being optimized for the center of the image).
This relationship, when looked at backwards, also makes lenses for a longer register relatively more valuable, as they can be converted to more systems. This was the entire premise behind the ideation of the T mount and the Adaptall series.
Using a mount conversion shouldn't degrade the image quality since there isn't any optics involved, just the mount. Be aware that the older lenses may not have the same level of optics you may be used to on current lenses.
You'll want to make sure you can change the aperture on the lens when you are using a converter since your camera won't be able to do so (since electronics aren't passed through) which make the older F-mount Nikon primes lenses great for these kinds of uses.
Nearly all good camera stores will have some old non-AI Nikkor lenses, the 50mm F1.4 is a really nice lens that will be well under $200. But, the non-AI lenses have coupling prong that may prevent it from mounting on your D7000. The F-mount is the same, but the prong can have interference problems with the pentaprism. You really have to try a lens or two with your body.
The second big problem is that modern DSLRs do not have a focusing screen that makes it easy to use manual focus. The old SLR focused wide open and had micro-prism or split-prism focus screens, which would snap when the lens was in focus.
