Originally posted by johnyates
AFAIK, the only disadvantage to using FF lenses on crop cameras is that the FF lenses admit more non image forming light into the camera which can cause flare. However, by using a sufficiently long lens hood, this problem is mitigated.
And of course the big advantage is the crop sensor gets the best image quality from the centre of the lens.
I'm quite satisfied with what I get from my FF lenses--I only own one DA lens.
Finally I found a post that highlights the same points I had in mind. This will spare me cramps, typing on the virtual keyboard of my cellphone
Just two more details:
1) most FF optics available in PK mount are film-era lenses; their design, not their image circle, is the problem (especially true for zooms and super wides)
2) a good multicoating on the back glass and appropriate baffles on both camera and lens bodies should tame the non-image-forming light rays coming from an FF optic. Of course a suitable lens hood is the first step. Though I've seen vintage film lenses with poor blackening, and adapters that are a total disaster in this regard.
A typical example would be a macro lens for Pentax 6x7, Pentax 645 or Mamiya 645. All of them are still sharp enough (the 6x7 one a little less, IMHO), but the adapter is the weak link. Chinese rings are generally good and cheap, but sometimes need to be closely inspected for light leaks and reflections (shiny, non blackened material).
With some M42 lenses any adapter ring, even the original Pentax, lets plenty of light in. My remedy is an O-ring of appropriate size, or a black hair band.
Many times I found that a supposedly poor lens was plagued by light leaks or inter-reflections. Once the problem was solved the same lens proved much better!
The area of the APS-C sensor doesn't differ so much from the FF/135mm, I don't think it would justify going for different levels of sharpness, at the design level. I don't think modern day FF lenses, of any maker, are designed to be less sharp than APS-C optics. Though the larger the format they are designed for, the more light enters the camera. Proper construction and appropriate lens hoods are paramount.
EDIT:
The last message, posted by @Digitalis, reminded me that sometimes 8x10" optics could give a certain level of veiling flare, if used on 4x5" or film backs. Even a black pleated bellows can cause inter-reflections, if the material is not fully matte.
I'm sure modern time Sinar or Linhof cameras have addressed the problem (and recent LF lenses are multicoated!), but I own a few vintage cameras that have the bellows made of a kind of shiny black material, and some areas that should be blackened, unfortunately are not. On such cameras it's better to use lenses with appropriate coverage, and it's always a good idea to use the kind of adaptable lens shades used in cinema, instead of a conventional screw-in hood.
Cheers
Paolo