Question for all - has anyone found any old film "vintage" lens to be significantly superior to a current day sister lens in terms or image quality on a canon DSLR? Or has it just been a matter of price for those or who gone down this route?I personally don't buy into the "made for digital" scenario or that today's lenses have to be better because DSLRs are oh so much sharper and better than film. I don't think that holds water but it is very difficult to test or proof. I actually have seen tests that seem to indicate that good film cameras with good film processed correctly are still resolving more detail than good digital cameras.
I'm still shooting film (135, 6x7, 4x5) and I use a mix of modern (low end) and vintage lenses on my t2i so I can actually address this pretty directly.
"Made for digital" (beyond EF vs EF-S, which is obviously a big deal) boils down mostly to a better coating on the rear element to resist flare. Digital sensors are brighter and reflect more light into the camera than film (which is matte dark gray) so this is one factor. Not sure it makes a big difference. My guess is some of Canon's very latest zooms are optimized for the 7d/60d/t2i's ultra-dense sensor, though, but I'm not sure since I won't spend that much money to find out. Film has never had this much resolution so this may be the case.
Which brings me to: the sharpest scan of the sharpest color film with the sharpest lenses at full frame (36x24mm) picks up details only a 24MP (maybe) full frame camera would pick up. However, they are extremely fuzzy and full of grain because film's mtf curve is much more gradually soft than digital as it approaches extinction and film is just, well, really grainy. So even if the detail is technically there sometimes, perceptually full frame digital is closer to 6x7 film and yes, I have compared prints. Some have said it surpasses 4x5 and 8x10. I'm not convinced one bit, maybe it's close to 4x5 but not 8x10, no way.
Old lenses do pretty well on digital.. The 50mm nikkors are all great, but their others are a bit softer. The 35mm f1.4 is a fantastic lens at f4, but on digital f1.4 is not good. All these ultra-fast primes were made for shooting 100ISO film and so light sensitivity mattered way more than sharpness. Once stopped down, old lenses are sometimes very, very impressive, even those from the 1960s. Very wide fast lenses and zooms (anything requiring aspherical elements and computer-aided designs) are MUCH better now than they used to be. There were two big innovations in design: multicoating, which allowed for lots of elements to correct aberrations (this happened in the 1960s, about) and in the past 20 years computer-aided design. Multicoating resulted in good, fast primes with decent contrast and correction for aberrations. Computer-aided design resulted in new, amazing zooms, and the rare mega-element fast wide angle prime. Most modern primes in normal focal lengths are only trivially better than older versions, at best, and retain very similar, rather simple optical designs. I do have the feeling that Nikon's new G series primes finally put to rest its earlier versions by being so much sharper at f1.4 with less coma, but look at the prices! And most of my nikon lenses from 20-40 years ago are still as sharp at f4 or f5.6 and they were really inexpensive (I bought them years ago, before prices went up).
One last note: imperfect infinity focus on a lens adapter is usually something one can deal with by focusing with live view, BUT it screws with close focus correction (floating elements) pretty badly so most newer wide lenses and anything that focuses internally gets bad results with it, generally.