Is there much of a difference in image quality using old manual lenses vs. modern day ones on Sony FF?
It really depends on the lens. A good vintage lens will outperform a mediocre modern lens, but wouldn't touch something like a Sigma Art in terms of corner-to-corner sharpness at wide apertures. That said, there are a few general trends:
- More precise manufacturing techniques for the glass itself have meant that new lenses are, in general, sharper.
- Lens coatings have improved, so flare is less of an issue for modern lenses (even relatively cheap ones).
- Computer aided design has changed the design process of new lenses, which I feel has made the biggest difference to zoom lenses. In general older zoom lenses aren't great (although there are some notable exceptions), but improved quite a lot from the 90s onwards. The complexity of zoom designs probably made it quite difficult to design good ones pre-CAD. This hasn't had quite as much of an impact on prime lenses, as they tend to have simpler designs. Many modern primes still use a
double-gauss design, for example, which is over 120 years old now. That said, the newest high end primes from the likes of Zeiss, Leica and Sigma, have likely benefited quite considerably from CAD, as they're using more complex designs to control for distortion, CA, etc, while retaining an extremely high degree of sharpness.
Philipreeve.net is a good resource for reviews of both old and new lenses on the full-frame Sonys, with comparisons at the end of the reviews which often include both modern and vintage lenses.
Good lenses are expensive on any system. No doubt you have to factor in the cost of them, but I think it's wrong to say everything is expensive.
Canon EF has some cheaper, still decent lenses like the 85mm f1.8 and 40mm f2.8 and there are cheaper options if you can settle for less IQ. The old Sigma 15-30 seems to be a good wide angle for the price.
Standard wise you can go really cheap for the 28-105 which is probably better optically than typical APS-C kit lenses and has USM or step up to the 24-105 IS STM which looks like a decent, if slow, lens. There are probably some Sigmas and Tamrons I'm missing here, but for sharpness a used 24-70 f4L is expensive, but not stupidly so. It's sharper than the 24-105 f4L too.
For tele zooms there's the old 70-300 IS or you could step up to the Tamron 70-300 VC. The Canon 70-200 f4 without IS is not that expensive used either if you want something sharper.
None of them set the world on fire, but I think they're reasonably competitive and you can make a decent budget FF setup for $1000. The EF mount is almost 30 years old so there's plenty of stuff I didn't cover, but look them up before buying as there are some dogs. In the 1990s a 24-70 was seen as wide so not surprisingly they didn't do a great job with wide angle zooms.
If you want low light then obviously a more modern APS-C with faster lenses will do a better job, especially if you get something 2010 or later like the Pentax K5 and Nikon D7000 (or smaller brothers with the same sensor).
I'm more talking in the context of a 20D owner who only wants to spend a couple of hundred dollars for an upgrade. They may be willing to stretch a bit to a 5D, but probably not that much further for lenses on top of that. APS-C cameras, by comparison, can often be found with kit zooms at little to no extra cost, and most modern kit lenses are actually decent enough. If you're talking a ~$1K budget then you can definitely piece together a few decent lenses on top of a 5D body.
The 28-105 is actually a pretty good deal, though, I hadn't realised it was available that cheap until I just looked it up. I'd be a little wary of the Sigmas and Tamrons from that era, though, as they reverse-engineered the EF mount and in many cases they don't work properly with newer (i.e. digital) Canon bodies, for example not autofocusing or an inability to stop down the aperture.
