My question to those more knowledgeable;
Is having an APS-C lens going to be better or worse in terms of image quality performance compared to a Full-Frame lens on a crop sensor camera like the a6500?
Theoretically, then can be better (at least in some ways). It's widely believed that it's easier to make sharper lenses when you're dealing with a smaller image circle (lenses for tiny sensors outresolve lenses for bigger sensors, generally). But that's the general case and doesn't always apply. On the other hand, you're likely to see more consistent performance across an APS-C frame (and less vignetting) with a FF lens.
There are a few lenses I am looking at (particularly primes) that are offered for both APS-C and FF, but not sure which would be the smarter choice.
I base it on more practical considerations - certain focal lengths, particularly WA lenses for APS-C, are only going to be available as crop lenses (like a 16-50 or 18-55) while others may be available, but the FF version is bigger and more expensive (in the case of wide angle lenses, they have to be bigger to cover the image circle). Consider whether you're likely to upgrade to FF - and, importantly, if you were to upgrade to FF, whether the lens is a lens you'd want to use on FF.
Generally, if there are both an APS-C and FF version of a lens, I'd take the APS-C version - it should be smaller, lighter, cheaper, possibly sharper, though not necessarily. Even then, it's not a slam dunk. I currently own both the Sigma 30/1.4 and the Sony 28/2 (full frame). The Sigma is probably sharper, but the Sony is sharp enough to not care (for me). The Sony has (slightly) quicker AF - usefully quicker AF in low light. The Sigma is f/1.4 which allows shallower DOF. The Sony is actually smaller (it is 1 stop slower, but it's still a small lens compared to, say, the Nikkor 28/1.8). I haven't figured out which, if either, I should get rid of because they both have strengths.
- Dennis
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