- Is the IQ of the Sony A7 better than the X-T20? They are both 24MP. Is the sensor size making a difference at usual ISO values, like say 400 ISO?
Yes, it is -- and most of the difference is in how they use a lens designed for FF, although low light performance is probably a bit better too.
Here's the DPReview article I wrote on old lenses with APS-C vs. FF. The difference is small, and sometimes APS-C with a focal reducer wins, but not usually and pretty much never stopped down. The most common FF lens defect is vignetting causing dark corners -- not blurry corners -- but stopping down fixes that.
- How are the jpeg? Can they be tuned to give very similar results, or are the compression algorithms different? If yes, is it the Fuji or the Sony that 'wins'?
As I said, Fuji's film emulation modes are quite accurate -- which means they result in images with blocked-up shadows, etc. If you prefer the film look, get a Fuji. However, the JPEGs from the Fujis don't have the dynamic range that Sonys do, and Sony is very good at reducing noise without artifacting (better than simple raw postprocessing is).
- Is the A7 giving good results in the corners with old lenses? I am using old lenses on M43. and better APS-C, and the results are often excellent, the cropped format removing the typically weak edges of the FF format. Are old lenses ok on a FF digital camera? Believe it or not, I never took a FF digital photo in my life, and it makes me nervous.
As I said above, and in my article, vignetting is the primary defect on FF (and it's somewhat correctable, burning a little dynamic range). On APS-C, it's lack of resolution (which isn't correctable, but also isn't obvious in viewing scaled-down images). The slight crop from native FF when you use a focal reducer on APS-C, about 1.1X, usually is enough to remove any annoying vignetting, but then so is stopping down a little on FF. On MFT, the resolution is quite demanding (16MP is like 26MP APS-C or 61MP FF, 20MP is like 33MP APS-C or 76MP FF). Additionally, MFT specifies a very thick sensor stack that makes dust spots less obvious, but causes CA and off-axis issues (good thing it never gets too far off axis). In short, lenses really need to be designed differently for top performance on MFT, and no old lenses were designed that way.
All that said, you'll mostly notice the cropping. For example, a 35mm acts like a normal lens on APS-C and a portrait telephoto on MFT.