Pretty much the same story for the 500, softer, more expensive, and more CA. Very slightly more transmission and slightly less vignette, also more distortion. Pretty much exactly the same as how the 300 mm lenses match up. Of course it may be the A99 limiting these lenses with a strong AA filter or something... but it has a higher resolution sensor than the 5DIII.
A99 is 24M camera, so pretty close to 5DIII.
Yes, I know overall inferior lens quality (comparing to Nikon and Canon). But it is not like total unavailability of some lenses for this cam. And don't forget about third parties. Sigma beats 1.4 lenses from C & N in many aspects. Some long lenses are fine from them too.
Time flows, and world is changing. Lots of young people plays with Sony these days. Not sure if they will to swap their camera systems as they grow up.
So far n+1 iteration of Canon sensors suffers from the same issue. I know, this issue is problem when user does not know, how to expose properly, but those of us, who remembers methods of lifting shadows in B&W process tends to forget that it is a sin.
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Marcin_3M