The lens is the Sigma 30mm f/2.8 Art (E-mount fit), which does have a reputation for being incredibly sharp. As far as I know I'm not using an CA correction (not in processing anyway - the camera might be applying some by default but the manual's so poor I wouldn't know!). I have turned the jpeg sharpness down to -2 in camera (it was set to 0) so will see if that helps.
I'm inclined to agree that it's likely not the sensor - I suspect the jpeg engine is 'over-processing' - however this issue does seem to be most prominent with 20mp sensors (in my experience). The worst offenders I've seen are the 6D, 70D and now the A5000.... all of which just happen to be 20mp. Coincidence?
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