Interesting you mention the maximum pixel density per unit on a sensor. Some time back the APS crop sensor (aka D300) was considered at the maximum then. 12-14MB was allegedly the limit as higher than that would result in too much distortion etc as the pixels got smaller and there were more edges and less info gathering pixel. Seems technology has worked a solution.
Can't help but wonder what a full size sensor at the current APS density would equate to.
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