[ddalex]

Aren't all sensors square already (well 4:3, or 3:2) , and fitting the circle ?

[rhplus]

No, 4:3 is not a square.

I found a discussion on the topic here[*]. The consensus seems to be that “wasted” sensor area outside the circle would not be marketable.

https://photo.stackexchange.com/questions/109452/is-there-an...