Aspect Ratio
Aspect ratio is the proportional relationship between an image's width and height. Most full-frame and APS-C cameras shoot 3:2 (inherited from 35mm film's dimensions); Micro Four Thirds and many medium-format cameras shoot 4:3; some cameras offer a true 1:1 square option; video work commonly uses 16:9.
The same total megapixel count produces different pixel width and height depending on aspect ratio, which changes achievable print dimensions slightly even at an identical pixel count. Aspect ratio is independent of sensor physical size — two different sensor sizes can share the same aspect ratio, and the same sensor size can be cropped in-camera to a different one.
Frequently Asked Questions
Why do Micro Four Thirds cameras use 4:3 instead of 3:2?
It follows from the sensor's own physical proportions, which were set closer to a 4:3 relationship from the format's original design rather than matching 35mm film's 3:2 ratio the way full frame and APS-C sensors do.
Does changing aspect ratio in-camera reduce my resolution?
Yes, typically — cropping to a different aspect ratio (square, for example) discards pixels from the sensor's native shape, reducing total resolution compared to shooting the sensor's full native aspect ratio and cropping later in post-processing if you decide you need it.