Magnification Ratio
The ratio between the size of a subject and the size of its image on the sensor.
Formula / rule
m = image size / subject size
Measured in
ratio
In depth
A ratio of 1:1 means life size, where a 10 mm subject projects a 10 mm image; 1:2 is half life size and 2:1 is twice. Because sensor size varies, the same ratio fills a smaller sensor more completely, so a 1:1 image on APS-C looks more magnified in the final picture than the same ratio on full frame.
Examples in practice
At 1:1 on a full-frame sensor, a subject 36 mm across exactly fills the long edge of the frame.