EOS M-aths
How to work out the view offered by an APS-C sensor
You’ll need to take into account the 1.6x crop factor of the smaller APS-C imaging sensor inside an EOS M when you’re selecting a lens, whether it’s a full-frame EF lens or a smaller EF-M one. For example, if you mount a lens with a focal length of 20mm onto an EOS M, you’ll get the equivalent view that you’d get from a focal length of 32mm on a full-frame DSLR (as 20mm x 1.6 = 32mm).
In order to get the equivalent view of a 20mm lens on a full-frame camera, you’d need to use a focal length of 12.5mm on an EOS M (12.5mm x 1.6 = 20mm).
As highlighted earlier, the advantage for long-lens photography is it’s easier to fill the frame with distant wildlife and sports action using shorter and (often) lighter lenses. It’s not so great for wideangle work though, and you may need to invest in a dedicated EF-M or EF-S ultra-wide lens if you’re adding an EOS M to a full-frame lineup.