>>2660957This is affected by the size of your sensor in your camera. Some cameras (mostly professional level bodies) have a "full frame" sensor which is an image sensor the same size as a 35mm negative (from back in the film days) and most people talk about lenses in regards to that sensor size relationship. A 50mm lens is a 50mm lens.
However, to save costs, weight, and speed, lower end cameras have smaller sensors in them. APS-H, APS-C, micro 4/3, 1", all the way down to a sensor the size of a pencil eraser, like in your cell phone.
These different sensor sizes don't relate to lenses in the same way. A 50mm lens still puts out an image the same way it does on a full frame sensor, but an APS-C sensor is smaller, so it sits in the middle of the projected image, and the outside edges of the scene don't fall on the sensor. (pic related) so the scene APPEARS to be "zoomed in" a bit.
This level of "zoom" is constant between sensor sizes, so it can be measured and predicted.
A 50mm on a full frame camera gives a 50mm equivalent
A 50mm on an APS-C camera (like a Canon 50D, 7D, T5i, or a Nikon D7100, or a Pentax K5, and many others) will "crop" the image by 1.5x giving the equivalent focal length of 75mm (on a full frame camera)
On M4/3 (Like an E-M1) it crops by 2x. So a 50mm lens behaves like a 100mm lens on full frame.
Because focal lengths are used to communicate the general field of view (or general usage) of a lens, it can be helpful to convert your actual focal range to the equivalent focal range in conversation. For instance, a lot of portraits are taken at 85mm (on a full frame camera) and many people feel that 40mm is too wide-angle for portraiture. So if you're on M4/3, and you say "look at this portrait! I shot it on my 40mm lens!" people might get confused. So you could give the "full frame equivalent" focal length of 80mm, to make it more clear.
