Crop Factor

Crop factor describes the relationship between a camera sensor's size and the 35mm film format, which is the standard reference for comparing camera systems. The Micro Four Thirds sensor has a crop factor of 2x, meaning it is smaller than a 35mm frame by a ratio that doubles the effective field of view of any lens. A 50mm M43 lens captures the same angle of view as a 100mm lens on a full-frame camera. To find the full-frame equivalent focal length of any M43 lens, multiply its actual focal length by 2.

The crop factor gets its name from the way a smaller sensor crops the image circle projected by a lens. A 50mm lens produces an image circle large enough to cover a 35mm frame. When mounted on an M43 body, only the central portion of that image circle falls on the sensor, producing a narrower field of view. The effect is identical to cropping a full-frame image to the same area, which is why the term persists despite the sensor not literally cropping anything.

For telephoto photography, the M43 crop factor is a practical advantage. A 300mm f/4 M43 lens delivers a 600mm full-frame equivalent field of view in a much smaller and lighter package than a 600mm lens built for full-frame. A 100-400mm M43 zoom covers 200-800mm equivalent. This reach advantage is one of the primary reasons wildlife and sports photographers choose M43, as the system delivers telephoto performance at a size and weight that larger formats cannot match.

The crop factor also affects depth of field comparisons between formats. A 25mm f/1.4 M43 lens gives a 50mm full-frame equivalent field of view, but the depth of field at f/1.4 on M43 is equivalent to approximately f/2.8 on a full-frame camera at the same framing and subject distance. To produce depth of field equivalent to a full-frame f/1.4 lens, an M43 lens would need to be f/0.7, which does not exist in practical autofocus form.