The same lens produces different results on cameras with different sized sensors. As a result, it can be tough to cross-shop Micro Four-Thirds, APS-C, Full Frame, and Medium Format cameras.
Crop Factor is a conversion that makes it simple to determine the exact results you'll get with any given lens, regardless of your camera's sensor size.
By multiplying your camera's crop factor by a lens' focal length AND aperture, you can determine the full frame equivalent angle of view, depth-of-field (background blur) and a good estimate of the low-light noise.
Here are common crop factors: