Crop factor describes the difference in sensor size relative to a full-frame sensor. It helps you compare the field of view produced by the same focal length on different camera formats.
The lens does not change focal length
A 50mm lens is still a 50mm lens on an APS-C camera. The smaller sensor simply captures a smaller central portion of the image projected by the lens.
Canon APS-C example
Canon APS-C cameras commonly use a crop factor of about 1.6x. A 50mm lens on one of those cameras gives a field of view similar to an 80mm lens on a full-frame camera.
50 × 1.6 = 80
Sony, Nikon, and Fujifilm APS-C example
Many other APS-C systems use a crop factor of about 1.5x. A 50mm lens then gives a field of view similar to about 75mm on full frame.
What crop factor does not do
- It does not physically turn a 200mm lens into a 320mm lens.
- It does not change the lens aperture marking from f/2.8 to something else.
- It does not create extra optical magnification.
Why this matters for shopping
If you want a wide field of view on APS-C, you often need a shorter focal length than you would on full frame. If you photograph distant subjects, the narrower field of view can be useful because the subject occupies more of the frame at the same focal length.
This is one reason lens recommendations should always consider the camera body and sensor format, not just the lens by itself.