Focal Length
Focal length is a fixed physical property of a lens, measured in millimeters, describing how strongly it bends light to a focus point. A 50mm lens is 50mm on any camera it's mounted on — what changes between sensor sizes is how much of the projected image the sensor actually captures, not the focal length itself.
Shorter focal lengths (wide angle) capture a broader field of view; longer focal lengths (telephoto) capture a narrower one and magnify distant subjects more. At a fixed real subject distance and aperture, a longer focal length also produces shallower depth of field — a real, separate physical effect from the field-of-view change, not just an appearance caused by tighter framing.
Frequently Asked Questions
Does focal length change if I switch camera bodies?
No — a lens's real focal length never changes based on what it's mounted on; only the field of view captured by a specific sensor size changes, described through the separate concept of crop factor.
Is a 'normal' focal length the same on every sensor size?
No — 50mm is normal on full frame, but the real focal length that reads as normal on a smaller sensor is shorter, scaled down by that sensor's crop factor.