What is crop factor, in plain terms?

Crop factor is a single number that compares a sensor's diagonal measurement to full frame's diagonal (43.27mm), and tells you how a given lens's field of view changes when mounted on that smaller sensor instead. Full frame sits at exactly 1.0x by definition. Nikon, Sony and Fujifilm's APS-C bodies land close to 1.5x; Canon's own APS-C sensor is slightly smaller still, at roughly 1.6x. Micro Four Thirds doubles that to 2.0x, and a typical 1-inch compact-camera sensor sits around 2.7x.

A lens's real, physical focal length never changes based on what camera it's mounted on — what changes is how much of the projected image actually lands on the sensor. See what this does to field of view and depth of field for the full genuine mechanism. In practical terms: a 50mm lens on this site's 1.5x-crop APS-C format (1.53x computed precisely) frames like a 77mm lens would on full frame — a real, useful field-of-view figure worth knowing at a glance.

Crop factor changes field of view and, when matched for equivalent framing, depth of field — it does not change exposure, a genuine distinction covered in full detail on does crop factor change exposure.

Crop factor also runs below 1.0 for any sensor physically larger than full frame — 44x33mm medium format sits around 0.79x, and large-format sheet film drops well under 0.3x. Below 1.0, the usual direction flips: a given real focal length frames wider on the bigger sensor rather than tighter, which is why a 645 film camera's normal 75-80mm lens covers roughly what a 47-50mm lens does on full frame, the opposite relationship from mounting the same lens on APS-C.

For the full breakdown of field-of-view versus depth-of-field equivalence, with your own real numbers, use the Crop-Factor & Equivalence Converter directly.