Camera sensor sizes: why "1-inch" is not an inch

A unit borrowed from vacuum tubes

Sensor sizes such as "1-inch" or "2/3-inch" do not describe the sensor at all — they describe the outside diameter of the glass vacuum tube that a video camera would have used to capture the same image area in the 1950s. The actual light-sensitive area was always smaller than the tube's outer glass, by a fixed, roughly one-third margin, and digital sensors kept the old tube naming even though the tube itself disappeared decades ago.

That inherited scale factor is why a "1-inch" sensor measures 13.2 × 8.8 mm rather than 25.4 mm across — its diagonal is close to two-thirds of an inch, not a full inch. Once you know the trick is a fixed ratio rather than a literal measurement, the naming stops being mysterious and starts being predictable.

Real dimensions by format

What crop factor actually converts

Full frame — 36 × 24 mm, the size of a frame of 35 mm film — is the reference point every other format is compared against. Crop factor is simply the ratio of the full-frame diagonal to the sensor's own diagonal, and it is what lets a 50 mm lens on an APS-C body behave, in field of view, like a 75 mm lens would on full frame: multiply the focal length by the crop factor (1.5×) to get the full-frame-equivalent field of view.

This is a genuine unit conversion in the same sense as the site's other pages, just applied to angle of view rather than length — the millimetre focal length printed on the lens never changes, only the portion of the image circle the smaller sensor is able to record.

Why sensor area matters more than the megapixel count

Two cameras can share a megapixel count on very different sensor areas, and the physical size of each individual pixel — not the total count — is what mostly governs how much light it gathers and therefore how clean the image looks in low light. A 13.2 × 8.8 mm sensor and a 36 × 24 mm sensor at the same 24 megapixels have individual photosites roughly nine times larger on the full-frame chip, since sensor area scales with the square of the linear dimensions.

The takeaway

Ignore the marketing fraction and look up the actual millimetre dimensions — the table above converts every common name into the real numbers, the same way any other nominal-versus-actual measurement on this site does.