How this instrument works
Crop factor compares a camera's sensor to the full-frame 35mm format (36 x 24mm) that photography has used as its reference point since long before digital sensors existed. It's the full-frame diagonal divided by your sensor's own diagonal, and this instrument builds that diagonal for you from the two numbers actually printed in camera spec sheets — sensor width and height — rather than making you compute a diagonal by hand first.
The number matters because it tells you how a lens's field of view changes when moved between formats. A crop factor of 2.0x, typical of Micro Four Thirds cameras, means a lens captures the same angle of view it would at twice the focal length on a full-frame body — a 25mm lens frames like a 50mm 'normal' lens, even though its actual focal length and physical properties, like depth of field, haven't changed at all. A sensor smaller than full-frame gives a crop factor above 1.0x; a sensor the same size or larger gives one at or below 1.0x.
Crop factor is purely a framing comparison, not a statement about image quality. A 50mm lens is still a 50mm lens regardless of the body it's mounted on — only the slice of its projected image that the sensor actually captures, and so the apparent field of view, changes with the format underneath it.
- Enter Sensor width (mm) — from your camera's published specifications or a camera-format reference table.
- Enter Sensor height (mm) from the same source.
- Read Crop factor beneath the inputs — the multiplier relative to full-frame 35mm's 36 x 24mm reference.
- A result above 1.0x means a smaller-than-full-frame sensor; near 1.0x means full-frame; below 1.0x means a larger sensor, such as medium format.
Worked example — a 23.6 x 15.7mm APS-C sensor
Enter 23.6mm width and 15.7mm height, a common APS-C spec used by Nikon and Sony DX/APS-C bodies. Diagonal = √(23.6² + 15.7²) = √(556.96 + 246.49) = √803.45 = 28.345mm. Crop factor = 43.3 / 28.345 = 1.528 — matching the well-known 'APS-C is roughly 1.5x' figure quoted throughout camera reviews and manufacturer specs.
That means a 35mm lens on this sensor frames a scene the way a 35 x 1.528 = 53.5mm lens would on a full-frame body — close enough to 'normal' that APS-C shooters often just round it to 1.5x and do the mental math on the fly.
Questions
Why is 43.3mm used as the reference diagonal for crop factor?
Because 43.3mm is the conventionally rounded diagonal of the full-frame 35mm sensor format (36 x 24mm) — the square root of 36² + 24² works out to 43.2666mm, which the photography world universally rounds to 43.3mm. Full-frame 35mm has been the reference format since it inherited its dimensions from 35mm film, so every other sensor size is compared back to it by convention, not because it's inherently a superior size.
Does a true full-frame sensor give exactly 1.0x?
Not quite — and this calculator shows the honest answer rather than rounding it away. Feed in 36 x 24mm and the diagonal comes out to 43.2666mm, but the reference constant used everywhere (including here) is the rounded 43.3mm, so 43.3 / 43.2666 = 1.0008x, not a clean 1.0000x. It's a rounding artifact of the convention, not a real optical difference — for any practical purpose, full-frame is 1.0x — but it's worth knowing why the digits after the decimal don't land on zero if you ever check the math yourself.
What crop factor does a Micro Four Thirds camera have?
Roughly 2.0x. A Micro Four Thirds sensor measures about 17.3 x 13.0mm, giving a diagonal of 21.64mm, and 43.3 / 21.64 works out to 2.001x — the well-known 'MFT is a 2x crop' figure quoted throughout that format's marketing and reviews. A 25mm Micro Four Thirds lens therefore frames a scene the same way a 50mm lens would on a full-frame body.
Does a higher crop factor mean worse image quality?
Not directly — crop factor by itself only describes a framing relationship, not image quality. It does correlate with sensor size, though, and larger sensors (lower crop factor) generally capture more light per pixel at a given resolution, which typically improves noise performance and dynamic range in low light. The crop factor number is the geometry; the image-quality difference comes from the physical sensor area it implies.
Where do I find my camera's sensor width and height?
Check the manufacturer's published specifications — sensor dimensions (width x height in mm) are standard entries on spec sheets and sensor-format reference tables, often alongside the sensor's common name (APS-C, Micro Four Thirds, full-frame, medium format). If a spec sheet lists only a diagonal or a crop factor directly, you can skip this instrument and use that figure as-is.