Crop factor is a single number that compares a camera’s sensor with the full-frame 36 × 24 mm format of 35 mm film. It is the ratio of the two diagonals: crop = 43.27 mm ÷ sensor diagonal. A smaller sensor "crops" the centre out of the image circle that the lens projects, which makes the lens look longer than it would on full frame.
| Format | Size (mm) | Crop factor | Circle of confusion |
|---|---|---|---|
| Full frame | 36 × 24 | 1× | 0.030 mm |
| APS-C (1.5×) | 23.5 × 15.6 | 1.53× | 0.020 mm |
| Canon APS-C (1.6×) | 22.3 × 14.9 | 1.61× | 0.019 mm |
| Micro Four Thirds | 17.3 × 13 | 2× | 0.015 mm |
| 1-inch | 13.2 × 8.8 | 2.73× | 0.011 mm |
| Medium format (44×33) | 43.8 × 32.9 | 0.79× | 0.038 mm |
| Phone main camera (1/1.3″) | 9.8 × 7.4 | 3.52× | 0.009 mm |
Equivalent focal length
Multiply the real focal length by the crop factor to find the full-frame lens that gives the same field of view. A 23 mm lens on a 1.53× APS-C camera frames like 35 mm on full frame; a 25 mm lens on Micro Four Thirds like 50 mm; a 600 mm lens on Micro Four Thirds like 1,200 mm. Going the other way, a medium format 63 mm lens behaves like a 50 mm on full frame.
Notice what is not happening: the lens is not magnifying more. Projected onto the sensor, a distant tree is exactly the same size with a 50 mm lens on any camera. The smaller sensor simply records less of the surrounding scene, and when both images are shown at the same size, the cropped one looks magnified. That is why crop cameras are popular for wildlife and sports — the "reach" comes free with the smaller sensor, as long as it has enough pixels.
Does crop factor change the aperture?
This is the most argued-about question in camera forums, and the answer is "for exposure, no; for depth of field and total light, yes".
Exposure stays the same
The f-number describes light per unit area. f/2.8 at 1/125 s and ISO 400 gives the same brightness on a phone, an APS-C body or a medium format camera. You never need to adjust exposure settings because of crop factor, and light meters and the exposure calculator work identically for every format.
Depth of field scales with the crop factor
To compare the look of two formats at the same framing, multiply the f-number by the crop factor as well. A 25 mm f/1.8 on Micro Four Thirds frames like a 50 mm and has the depth of field of about f/3.6 on full frame. A Fujifilm 56 mm f/1.2 is equivalent to roughly 85 mm f/1.8. A phone’s 7 mm f/1.8 main camera behaves like a 24 mm f/6.3 on full frame for depth of field — which is why phones compute background blur in software.
Total light and noise
At the same f-number, a full-frame sensor collects more total light than a smaller one because it is bigger, so at the same output size its images are cleaner at the same ISO — roughly by the crop factor squared, or about one stop for APS-C and two stops for Micro Four Thirds. Equivalent aperture captures this too: f/1.8 on Micro Four Thirds collects about as much total light as f/3.6 on full frame.
What crop factor does not change
- Perspective. Perspective depends only on where you stand. A 50 mm on APS-C and a 75 mm on full frame from the same spot produce the same perspective.
- The lens’s focal length. It is a physical property printed on the barrel.
- Exposure settings, as explained above.
- Image stabilisation and handholding — except that the old "1 ÷ focal length" rule should use the equivalent focal length, because the effective magnification is higher.
Field of view
Field of view is the angle of the scene a lens covers: 2 × arctan(sensor dimension ÷ (2 × focal length)). A 50 mm lens on full frame covers about 40° horizontally and 47° diagonally. On APS-C the same lens covers about 26° horizontally — similar to a 75–77 mm lens on full frame. Our crop factor calculator shows horizontal, vertical and diagonal angles for any lens and sensor.
Common equivalents
| Full frame | APS-C (1.5×) | Canon APS-C (1.6×) | Micro Four Thirds |
|---|---|---|---|
| 14 mm | 9 mm | 9 mm | 7 mm |
| 24 mm | 16 mm | 15 mm | 12 mm |
| 35 mm | 23 mm | 22 mm | 18 mm |
| 50 mm | 33 mm | 31 mm | 25 mm |
| 85 mm | 56 mm | 53 mm | 43 mm |
| 200 mm | 131 mm | 124 mm | 100 mm |
Real lens ranges follow these numbers: an 18–55 mm APS-C kit zoom is a 27–84 mm equivalent, and a 12–40 mm Micro Four Thirds zoom is 24–80 mm. For exact figures for a specific body — Sony a6400, Canon EOS R50, Fujifilm X-T5, RX100, iPhone and more — see crop factor by camera.
So which sensor size is "best"?
None — each trade-off suits a different photographer. Full frame and medium format make shallow depth of field and low-light work easier. APS-C and Micro Four Thirds give smaller, lighter telephoto lenses and more depth of field at a given aperture, which landscape, macro and travel photographers often welcome. 1-inch sensors and phones fit in a pocket and keep almost everything in focus. Understanding crop factor lets you compare them honestly, and translate any advice — "use an 85 mm f/1.8 for portraits" — into your own system. See each format’s numbers on its sensor page.