The term crop factor refers to the ratio of a specific sensor to a 35mm full frame sensor. This factor determines the equivalent field of view of a lens when used on a camera with a sensor that is either smaller or larger than our reference full frame sensor.

Many people are familiar with the two common APS-C crop factors: 1.6x for Canon, and 1.5x for Nikon, Sony and everyone else. But how do we get to this number, and how do you calculate the crop factor for other sensor sizes?

Crop Factor Calculation

The important thing to know is that the crop factor is the ratio of the diagonal dimension of the sensor. Manufacturers often provide the horizontal and vertical dimensions of a sensor, so we can use Pythagorean theory to calculate the diagonal dimension.

c2 = a2 + b2

therefore

c = √(a2 + b2)

Full frame sensor dimensions: 36mm x 24mm therefore diagonal dimension is  √(362 + 242) = 43.27mm

Crop Factor Calculation Example

Now you need to calculate the diagonal dimension of the sensor for which you are trying to find the crop factor. For our example, we’ll simply work with a Canon APS-C sensor which has the dimensions 22.2 x 14.8 mm.

Canon APS-C dimensions: 22.2 x 14.8 mm therefore diagonal dimension is  √(22.22 + 14.82) = 26.68mm

Crop Factor therefore equals: 43.27/26.68 = 1.621814 -> rounded to 1.6 for general usage.

Applying Crop Factor to Find Equivalent Field of View

The reason that we want to know the crop factor of a camera is to understand what kind of field of view we will get with a particular focal length. For example, placing a 50mm lens on an APS-C camera will actually give us a field of view that is equivalent to that of a much longer lens on a full frame camera. For a Canon APS-C camera, that would be 1.6×50 = 80mm.

In other words, a 50mm lens on an APS-C camera, delivers the same field of view as an 80mm lens on a full frame camera. If you wanted that “normal” 50mm framing on an APS-C camera, you would reverse the process and divide 50/1.6 giving you 31.25mm.  This means that you could use a lens that covered that focal length, perhaps a 24-70mm zoom, and set it to about 32mm to replicate the field of view of a normal 50mm lens if you were using an APS-C camera instead of a full frame camera.

Common Crop Factors

Below you can find some of the most common crop factors for modern digital sensors, but using this simple math technique that I have explained above, you can now calculate the crop factor of any sensor that you come across. You might also want to take a look at my other post comparing common digital sensor sizes.

Sensor SizeSensor DimensionCrop Factor
Medium Format53.7mm x 40.4mm0.64
Medium Format (cropped)43.8mm x 32.9mm0.79
Full Frame 35mm36mm x 24mm1
APS-H27.9mm x 18.6mm1.3
APS-C23.6mm x 15.6mm1.5
APS-C (Canon)22.2mm x 14.8mm1.6
1.5″18.7mm x 14mm1.9
4/3″17.3mm x 13mm2
1″12.8mm x 9.6mm2.7
2/3″8.8mm x 6.6mm3.9
1/2.3″6.17mm x 4.55mm5.6

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