"Crop Factor" refers to the sensor size of a camera relative to a 35mm (Full-Frame).
The crop factor determines how much smaller (or larger) a sensor is than a reference full-frame sensor (i.e. 35 mm film or a 24 mm * 36 mm sensor).
c = A35mm[mm²] / Acrop[mm²]
c is the crop factor, A35mm is the area (as in w [mm] * h [mm]) of the reference full frame sensor in square millimeters, Acrop is the area of the sensor we want to know the crop factor of in square millimeters.
Some of the most common crop factors are:
- APS-H: 1.29
- APS-C: 1.51 (Nikon, Sony) to 1.61 (Canon)
- 4/3": 2
- 1/2.3": 5.49
While the focal-length does not change, because a smaller sensor sees less of the image circle, it has a smaller field-of-view (FOV), for which you would have to use a longer focal length lens on a 35mm (full frame) camera:
FOV [°] = 2 * arctan ( d [mm] / (2 * f [mm]) )
FOV is our field of view in degrees, d is one of the dimensions of the sensor (height/width/diagonal) in millimeters and f is the focal length in millimeters.
Using the same lens (and therefore the same focal length), a sensor with 1/2 the size of a full-frame sensor would have a crop factor of 2 and therefore, the FOV of such a sensor would be half of that of a 35mm sensor. To achieve the same FOV with 35mm, we would need twice the focal length. Note that the actual focal length stays the same regardless the size of the sensor.
Further reading:
- What is crop factor and how does it relate to focal length?
- What is the crop factor for Four Thirds cameras considering aspect ratios? deals with the question of crop factors of non-3:2-ratio sensors (in this case: 4/3").
- How does crop factor affect perspective?
- Does my crop sensor camera actually turn my lenses into a longer focal length?
- Can a smaller sensor's “crop factor” be used to calculate the exact increase in depth of field?
