Crop Factor


This story was originally published in October 2014.

How is a crop factor caused? How does one calculate it?
Soumya H, via email

‘Crop factor’ came into use with the advent of the digital camera. Digital cameras and many DSLR sensors are smaller than the standard 35mm format. The lens for conventional 35mm film produces an image circle inside the camera that covers the 35mm frame perfectly.

But as a digital sensor is smaller than this frame, it will not be able to capture the entire frame, even as the lens forms an image meant to cover the larger sized 35mm film. Thus, there is a ‘crop factor’, because of the reduced field of view and it depends on the camera’s sensor size in ratio to the size of the original 35mm frame.

The Canon APS-C sensor has a crop factor of 1.6x, whereas Nikon has 1.5x magnification. The actual focal length delivered by the lens can be calculated, by multiplying the magnification factor with the focal length.

For example, if a 50mm lens is used on an AF-S sensor, you will get an image that corresponds to a focal length of 75mm (35mm equivalent).

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