crop factor

Crop factors can be used to approximate equivalencies between full frame and crop sensor cameras to take similar pictures using different hardware, and to compare and contrast the strengths and weaknesses of camera bodies and lenses.

For two cameras with a full frame and crop sensor respectively, but using lenses with equivalent focal lengths, the resulting field of view on the smaller camera will be reduced by its sensor’s crop factor. For example, a full frame camera with a 32 mm focal length will have a field of view 1.6x as narrow on an APS-C camera. To get an equivalent focal length, the APS-C camera would need a lens with a \(\frac{32\ \mathrm{mm}}{1.6} = 20\ \mathrm{mm}\) focal length, or the APS-C shooter would need to stand further back to capture more of the surrounding scene. This is simply due to the sensor being smaller than the sensor/film on a full frame camera, which is thus not able to take in the same amount of light per square millimeter.

Using shorter focal lengths to get an equivalent picture size on a crop sensor also makes the effective depth of field wider, as DoF is a product of focal length.

Adapting lenses

Dumb adapters can be used to mount full frame lenses on crop sensor cameras. These adapters must also place the lens in such a way that the convergence point is moved from its original location to what the new camera expects. For example, Micro Four Thirds cameras have a convergence point exactly 14 mm longer when compared to a full frame camera; so any dumb full frame lens adapter must hold the lens an additional 14 mm away from the camera sensor to be able to achieve infinite focus. This is why Micro Four Thirds dumb adapters are significantly longer than other adapters.

However, dumb adapters will necessarily not address the loss of light: since the sensor is smaller, it cannot take in all the light the lens projects, as the outer bounds of the lens’ projection do not land on the sensor and are effectively lost. This results in the image appearing magnified. In order to take in more light when using adapted lenses, thus increasing the field of view and providing an image closer to that of the lens when used on its intended camera, telecompressors can be used. Also known as focal [length] reducers or speed boosters, telecompressors shrink the projected image using their own independent lens elements. This also increases the lens speed by virtue of having more light hit the sensor.

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