Full Frame Vs. APS-C:The Cropping Factor Explained
What's a lens-cropping factor, and why don't full-frame cameras have one? The lens-cropping factor is a shortcut used to explain and predict the different fields of view captured by full-frame and APS-C cameras using similar focal-length lenses. Essentially, full-frame cameras actually have a cropping factor, but it's a 1:1 factor compared to factors ranging from 1:1.5 to 1:1.6 in most APS-C cameras, and typically written as 1.5X (Nikon, Pentax, Samsung, Sony) or 1.6X (Canon).
In the early days of digital, the cropping factor often was erroneously labeled the focal-length magnifier, as if it referred to the same changes one might expect when using a teleconverter to increase the reach of a lens. These changes included a reduction in a lens' maximum aperture, plus decreased field of view and depth of field at any given focal length and aperture. But with digital cameras, only the field of view varies, as lens focal length is based on optical formulas that don't factor in the size of the camera sensor. Therefore, a lens' maximum aperture and depth of field at any given aperture don't vary based on sensor size; only the field of view changes.
An easy way to picture this relationship is to imagine two different windows set in the same wall. Stand inside at the same distance from each window and look through the smaller window (APS-C), and you'll see a narrower field of view compared to the larger window (full frame), but the exact same depth of field and scene brightness.
To determine the cropping factor on any interchangeable-lens camera, you must divide the diagonal length of a full-frame sensor (43.3mm, the same as 35mm film) by the diagonal length of the smaller APS-C sensor. For APS-C sensors in a Nikon camera, the diagonal is 28.9mm, resulting in a cropping factor of 1.5X, while on a Canon APS-C-class sensor, the diagonal is closer to 27mm, resulting in a cropping factor of 1.6X.
Once the cropping factor is known, it can be multiplied by the focal length of the lens to determine the equivalent field of view recorded by the sensor. For example, a 50mm lens placed on a Nikon D90 (APS-C sensor) will show the same field of view on captured images as a 75mm lens mounted on a full-frame camera (50 x 1.5). However, the depth of field at any given aperture will remain the same for each of the captured images, while changing the actual focal length from 50mm to 75mm on the full-frame camera will result in a slightly decreased depth of field.
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