Understanding Crop Factor & Sensor Optics
In modern digital photography, the crop factor (also known as the focal length multiplier) describes the ratio between the diagonal of a standard 35mm full-frame film frame (36mm × 24mm) and the diagonal of any camera sensor.
The Crop Factor Formula
= 43.27 mm / √(Width² + Height²)
For instance, an APS-C sensor measuring 23.5mm × 15.6mm has a diagonal of 28.2mm, giving a crop factor of 43.27 / 28.2 ≈ 1.53×.
Equivalent Focal Length & Field of View
A 50mm lens mounted on an APS-C camera produces the exact same field of view as a 50mm × 1.53 = 76.5mm lens mounted on a full-frame body. The lens does not physically zoom in; rather, the smaller sensor records only the center portion of the lens image circle, effectively "cropping" away the outer edges.
Equivalent Aperture & Depth of Field
While an f/2.8 lens retains the same exposure brightness per square millimeter across all sensor sizes, the total light gathered and the depth of field are scaled by the crop factor:
Therefore, an f/1.4 lens on a Micro Four Thirds sensor (2.0× crop) yields the depth of field of an f/2.8 lens on a full-frame camera.