Convert any focal length to its 35mm equivalent using the crop factor of your sensor.
The 35mm equivalent is the focal length that would give the same field of view on a full-frame (36×24 mm) sensor. A 50 mm lens on an APS-C camera with a 1.5× crop factor gives the same framing as a 75 mm lens on full frame — a moderate telephoto instead of a normal lens.
This matters because focal length alone does not tell you how wide or narrow the image is. A 50 mm lens is wide on medium format, normal on full frame, and a short telephoto on APS-C. The 35mm equivalent normalises the comparison so you can talk about framing without specifying the sensor.
The field of view is calculated from the equivalent focal length and the 36 mm diagonal of a full-frame sensor: FoV = 2 × arctan(diagonal / 2f). It is the diagonal angle — the horizontal angle is narrower.
Equivalent = focal length × crop factor. FoV = 2 × arctan(36 / (2 × equivalent)).
No — it is the focal length that would give the same field of view on a full-frame sensor. The actual focal length of the lens does not change.
Indirectly. A crop sensor needs a shorter focal length for the same framing, and shorter lenses have more depth of field.
Typically around 5-6× for a smartphone main camera. The sensor is much smaller than full frame.
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