Effective focal length
Focal length is based on using a lens with a full-frame camera. Found in higher-end cameras, FX sensors measure 36x24mm, which is roughly the size of an old 35mm film negative. If you have a DX camera, such as a Z 50, D3xxx, D5xxx, D7xxx or D500, its DX sensor is smaller than an FX sensor, measuring 24x16mm, around the size of an APS-C film camera negative. The DX camera sensor is 1.5 times smaller than an FX sensor, and this alters the field of view and focal length when using any given lens on one of these cameras. This is known as the ‘crop factor’ and is used to work out the ‘effective focal length’, which is the focal length that you are effectively shooting at when using a DX camera. If you mount the same lens on a DX and FX camera, the FX camera will have a wider angle of view and ‘see’ more of the image projected by the lens. Conversely, on a DX camera the lens will have a narrower angle of view as the focal length is magnified by 1.5 times. With telephoto lenses this can be a benefit as it brings you closer to the subject. For example, a 70-300mm lens would effectively become a 105-450mm lens on a DX camera. But the crop factor can be a challenge at the wide-angle end, as a 14-30mm lens would give a 21-45mm effective focal length. To counter this, Dx-specific lenses have been developed, such as the Nikon 10-20mm f/4.5-5.6, which becomes effectively a 15-30mm lens due to the crop factor.