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A lens conversion factor—usually called crop factor or focal length multiplier—describes how a camera sensor’s size affects the field of view captured by a lens compared with 35 mm full frame. To find the equivalent field of view, multiply the lens’s actual focal length by the camera’s crop factor. The lens itself does not change.
What does lens conversion factor mean?
In photography, “lens conversion factor” usually means the camera’s crop factor: a ratio comparing its sensor size with a reference format. The common reference is 35 mm full frame. Nikon also calls this ratio a “focal length multiplier.” The factor indicates how the lens-and-sensor combination frames a scene relative to that reference format; it does not describe a change to the lens’s optics. Nikon’s glossary explains the relationship using sensor diagonals and field of view.
How do you calculate equivalent focal length?
Use this formula to calculate the full-frame-equivalent focal length for field-of-view comparisons:
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For example, Nikon gives a crop factor of approximately 1.52× for its DX format. Its rounded example uses a 1.5× multiplier: a 50 mm lens on DX frames approximately like a 75 mm lens on full frame (50 × 1.5 = 75). The result is an equivalent field of view, not the lens’s new physical or optical focal length. Nikon UK’s DX and FX explanation provides the example.
Does crop factor change a lens’s actual focal length?
No. Focal length remains the lens’s marked optical value. A 50 mm lens remains a 50 mm lens when used on a smaller sensor. The smaller sensor records a narrower portion of the image the lens projects, so the framing looks tighter than it would on 35 mm full frame. Canon distinguishes the lens’s actual focal length from its conversion factor in its lens guide.
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Focal length versus equivalent focal length
Focal length, measured in millimeters, is an optical property of the lens; it is not the lens’s front-to-back physical length. It determines angle of view in combination with the image format. Equivalent focal length is a comparison figure: it tells you what focal length on the 35 mm full-frame reference would give a similar field of view. Nikon’s explanation of focal length discusses its relationship to angle of view.
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- Actual focal length: the lens’s optical specification, such as 50 mm.
- Crop factor: the sensor-size ratio relative to the chosen reference format.
- Equivalent focal length: actual focal length multiplied by crop factor, used to compare field of view with full frame.
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