How this instrument works
35mm-equivalent focal length translates a lens's actual focal length into the focal length that would produce the same field of view on a full-frame 35mm sensor. Because smaller sensors capture a narrower slice of a lens's projected image circle, the same physical lens appears to have longer reach -- a 'tighter' field of view -- on a smaller sensor, and equivalent focal length is how photographers describe that effect using one comparable number.
The calculation is a direct multiplication: actual focal length times the sensor's crop factor. A 50mm lens on a camera with a 1.5x crop factor, the common APS-C figure, behaves, in terms of framing, like a 75mm lens would on a full-frame body -- even though the lens itself is still physically a 50mm lens with unchanged optical properties like maximum aperture and depth of field.
This is purely a field-of-view comparison, not a change in the lens's actual optics. Equivalent focal length lets photographers moving between formats, or comparing gear across formats in reviews, talk about 'reach' using one shared reference point, since 'a 50mm lens' means a different field of view depending on what sensor sits behind it.
- Enter Actual focal length (mm) -- the lens's true, physical focal length as printed on its barrel.
- Enter Crop factor -- your camera's crop factor relative to full-frame 35mm; use this site's crop factor calculator if you only know your sensor's diagonal.
- Read 35mm-equivalent focal length (mm) beneath the inputs -- the focal length that would frame the same scene on a full-frame body.
- A crop factor of exactly 1.0 (full-frame) returns the actual focal length unchanged, since there's no format difference to convert.
Worked example -- 50mm lens on a 1.5x crop body
Enter a 50mm actual focal length and a 1.5 crop factor, typical of a Nikon DX-format APS-C camera. 35mm-equivalent focal length comes out to 50 x 1.5 = 75mm -- meaning the lens frames a scene the way a 75mm lens would on a full-frame body, noticeably tighter than the 50mm 'normal' field of view it would give on full-frame itself.
Questions
What crop factor should I use for my camera?
It depends on your sensor format: full-frame (36 x 24mm) sensors use 1.0, Canon APS-C sensors typically use 1.6, Nikon and Sony APS-C sensors typically use 1.5, and Micro Four Thirds sensors use 2.0. If you know your sensor's diagonal but not its crop factor directly, this site's crop factor calculator converts one into the other.
Does a smaller sensor actually make my lens zoom in more?
No -- the lens's actual focal length, maximum aperture, and optical magnification never change, regardless of what body it's mounted on. What changes is how much of the lens's projected image circle the smaller sensor captures: it crops in on the centre, which looks similar to a longer lens's field of view but isn't the result of any actual optical magnification.
Does equivalent focal length affect depth of field too?
Not directly through this formula -- equivalent focal length only describes field of view. Depth of field for a given framing does differ between formats, but that's a separate effect driven by the different actual focal length and, often, a different aperture needed to match framing between formats -- not something this calculator's multiplication captures on its own.
What's the equivalent focal length of a lens on a full-frame camera?
The same as its actual focal length -- a full-frame sensor has a crop factor of exactly 1.0 by definition, since full-frame 35mm is the reference format every other crop factor is measured against. Multiplying any focal length by 1.0 leaves it unchanged, so a 50mm lens on full-frame is simply a 50mm-equivalent lens.
Why do camera reviews quote equivalent focal length over actual focal length?
Because it gives readers a single, familiar reference point for comparing field of view across different sensor formats, even if they've only ever shot full-frame. A '24-70mm equivalent' zoom means the same thing to a reader regardless of whether the actual lens is 16-47mm on Micro Four Thirds or 24-70mm on full-frame -- the equivalent number describes the experience of using it, not the lens's physical specification.