Focal length calculator
Tell this tool one thing you know about your lens - focal length, the field of view you want, or the full-frame equivalent - and it solves the relationship for your sensor.
Lens & sensor
Pick a preset or choose “Custom sensor size…” and type the width and height below.
Result
Solved from what you know
How it works
A lens projects a cone of light onto your sensor, and the sensor’s size decides how much of that cone you actually capture. The wider the sensor dimension and the shorter the focal length, the bigger the angle of view. The calculator only needs one known quantity to reconstruct the rest of the relationship.
Here f is the actual focal length in mm, d is the sensor dimension you measure across (width, height or diagonal), and fov is the angle in degrees. The full-frame equivalent is then f × crop factor.
Example
You want a horizontal field of view of 39.6° on a full-frame body, whose sensor width is 36 mm. The half-angle is 19.8°, so f = 36 ÷ (2 · tan 19.8°) = 50 mm. On full frame the equivalent is the same 50 mm; mount that same lens on an APS-C Canon body and the framing becomes ~80.7 mm full-frame equivalent.
Useful combinations
- Turn a desired framing (degrees) into a shopping decision in millimetres.
- Understand what an equivalent focal length really means when moving between sensor sizes.
- Cross-check the field of view your current lens already gives on your body.
Related optics maths
Curious how the crop factor is derived? See the crop factor calculator. To convert a known lens to another sensor, use the equivalent focal length calculator, or jump straight to angles with the field of view calculator. Focal length also drives how shallow your focus can be - the depth of field calculator shows the connection.
Frequently asked questions
How do I find the focal length from a field of view?
Measure the sensor dimension you are framing across (usually the width, or the diagonal for the quoted spec) and enter your desired angle. The calculator uses f = d / (2·tan(fov/2)), where d is that dimension in mm and fov is in degrees.
What is the difference between actual and full-frame equivalent focal length?
Actual focal length is a physical property of the lens. The full-frame equivalent is that number multiplied by your sensor's crop factor, and it describes what focal length would give the same angle of view on a 36 × 24 mm full-frame sensor.
Why is diagonal field of view the number quoted on lenses?
Manufacturers quote the diagonal angle because it is the widest framing of the lens and stays consistent regardless of whether you shoot landscape or portrait orientation. The horizontal and vertical angles are always narrower.