How to use the crop factor and equivalent focal length calculator
- Choose the sensor size of your camera from the list. If your camera is not listed, choose Custom size and enter the sensor width and height in millimeters from the camera specifications.
- Enter the focal length printed on the lens, not an equivalent value that was already converted.
- Optionally enter the aperture to see the equivalent aperture for depth of field.
- Read the crop factor, the equivalent focal length and the table that shows your lens on the example sensors.
Formula
Full frame diagonal = square root of (36^2 + 24^2) = 43.27 mm
Crop factor = full frame diagonal / sensor diagonal
Equivalent focal length = focal length x crop factor
Equivalent aperture = f-number x crop factor
Full frame is defined as a 36 x 24 mm sensor. A crop factor above 1 means the sensor is smaller than full frame.
A 35 mm f/1.8 lens on a 23.5 x 15.6 mm sensor
The sensor diagonal is 28.21 mm, so the crop factor is 43.27 / 28.21 = 1.53. A 35 mm lens then frames a scene like a 53.7 mm lens on full frame.
Multiplying the f/1.8 aperture by 1.53 gives f/2.8. That is the aperture that would give similar depth of field and a similar amount of total light on a full frame camera. The same 35 mm lens on a Micro Four Thirds sensor (17.3 x 13 mm) has a crop factor of 2.00 and frames like a 70 mm lens.
What equivalent focal length means
A lens has one focal length. What changes with the sensor is how much of the image circle the sensor records. A smaller sensor records a smaller center part, so the picture looks more zoomed in. Equivalent focal length tells you which lens on full frame would give the same field of view.
It does not change perspective, which depends only on where you stand. And it does not change the exposure you need. At the same f-number, shutter speed and ISO, the picture is exposed the same on any sensor. The equivalent aperture only describes depth of field and noise from total light.
Sensor sizes are nominal
The sizes in the list are example dimensions. Cameras labeled with the same sensor type can differ by a fraction of a millimeter, which shifts the crop factor in the second decimal. For exact work, use Custom and type the dimensions published for your camera.
To judge how deep the sharp zone is for a given lens, the depth of field calculator takes the focal length and aperture, and the ND filter exposure calculator helps plan long exposures.
Assumptions and limits
- The reference is a 36 x 24 mm full frame sensor with a diagonal of 43.27 mm.
- Listed sensor sizes are nominal examples. Use Custom for the exact size from your camera specifications.
- The tool compares diagonals, so it assumes the sensor and the reference have the aspect ratio you entered.
- Equivalent aperture applies to depth of field and total light only, not to the exposure settings.
Frequently asked questions
How do you calculate crop factor?
Divide the diagonal of a full frame sensor, 43.27 mm, by the diagonal of your sensor. A sensor measuring 23.5 x 15.6 mm has a 28.21 mm diagonal and a crop factor of 1.53.
What is equivalent focal length?
It is the focal length times the crop factor. It tells you which lens on a full frame camera would show the same field of view as your lens on the smaller sensor.
Does crop factor change my exposure?
No. For a correct exposure you use the same f-number, shutter speed and ISO as you would on any camera. The equivalent aperture only helps compare depth of field and light gathered across the whole image.
Why does my camera maker list a slightly different number?
Sensor sizes differ slightly by model, and makers round the crop factor. Entering the exact sensor width and height under Custom gives the most accurate figure.