What is the Focal Length Calculator?
The Focal Length Calculator is an essential tool for photographers and cinematographers. It calculates the effective field of view of a camera lens by determining the "crop factor" of your specific camera sensor, helping you buy the right lens for portraits, landscapes, or sports photography.
How to Calculate Effective Focal Length (Formulas)
Because camera lenses are universally labeled based on the standard 35mm "Full Frame" sensor size, putting a full-frame lens on a smaller sensor "crops" the image, making the lens appear more zoomed in.
- Crop Factor Formula: Diagonal dimension of 35mm sensor / Diagonal dimension of your specific sensor.
- Effective Focal Length Formula: Lens Focal Length (mm) * Camera Sensor Crop Factor.
Frequently Asked Questions
What is an APS-C crop factor?
An APS-C sensor (common in entry-level and mid-range DSLRs/Mirrorless cameras like the Sony a6000 or Canon Rebel) is physically smaller than a full-frame sensor. Nikon, Sony, and Fuji APS-C cameras have a 1.5x crop factor, while Canon APS-C cameras have a 1.6x crop factor.
What does a 50mm lens look like on a crop sensor?
If you put a 50mm "nifty fifty" lens on a Sony APS-C camera, you multiply 50mm by the 1.5x crop factor. The lens will produce an image that looks exactly like a 75mm telephoto lens would on a full-frame camera, making it excellent for portraits but poor for wide indoor shots.
Does crop factor change the aperture or depth of field?
Crop factor does not change the physical light-gathering ability of the lens (an f/1.8 lens is still an f/1.8 lens). However, because you have to stand further away to get the same framing, the perceived background blur (depth of field) behaves similarly to the crop factor multiplier.