Understanding Crop Factor
Crop factor describes how much smaller a camera's sensor is compared to a full-frame 35mm sensor, and how that difference changes the field of view and depth of field you get from a given lens. It is a ratio of sensor diagonals, and once you know it you can translate any lens's specs into the "full-frame-equivalent" numbers that photographers commonly compare against.
The formula
A full-frame sensor measures 36mm x 24mm, giving a diagonal of sqrt(36² + 24²) ≈ 43.3mm by the Pythagorean theorem. For any other sensor, the same formula gives its diagonal from its width and height. Crop factor is then:
- Sensor diagonal = √(width² + height²)
- Crop factor = 43.3mm ÷ sensor diagonal
- 35mm-equivalent focal length = actual focal length × crop factor
- 35mm-equivalent aperture = actual f-number × crop factor
Why it matters
A smaller sensor with the same lens sees a narrower slice of the scene, which is mathematically identical to using a longer lens on a full-frame body. A 50mm lens on a sensor with a 1.5x crop factor frames like a 75mm lens would on full frame. The equivalent-aperture figure is a separate comparison used for depth of field and background blur: because a smaller sensor needs a wider physical aperture to produce the same blur as a larger one, multiplying the f-number by the crop factor estimates the full-frame aperture that would give a similar look. It does not change the actual exposure math, which still follows the printed f-number.
Typical sensor formats
Common formats and their approximate crop factors: full frame (36 x 24mm) is 1.0x by definition; APS-C sensors are roughly 1.5x (Sony, Nikon, Fujifilm) or 1.6x (Canon); Micro Four Thirds is about 2.0x; and 1-inch sensors found in many compact cameras are about 2.7x. Enter your own sensor's width and height for an exact figure rather than relying on a rounded label.
Frequently Asked Questions
What is crop factor?
How do I find my camera's crop factor?
How does crop factor change my effective focal length?
Does crop factor affect aperture and depth of field?
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