Crop factor is a number that compares the size of your camera sensor with a full frame sensor, which measures about 36 by 24 millimeters. A smaller sensor sees a narrower slice of the image the lens projects, so the same lens looks more zoomed in. Multiplying the focal length by the crop factor tells you the full frame equivalent field of view.
How Crop Factor Is Calculated
Crop factor is the diagonal of a full frame sensor divided by the diagonal of your sensor. Makers publish it, so you rarely need to work it out yourself. Common approximate values are listed below, and you should confirm yours on the maker’s page.
| Sensor type | Approximate crop factor |
|---|---|
| Full frame | 1.0 |
| APS-C (most brands) | about 1.5 |
| APS-C (Canon) | about 1.6 |
| Micro Four Thirds | 2.0 |
What It Does to Your Lens
A lens does not change its real focal length when you move it to a smaller sensor. What changes is the framing. A 50mm lens on a body with a crop factor of 1.5 frames the scene much like a 75mm lens on full frame.
- Wide lenses become less wide, which can make landscape and interior work harder.
- Telephoto lenses reach farther, which wildlife and sports shooters often like.
- Depth of field and light gathering are not multiplied in the same simple way, though many people also apply the crop factor to aperture when comparing background blur.
Working Out Equivalent Focal Length
- Find your camera’s crop factor.
- Take the lens focal length printed on the barrel.
- Multiply the two numbers.
- Read the result as the full frame lens that would show a similar angle of view.
For example, a 24mm lens on a 1.6 crop body gives roughly 38mm of equivalent view, and a 100mm lens on a 2.0 crop body behaves like about 200mm.
Practical Tips
- When buying a wide lens for a smaller sensor, look for a shorter real focal length or a lens made for that sensor size.
- Lenses designed for full frame work on smaller sensors, but not always the other way around without cropping.
- Use equivalent values only as a guide to framing, not as a rule for image quality.
FAQ
Does crop factor make my lens sharper?
No. It only changes how much of the image circle the sensor records. Sharpness comes from the lens design and your technique.
Do I need to think about crop factor with a phone?
Phone makers often quote equivalent focal lengths for you, which are based on the same idea.
The Short Version
Crop factor compares sensor sizes and tells you how much narrower your view is than full frame. Multiply the focal length by it to estimate the equivalent field of view, and use that number to choose lenses for your framing needs.
