Crop Factor and Equivalence, Explained

Crop factor lets you compare lenses across different sensor sizes. Once you know the one number for your camera, you can translate anything.

Crop factor in one sentence

Crop factor is the diagonal of a full-frame sensor (about 43.3 mm) divided by the diagonal of your sensor. A 1.5× sensor sees the narrower slice of the image a lens projects, so a 35mm lens frames like 52mm on full frame.

Converting focal length

Multiply by the crop factor.

• 35mm on APS-C (1.5×) → about 52mm full-frame equivalent
• 25mm on Micro Four Thirds (2×) → 50mm equivalent
• To match an 85mm full-frame framing on a 1.5× APS-C body, use about 56mm

The lens does not change. Only the framing does.

Converting aperture

For depth of field and total light, multiply the f-number by the crop factor too. A 35mm ƒ/1.4 on APS-C gives the depth of field of about a 52mm ƒ/2.1 on full frame. Your exposure settings do not change: ƒ/1.4 on a 1.5× sensor exposes like ƒ/1.4 on any other sensor.

Converting ISO

To match noise and exposure across formats, multiply ISO by the square of the ratio. ISO 100 on a 1.5× sensor lines up with about ISO 225 on full frame for equal light per area. This is why larger sensors tend to look cleaner at the same framing and depth of field.

Why equivalence is only part of the story

Equivalence answers a specific question: what do I need on another format to get the same picture? It does not say which camera is better. Smaller formats give you smaller lenses, lower prices and more depth of field when you want it. Bigger formats give you more light and background separation. Both are real advantages.

Try it yourself

Use our equivalence calculator to compare any two formats in seconds.

Frequently asked questions