What does crop factor mean and what is my lens equivalent to?

Almost all photography advice is written in full-frame terms, and almost all cameras people buy have smaller sensors. Crop factor is the conversion, and understanding what it does and does not change removes a lot of confusion about lens choice.

Step by step

1. Picture what is actually happening.The lens projects a circular image. A full-frame sensor takes a large rectangle out of the middle of it. A smaller sensor takes a smaller rectangle from the same projection, so it captures a narrower field of view — like cropping the picture, which is where the name comes from.
2. Learn the three numbers that matter.APS-C is roughly 1.5, Canon's APS-C 1.6, and Micro Four Thirds 2.0. Compact cameras and phones have much larger factors, which is why a phone's main camera is physically a few millimetres and quoted as a 24mm equivalent.
3. Convert focal length for field of view.Multiply. On APS-C, an 18-55mm kit zoom behaves like a 27-83mm. A 50mm behaves like a 75mm, which is why it feels awkwardly tight indoors when the advice said it was a standard lens.
4. Apply the same factor to depth of field.To frame a subject identically on a smaller sensor you stand further back or use a shorter lens, and both deepen the depth of field. The practical effect is that f/1.8 on Micro Four Thirds separates a background about like f/3.6 on full frame.
5. Do not apply it to exposure.An f-number describes the ratio of the aperture opening to the focal length, and it sets the brightness of the projected image regardless of what is behind it. f/2.8 at 1/250 and ISO 400 is the same exposure on every sensor size. Only the noise at that ISO differs.
6. Use it when reading recommendations.Advice to use an 85mm for portraits, a 24mm for interiors or a 35mm for street means the equivalent view. Divide by your crop factor to find the lens you actually need — a 56mm class lens on APS-C for portraits, a 16mm for interiors.
7. Check which lenses fit and which cover.Lenses designed for smaller sensors do not project a large enough circle for a full-frame body, and produce dark corners or an automatic crop. Full-frame lenses work fine on smaller bodies, just heavier and tighter than their marked focal length.
8. Remember what it does not change.Perspective is set by where you stand, not by the sensor. A face photographed from a metre away looks the same shape on any camera. Crop factor changes what fits in the frame from that spot; it does not change the shape of what is in it.

Tips

• For a standard 50mm-equivalent view, buy a 35mm on APS-C, a 25mm on Micro Four Thirds, and a 50mm on full frame.
• Reach for wildlife and sport is the one place a crop is a genuine advantage — the same lens frames tighter with no loss of detail.
• Phone specifications quote equivalent focal lengths precisely because the real numbers would be meaningless to compare.

Common mistakes

• Applying crop factor to the f-number for exposure — It is a real conversion for depth of field and for the light gathered by the whole sensor, and it is not a conversion for exposure. Metering, shutter speeds and flash calculations use the marked f-number on any camera.
• Buying a 50mm for portraits on an APS-C body — It frames like a 75mm, which is workable but tight, and awkward in a small room. A 35mm gives the standard view and a 56mm class lens gives the classic portrait view.

Questions people ask