What does crop factor mean, and how does it affect a 50mm lens on APS-C vs full frame?

Question

I often see 50mm lenses recommended as a first prime because they give a “natural” view. But on most APS-C DSLRs, people say the image is cropped by about 1.5–1.6×, so a 50mm behaves more like an 80mm lens. I’ve also read that crop factor does not actually change the focal length or perspective.

Can someone explain the difference between:
- a 50mm lens on an APS-C DSLR, and
- an 80mm lens on a full-frame/35mm camera?

If the framing looks similar, what is actually changing: focal length, angle of view, perspective, or depth of field?

user115 · Accepted Answer

You can find detailed definition of Crop Factor in Wikipedia there is also a good explanation on dpreview site where it is referred to as "Focal Length Multiplier"

In short in your scenario if you have one full frame camera (crop factor 1) with 80mm lens and a second camera with 1.6 crop factor and 50mm when taking photos from the same position you will get the same frames (80 x 1 = 50x1.6 = 80)

That does not mean however that the photos will be identical. The depth of field for example if shooting with the same aperture will be different as it is still dependent on the (actual) focal length of the lens, that is the reason why people who are interested in achieving shallow depth of field tend to use full frame cameras.

Also the camera with crop factor 1.6 has a smaller sensor (see crop factor definition) - so assuming the both have same resolution say 10 Mega Pixels, and use the same technology the full frame camera will have bigger pixels, each would capture more light and that would usually translate into better high iso performance and better dynamic range.

What does crop factor mean, and how does it affect a 50mm lens on APS-C vs full frame?

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