Why does a 70–300mm lens seem longer on APS-C than on full frame?
Asked 9/12/2012
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In a lens review, the presenter said a Tamron 70–300mm lens gives 300mm at the long end on full frame, but on an APS-C camera it is like 480mm. How can the same lens appear to have a different focal length depending on sensor size? Is the focal length actually changing, or is this related to crop factor and field of view?
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Photography Stack Exchange contributor
13y ago
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The focal length does not change. A 70–300mm lens is always 70–300mm, no matter what camera you mount it on.
What changes is the field of view. An APS-C sensor is smaller than a full-frame sensor, so it captures a smaller central portion of the image projected by the lens. That makes the scene look more "zoomed in" compared with full frame.
This is why people often talk about a full-frame equivalent focal length. If your APS-C camera has a 1.5x crop factor, 300mm gives a field of view similar to a 450mm lens on full frame. If it has a 1.6x crop factor, 300mm looks like about 480mm on full frame.
So:
- focal length = physical property of the lens
- crop factor = how much smaller the sensor is than full frame
- equivalent focal length = a comparison of field of view, not a change in the lens itself
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