How does a change in FL relate to a scalar change in FoV?

Started Jul 28, 2013 | Questions thread
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How does a change in FL relate to a scalar change in FoV?
Jul 28, 2013

Suppose I'm thinking about buying a longer lens and I want to visualize the effects of its narrower field of view by cropping-and-upsizing some already taken shots . Then,

  • If I have a picture W pixels wide shot with a L-millimeter-long lens, to which W' should I crop the picture to match the FoV of an L' focal length which is n times M?

(Sorry if I'm not trying to figure this formula out myself, but still haven't found the rusty trigonometry that hopefully lies like flotsam in some backwater corner of my mind. Just wondered someone would know it more or less by heart?) Thanks!

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