by ʇolɐǝz ǝɥʇ qoq

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I have a photo having H x W (height x weight) pixels and I know, based on the EXIF, that it was shot at F1 focal length.

How many pixels I must crop (or add) in order to see how it would be the photo if it would be shot at F2 focal length?

All the other settings (camera, stage, POV etc.) will be the same.

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up vote 3 down vote accepted

It's a simple linear relationship, the number of pixels to crop depends on the ratio of the two focal lengths, i.e. if F1 is half the value of F2, then you need to chop the image in half. Or more formally:

new height = H * (F1/F2)
new width  = W * (F1/F2)
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