How do I crop an image to simulate a different focal length?

Asked 8/15/2012

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I have a photo with dimensions H × W pixels, and the EXIF says it was shot at focal length F1. If the camera position, subject distance, and everything else stay the same, how can I calculate how many pixels to crop (or add) to make it look like it was taken at focal length F2?

field-of-view crop framing focal-length

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

14y ago

2 Answers

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)

Originally by user1375. Source · Licensed CC BY-SA 4.0

user1375

14y ago

AI Answer

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If the camera position and subject distance stay the same, the field of view scales approximately linearly with focal length. To simulate focal length F2 from an image shot at F1, crop the image by the ratio F1/F2:

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

So if F2 is longer than F1, you crop tighter. Example: going from 50mm to 100mm means cropping to 50/100 = 1/2 of the original width and height.

If F2 is shorter than F1, you would need to add image outside the frame, which isn’t possible from the original photo alone.

This assumes the same camera, sensor, viewpoint, and a normal rectilinear lens behavior. Cropping changes framing/field of view, but it does not recreate other effects of a different lens, such as perspective changes from moving the camera.

How do I crop an image to simulate a different focal length?

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