How do I crop a photo to match the field of view of a longer focal length?
Asked 4/26/2024
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I have an image that is 2560 × 1920, taken at 2.8mm. Can I crop it so the result matches the field of view of a much longer focal length, such as 94mm? If so, how do I calculate the crop dimensions in pixels?
Originally by Daniel Jørgensen. Source · Licensed CC BY-SA 4.0
Daniel Jørgensen
2y ago
2 Answers
3
Yes, since the spacial relationship of objects in the scene is dictated only by the perspective (distance), you can crop a wider recorded FOV for the same resulting image composition as if a longer FL was used instead.
You can use a field of view calculator to determine the relative amount of the scene that would remain if a longer FL was used. Or, double the FL is half the FOV.
The only issue is that the declared FL is often a result of rounding and not exact.
Originally by Steven Kersting. Source · Licensed CC BY-SA 4.0
Steven Kersting
2y ago
0
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Yes. Cropping a photo taken with a wider lens can simulate the narrower field of view of a longer focal length, as long as the camera position stays the same. Perspective will not change—only the amount of the scene you keep.
Use the focal-length ratio as the crop factor:
crop factor = original focal length / target focal length
So here: 2.8 / 94 ≈ 0.0298
Multiply the original dimensions by that ratio:
- width: 2560 × 0.0298 ≈ 76 px
- height: 1920 × 0.0298 ≈ 57 px
So you would crop to about 76 × 57 pixels to match the field of view of 94mm.
That also shows the limitation: going from 2.8mm to 94mm is such a huge jump that the remaining image will have extremely low resolution.
In general, if you double focal length, you keep about half the field of view in each dimension, so the crop dimensions are halved. For more exact results, a field-of-view calculator can help, especially since stated focal lengths may be rounded.
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