How can I crop one camera's photo to match another camera's field of view?
Asked 5/8/2020
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2 answers
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I want to compare photos of the same scene taken from the same position with different cameras or phones, then crop the wider image so it contains the same framing as the narrower one. The cameras may differ in focal length, sensor size, aspect ratio, and pixel density.
What is the correct way to calculate the crop? Is field of view the key value, and if so, can I derive the crop from the horizontal or vertical angle of view alone? For example, if one camera reports a 70° field of view and another reports 60°, can I just crop the wider image by 6/7 in each dimension, or is trigonometry required?
I'm especially interested in phone cameras, where reported field-of-view values don't always seem to match real-world results.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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while the theoretical approach is interesting I think you will have better results with a more empirical approach.
Setup your cellphones / cameras on a tripod marking clearly its position so it is not going to move in between camera changes.
At a fixed distance setup a board with clearly printed markings. You don't need to do a lot of them. Just enough, especially towards the border.
Take pictures of this board with the different cameras.
Check the results in your computer and write down your findings. How much do you have to crop out in order to go from camera A photo to camera B?
Make a matrix of this crop factors for all of your cameras combinations (which I presume are not too many to do this).
Originally by user92003. Source · Licensed CC BY-SA 4.0
user92003
6y ago
0
Generated from our catalog & community — verify before relying on it.
You can’t crop by a simple ratio of the angle values themselves (so 60/70 is not correct). Field of view must be converted geometrically, using trigonometry, and you must compare the same dimension: horizontal-to-horizontal, vertical-to-vertical, or diagonal-to-diagonal.
In practice, though, the bigger issue is that phone-reported FOV values may not match the actual captured image well enough to rely on. So even with the right math, your result may still be off.
The most reliable method is empirical: mount each camera in the same position, photograph a test target with clear edge markings, then measure how much of the wider image must be cropped to match the narrower one. Record those crop factors for each camera pair.
So:
- don’t use raw angle ratios
- use trig if you have trustworthy FOV data
- but for phones, real-world calibration is likely the best solution
If aspect ratios differ, match one axis first and then crop the other as needed.
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