posted on Dec, 3 2017 @ 10:53 PM
This is easy to compute. A large plane (A380 or 747) is about 230ft long. We need to complete the angle of view. Using the small angle approximation,
the angle of view is 230/40000. That is in radians. Multiply by 180/pi to get it in degrees. The result is approximately 0.33 degree.
For a 400mm lens on a full frame sensor, the vertical angle of view is 3.44 degrees. The percentage of the vertical field that the image will occupy
I suppose you could crop out something. A Canon 5d IV is about 4400 pixels in the vertical, so you would have 400 pixels in the image.
Note you can reverse my math. If you know the dimensions of the target and how many pixels it occupies on the screen, you can compute the distance. At
some point you may not want to use the small angle approximation.