# Image processing and Camera Calibration

I have several related questions about camera resolutions.

1. If I calibrate my camera to a particular resolution say 640x360, can I use it for another resolution like 1024x768?

2. Also, I want to know as to how many centimeters does 1 pixel contain in my image. It varies from system to system. How do I find that?. Also, it is not compulsorily square in shape? So I have to find the length and width of it. How do I do that?

3. I am using a logitech c170 which is a low speed cam. Is it okay to get an error around 8mm when I am trying to measure the distances in the image and compare them with real-time distances?

1. Yes, the center pixel and focal length in pixels will change, as described in the link above. However, if you learn distortion parameters (radial and tangential) then they shouldn't change as resolution changes because they operate on the projective image plane (before multiplying by camera matrix) instead of pixel coordinates (after multiplying by camera matrix).

2. Size of pixel (if i understand you correctly) will depend on the distance of the object at that pixel, and the camera field of view. In some cameras, pixels are not exactly square, but that will be determined by camera calibration.

For instance, if you're viewing a plane at 1m away, and your camera has a horizontal field of view of 57.5 degrees, you can use the tangent function to find the total width of the image at that distance, and then divide by resolution to find pixel width (say, 640) :

$$\frac{distance \times 2 \times tan(\frac{\theta}{2}) } {horizontal resolution} = \frac{1m\times 2\times tan(\frac{57.5}{2}) } {640px} = 0.0016m/px = 1.6mm/px$$

3.This could be caused by not knowing the exact distance from the camera, which will have a huge effect on any measurements you're trying to get out of pixel coordinates.