I have to calibrate a camera with a variable focal length and that I'm going to use for long focal distances (>3m).

I have a big chessboard target, but if I position it at more than 3m from the camera it doesn't cover a big part of the image, let's say a quarter of it. If I position the target closer to the camera, the situation improves but the chessboard is out of focus so the corners as not as well defined.

I can't use a bigger target, so my question is if it is possible or a good idea to focus the camera at a shorter focal distance and take the images that way, and then apply a proportional factor to the values in the intrinsics matrix since the diagonal values should be the focal length? I will have an idea of the proportion between the focal distances from the camera controls that I will use to change the focal length.

Or would it be just better to image the target as small as it is and try to cover all parts of the image with it?


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