Movement of "X" and "Y" as the image is moved in "Z" axis

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My question is based on a calibrated Camera. Once a camera is calibrated the images are properly aligned in "X" and "Y". The "Z" axis is unknown, but any movement in "Z" will mean that the position of objects will scale the same in both axis. Therefore the movements in the "X" and "Y" will also move as a scale-able amount as the "Z" axis changes. I need to find a method to determine that scaling as a function of the "Z" axis movement. This would equate to an angle at which the rays of light enter the camera. If the camera is calibrated the rays of light entering the camera are on similar amounts of angle. can I calibrate the amount of pixel spread occurs per a change in "Z" movement??? Thank You