Calibration Bad Targets
On the theory that we learn from our mistakes, the following images were submitted but could not be used for calibration. The photographers have requested anonymity.
Illustration (a). The brick wall has too many imperfections. No brick walls please. This is especially apparent when viewed at 100%, which is what I do when calibrating an image. Modern buildings make better targets. To make matters worse, the camera to subject distance is too short. It's best to stand at least 25 feet away. This looks like about 3 feet!
Illustration (b). Well, we have good distance in this shot. But where are the horizontal features?
Illustration (c). Good distance and shows excellent horizontal features. Notice how the lines appear to converge at the top of the frame. This is perspective distortion and is caused by camera tilt. A small amount of tilt is okay but this is excessive.