The difference between what the viewfinder shows and what the lens records. It happens in cameras where the viewfinder and the taking lens are in different positions, such as rangefinder cameras (for example Leica M models), twin-lens reflex cameras and simple compact cameras. The error is small for distant subjects but grows at close distances, so parts of the subject can be cut off. Many cameras show frame lines that shift to correct it. Single-lens reflex and mirrorless cameras do not have this problem, because you view the scene through the taking lens.
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