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Abstract
A rule has been derived mathematically, which states that “If an image is a certain fraction toa large/small, correct size will be obtained by increasing/decreasing the lens-to-original distance by that fraction of the focal length divided by the scale of reproduction and decreasing/increasing the lens-to-image distance by that fraction of the focal length multiplied by the scale of reproduction”. Examples are given showing the accuracy obtained when the rule is applied to same-size, 1/4 size and 1/25 size images.