Photophys.com: The Science of Photography

Forget equivalent, forget crop factor, none of that matters. Just measure the size of the image of an object on your camera’s sensor or film. Use height, width, or any other dimension you like. Then measure the size of the object you photographed. Divide the size of the image by the size of the object and voilà, you have the magnification. If you really want to measure the magnification factor of your lens, the easiest route is to photograph a ruler marked in millimeters (mm). If you know the dimensions of your sensor in mm, you can easily determine the size of the image.

This is all old hat. Why am I mentioning it here? Simply because I keep seeing the term magnification misused. This occurs in lens descriptions and ads. The most common abuse of the term seems to appear in descriptions of four thirds lenses where we find nonsense such as: “Maximum image magnification = 1.0x (2.0x : 35mm equivalent).”

The time when the definition is most important is when one is buying a macro lens. True macro lenses permit magnifications to 1x, also known as 1.0, 1:1, or lifesize. You really need to know that the lens gives 1:1 and not 1:2 or 1/2x without any malarkey about equivalent magnification. The term “35mm equivalent” when referring to magnification is misleading and in some cases deceptive. Please help stamp out magnification abuse.