Macro photography is a kind of close-up photography. The classical definition is photography in which the image on film is at least as large as the subject. Therefore, on 35mm film (for example), the camera has to have the ability to focus on an area at least as small as 24×36 mm, as this is the size of the image on the film. This is a magnification of 1:1.
For example, suppose you take a macro photograph of a coin on film. You set the lens for a magnification of 1:1 and move the camera to and fro until the coin is in focus, then take the picture. After you have the film developed, you can place the coin on the film, and you'll see that the coin is exactly the same size as the picture of the coin on the negative or slide.
In recent years, the term macro has come to mean being able to focus on a subject close enough so that when a standard 102×152 mm (4×6 inch) print is made, the image is life-size or larger.
Going back to the coin, you photograph it from farther away. This time, you set the lens to a magnification of 1:4, move to and fro until the coin is in focus, and take the picture. When you compare the coin to the image on film, you see that the real coin is four times as big across as the image. Next, you have a 4× enlargement print made. The enlargement is about 100×150 mm. And when you place the coin on the print, you see that the size of the coin matches the size of the photo of the coin. That is, the image is life-size in the print.
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