In close-up and macro photography, the reproduction ratio (or magnification ratio, magnification factor, magnification) specifies the ratio of the size of an image projected on the film or sensor of a camera, and the size of the subject itself:
where R is the reproduction ratio, S the size of the subject and s the size of its image. As an example, if the length of an object is 100 mm and its image on the film or camera sensor is 10 mm, the reproduction ratio is 10/100=0.1. Thus, the reproduction ratio can be written as x0.1 or 0.1x, where x stands for "times" (i.e., the subject is magnified 0.1 times). In practice, however, it is customary to specify the reproduction ratio as a fraction (in this case, 1/10) or ratio (in this case, 1:10).
Typical examples of reproduction ratios are 1:3 (one-third the original size) and 1:100 (one-hundredth the original size). To obtain the original size of the subject, you simply calculate
When a photograph is reproduced in press (e.g., a book or magazine), the reproduction ratio R1 specified in a figure caption no longer applies to s as measured on the film or sensor, and instead applied to the size s1 of the photograph in print, i.e.:
When the subject and its image are the same size, the reproduction ratio is 1:1. This ratio is where macro photography starts. Examples of higher reproduction ratios are 2:1 (the image is twice the size of the subject, i.e., magnification is 2x) and 5:1 (the image is five times the size of the subject, i.e., the magnification is 5x). When the reproduction ratio is higher than 1:1, it is frequently expressed as an integer or decimal number rather than a fraction. For example, you will almost never see a reproduction ratio expressed as 12:5 or 12/5. Instead, this is expressed as 2.4x, which is much easier to understand.