The ratio of the subject size on the
film plane (or sensor plane) to the actual subject size is known as the reproduction ratio. Likewise, a macro lens is classically a lens capable of reproduction ratios greater than 1:1,
although it often refers to any lens with a large reproduction ratio, despite
rarely exceeding 1:1.
Outside of technical photography and
film-based processes, where the size of the image on the negative or image sensor is the subject of discussion, the finished print or on-screen image more
commonly lends a photograph its macro status. For example, when producing a 6×4 inch (15×10 cm) print using 135 format film or sensor, a life-size result is possible with a lens having only a
1:4 reproduction ratio.
Reproduction ratios much greater than
1:1 are considered to be photomicrography,
often achieved with digital microscope (photomicrography should not be confused with microphotography, the art of making very small photographs, such as for microforms).
Due to advances in sensor technology,
today’s small-sensor digital cameras can rival the macro capabilities of a DSLR with a “true” macro lens, despite having a lower
reproduction ratio, making macro photography more widely accessible at a lower
cost. In the digital age, a "true" macro
photograph can be more practically defined as a photograph with a vertical
subject height of 24 mm or less.
-Wikipedia-
