Macro photography is extreme close-up photography, usually of very small subjects, in which the size of the subject in the photograph is greater than life size (though macro photography technically refers to the art of making very large photographs). By some definitions, a macro photograph is one in which the size of the subject on the negative or image sensor is life size or greater. However, in other uses it refers to a finished photograph of a subject at greater than life size.
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 of at least 1:1, although it often refers to any lens with a large reproduction ratio, despite rarely exceeding 1:1.
Apart from 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 (3.6×2.4 cm) 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]