macro - What is the 'Maximum Reproduction Ratio' of a lens?

A review of a Sigma 19mm f2.8 lens contains the following line

It has a minimum focusing distance of 20cm /7.9in and a maximum reproduction ratio of 1:7.4.

So, as in the title, what is 'maximum reproduction ratio'? What does it mean for practical purposes?

Answer

The reproduction ratio means the largest that you can make a subject on the film/sensor compared to its real-life size. In the case of your lens, it means that the image on the film plane will be 1/7.4, or 5/37 of the actual size of the object when it is as close as you can possibly focus the lens.

If your camera has a full-frame (24x36mm), an object would have to be at least 177.6mm by 266.4mm to completely fill the frame using that lens at its closest focusing distance. A Nikon/Sony/Pentax APS-C (DX) sensor would be filled with an object 118.4mm x 177.6mm; a Canon APS-C would be filled with an object 111mm x 166.5mm. In this case, on a μ4/3 camera, that reproduction ratio will fill the frame with an object 99.9mm by 133.2mm.

A lens of this class is rarely used for extreme close-up or macro work due to the very small working distance between the lens and the subject, even if it could be made to focus more closely. (You can, however, achieve very high magnifications with short-focal-length lenses like this one by mounting them reversed on the camera using a special adapter, with or without extension tubes or bellows.) The lens can be considered a "short normal" or a "moderate wide angle", depending on who you ask, and is intended for general photography. In larger formats (35mm, medium and large format), a "normal" lens can often be pressed into macro service, but the "normal" focal length of the 4/3-sensor world means that the lens-to-subject distance gets very small, and keeping the camera from shadowing the subject becomes difficult.

Macro photography is a term used to describe reproduction ratios at or around 1:1. That is, the image on the camera's film/sensor approximates the actual size of the object. Lenses labeled "macro" usually have a reproduction ration of at least 1:4; many photographers wouldn't consider a lens to be a "true macro" unless it goes to at least life size (1:1). Microphotography refers to reproduction ratios significantly greater than 1:1 (the old definition used to start at 10:1; I don't know what the standard is today).