A way to describe magnification capability
This is a term mainly used for macro photography. It tells you in respect to a full frame sensor, how large the image you can capture can be. It is usually stated as 1:
number although I have seen it also represented as
ratiox where the
ratio is 1/
number of the other representation. So 1:4 and 0.25x represent the same reproduction ratio. This means that you can picture something that is 24mm x 4 == 10cm high and 36mm x 4 = 15cm wide that will fill the entire image.
Although the focal length seems to suggest a magnification ratio, that is misleading when you are trying to picture things that are not far away. The problem is a longer focal length usually involves also a longer minimum focus distance, so although you can magnify things, you are also further away from them. This makes perfect sense when what you want to photograph are sized normally.
Macro lenses work differently and allow you to focus closer. But there is also a compromise to be made just how small objects you want to image. The macro lenses are designed around the demand for the size of things that need to be photographed. Postage stamps, insects, coins, small flowers are all roughly around the size of a full frame sensor, so typical macro lenses use a reproduction ratio of 1:1 or 1:2.
This is only one part of the equation for a macro. The second part is the focal length which gives you the minimum focus distance. For example the Nikon 105, Sigma 150 and Nikon 200 all have a reproduction ratio of 1:1, regardless of their focal length they essentially are able to picture the same subject with the same size. Their minimum focus distance is different. See the pictures on this page.
These pages are for Amateur Photographers and not really for seasoned photographers and professionals. I have no affiliation or commercial interest with any brand/make. I write from my own experience. I ended up using mainly Nikon, so I am more familiar with this brand than others. See price for notes on pricing as well as photography related links.