Extension tubes

For a more in-depth discussion of macro photography and magnification rates check out John Shaw's 'Closeups in Nature'.

Extension tubes increase the distance between the optics in the lens and the digital sensor. This allow the lens to focus at a closer distance to the subject, hence increasing its size. An approximate magnification rate is given by total extension/focal length - immediately this tells you that greater magnification rates are achieved with shorter focal length lenses. 

Since there is only air between the end of the lens optics and the sensor I bought the cheaper Kenko extension tubes which come as a set of three - 12, 20 and 36mm. They allow the lens to communicate with the camera body so that autofocus and metering are not affected. With my 100m f/2.8 (a true 1:1 macro lens) putting on all of the above extension gives a magnification rate of .7-fold. 


The extension tubes can be used in conjunction with the extender I discussed in the previous post. If you attach extension to the lens first and then subsequently add the extender you will multiply the magnification by a further 2x. I took the photograph of the fly with a 100mm f/2.8 macro lens, 68mm of extension and a 2x extender!

Another result of adding extension to a lens is that minimum and maximum focusing distances are reduced. The former can be beneficial. I often use my 85mm f/1.2 L lens indoors with an extension tube to reduce the closest focusing distance of 0.95m - I can then take advantage of its excellent low-light capabilities in confined spaces.

Check out my photography here: wedding photographer Kent