Friday 18 July 2008

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


Dominic Alan said...

Hello there.

I note you have a Canon EF 100 Macro lens and wonder how you rate this (I guess you don't use it at Weddings?). I have a 24-70L and note that it too lets me get very close to subjects and I can get nice close-ups at 70mm end. Is the 100macro noticably better for close-ups?

Thank you

David said...

Hi Dominic,

I did used to take the 100mm macro lens to weddings but have sacrificed it recently to keep the weight of my bag down.

The 100mm f/2.8 macro lens is much better for close-ups than either the 24-70mm f/2.8 L or 35mm f/1.4 L - it can achieve 1:1 magnification. The other lenses can be used for close-ups of wedding rings etc since they have reasonably short minimum focusing distances, but for true macro photography the 100mm lens is far superior.

I wouldn't rush out and buy one, however, if you only intend to use it for wedding-related photography. They are great fun though!

All the best,


Dominic said...

HI again David,

I don't really need to splash out on the 100mm macro just want to shoot some nice macro style shots.

Perhaps I'd be better off with the canon 2x Extender mounted on an existing lens like my 70-200 F/2.8 IS ? Have you any examples of how close I could magnify in that set-up?

Thanks you!

David said...

Hi Dominic,

Are you clear on the difference between extenders and extension tubes? The latter are much more useful for macro photography and a lot cheaper!



Lucy Evans said...

Hi David,

How does the math work on these tubes, for example all 3 combined = 68mm, so is it simply a case of add 68mm to my lens? So a 100m lens would become a 168mm?

Sorry for asking a silly question if so!

David said...

Hi Lucy,

By definition the lens will now have a focal length of 168mm. It won't behave like a normal lens with focal length of 168mm however. The focal range will be incredibly limited (in the order of 10-20cm) and everything will be magnified!

Adding extension tubes is not a useful strategy for gaining increased focal length.

All the best,