Understanding Extension Tubes

I have been playing with my macro photographs lately and thought I’d give you a rundown on the use of extension tubes.

Extension tubes are fairly simple and inexpensive devices and worthy accessories to your photography kit.

They are available for just about every brand of interchangeable lens cameras, and are typically packaged together in different lengths.

Extension tubes typically have no optical components. You simply connect them to your camera lens mount and then attach your lens directly to the tube. What makes them useful is the fact that they will increase the magnification of your lens without adding optical distortions caused by additional glass or plastic between your lens and camera. Many come with electrical connections that allow your lens to communicate with your camera for electrical control of autofocus and aperture. In a manner of speaking, they are a poor man’s macro lens. You can use them in lieu of purchasing an expensive macro lens for your camera, or as an addition to your kit. Extension tubes will function with just about any lens you own, though their effectiveness will vary from lens to lens.

Understanding the basic magnification an extension tube will provide is a fairly simple math equation. I’ll give a few examples. Lets assume you’ve purchased a set of extension tubes rated at 31mm, 21mm and 13mm. Not all tubes are the same length, but you’ll often find them in sets of three at similar lengths. Lets also assume you are using a 100mm prime lens with the extension tube. To calculate the amount of magnification your configuration will generate, simply divide the extension tube distance by the focal length of your lens.

So, we place a 31mm extension tube on a 100mm lens, the amount of magnification gain that combination gives you is 31/100 =.31x

Place a 21mm extension tube on the same 100mm lens and you get a magnification gain of 21/100 = .21x

Place a 13mm extension tube on the same 100mm lens and the magnification gain is 13/100 = .13x

Lets use a different lens. Lets take a 400mm lens and use a 31mm extension tube. 31/400 = .0775x

Lets use a shorter focal length lens. Say a 24mm focal length and attach a 21mm extension tube. The magnification gain is 21/24 = .875x

As you can see, extension tubes provide more magnification to the image when used with shorter focal length lenses.

Other considerations you will need to account for are…

Extension tubes will provide you with a shorter minimum focusing distance but may prevent you from focusing at infinity or at long distances. The effect varies between the lens and the length of extension tube you select. Therefore, one wouldn’t generally want to use an extension tube for anything that was at a great distance.

Your actual aperture/depth of field will probably change from what the camera is configured for. It will modify your depth of field.

You can stack extension tubes, but you’ll find it a lot more picky about your distance to subject when trying to focus on the subject. You’ll lose even more focal range as you stack them.

Zoom lenses are much more touchy finding the correct focus length as you zoom in or out. You may have to reposition your camera to find the sweet spot for focusing.

They are cheap. You can find a set of extension tubes for under $25. Here’s a link for a set of Canon EF mount extension tubes on Amazon.

If you really want to understand them more in depth, here is a link to Cambridge in Colour website where they explain things in more depth. Cambridge in Colour is a great website for learning about many things photography related. Save the bookmark.