How Extension Tubes Work

There seems to be a lot of misconceptions out there regarding extension tubes and how they work, and if you're just getting started with them, it can be a little frustrating when you're not getting them to behave how you might expect.  I've talked a few people through it to clear up questions they had, so hopefully by writing up a post about it, it will help someone else one day.

Extension Tubes Are Just Spacers

An extension tube

Some years back, I ran across several product reviews for a set of extension tubes that were unnecessarily negative. The main reason was because the users expected to be able to "zoom in" farther with these tubes to allow the subject to better fill the frame. What they were probably thinking of was a teleconverter, which introduces additional lens elements to effectively increase the focal length of the lens it's attached to.  Extension tubes are different.  They have no optical elements at all; their only purpose is to add additional distance between the lens and the film plane. If you really wanted to, you could actually make your own extension tube with a cardboard toilet paper tube! That's how simple they are.

A bigger extension tube

Every mount type (Canon EF mount, Nikon F mount, Leica M mount, etc) has a specific distance in the camera between the film plane and the flange on the camera body that the lens attaches to (called the flange focal distance).  For example, the flange focal distance for all Canon EF-mount cameras is 44mm. The lenses are then designed specifically so that the full range of focus, from the minimum focusing distance to infinity, can be achieved at a point 44mm away (where the sensor or film is). Olympus OM series SLR's have a flange focal distance of 46mm, so all of the OM lenses are designed with that value in mind.

Extension tubes are just hollow spacers that increase that distance.

How Extension Tubes Affect Your Image

Since the lens is expecting a specific distance to the film plane, changing this distance impacts its performance in a big way.   

Focusing

Off-center focus by selecting different
AF points in-camera. (See photo in Flickr)

First of all, the lens's focus will not work as it did before.  The focusing range of the lens, from its minimum focusing distance to infinity, requires that exact flange focal distance.  By increasing that distance, the focusing range shifts dramatically closer.  If you had a lens that could focus from 18 inches to infinity, for example, adding an extension tube might change that range so that it's now able to focus from 4 inches to 6 inches (totally made up numbers by the way, just to illustrate the difference).

If your camera has autofocus and the lens is sufficiently fast, you may be able to retain some autofocus functionality, but the larger the tube, the less likely that becomes.  Any method you may use of focus/recompose is going to be much harder to do correctly because of the very narrow depth of field.  Better to compose first, then focus manually.  If you still have use of AF, select the AF point in-camera so that you don't need to recompose.

Image Circle

The added flange focal distance causes the projected image circle to increase in size.  I like to use the example of a projector that's projecting an image on a wall.  Let's say you move that projector really close to the wall so that the projection is really tiny, and then trace a border around that image.  That rectangle will be your camera's sensor. The projection fills your sensor perfectly.  Now move the projector back a few feet. The projection is now larger, flowing over the boundaries of the sensor.  Everything that's still in the sensor is now larger.

This was shot at f/16.
(See image on Flickr)

Exposure Time

Extension tubes will increase the exposure time required.  Think of the projector again.  The farther you move the projector from the wall, the darker the projection appears.  It's the same for the extension tube.  The larger the flange focal distance, the longer the exposure required. Depending on the conditions, this may mean that you need to use a tripod where you didn't need one before.

You'll also need to think about the depth of your subject (see the next section).  It's common, as in the case of the yellow flower, to stop way down just to get a depth of field of an inch or so.

Depth of Field

The depth of field in your image will be reduced dramatically because of the reduction in focus distance, so stopping down will be important. In the image of the little frog here, you can see how thin the depth of field is by looking at the ground below him (click on the image to view it larger). That's a 22mm focal length at f/4, which would normally have a much wider depth of field.

Image Quality

A very tiny frog. Image details on Flickr

The quality of the lens used will be even more important when extension tubes are used.  Resolution limitations, chromatic aberration, fringing, etc. are all magnified along with the image. When the extension tubes are used judiciously and with a reasonably capable lens, you won't have to worry to much about this.  However, it is worth mentioning.

Which Lenses To Use

Interestingly, the best lenses to use for maximum magnification are not the really long telephoto lenses like you might expect.  The longer the lens focal length, the less of an impact an extension tube makes on the resulting image.  On the other hand, really wide lenses are affected so much that they might be completely useless.  For example, if I put an extension tube on my 17mm lens, I can't focus on anything at all because the maximum focus distance is actually inside the lens!

A standard 50mm lens is a popular choice because you can get great magnification and still have the subject outside of your camera.  In fact, you could likely achieve a 1:1 magnification ratio on a 50mm lens while still being about 2 or 3 inches away from your subject.  That's obviously not nearly as good as the subject distance for a true macro lens, but you're also not paying for a true macro lens.

Sunflower; 50mm lens and extension
tube. Image details on Flickr.