Extension tubes and teleconverters are two methods of gaining magnification with lenses. Both accessories are designed to fit between the camera body and the lens. Many beginning photographers purchase these as affordable tools to expand the range of their kit lenses, but they also have applications for more experienced and well-equipped photographers.
Extension tubes are an affordable way to help any lens focus closer. They are simply a hollow tube that contain no optical glass elements. Because all lenses are manufactured to project an image circle a certain distance to cover the image sensor, adding extension increases the size of the image circle so that the sensor is sampling a smaller portion of the projected image. This shift also creates two changes to your original lens. It decreases the maximum aperture, darkening the image, and it shifts the focus range so that focusing to infinity is no longer possible. The decrease in aperture can also affect the auto-focus capabilities of the lens.
Extension tubes can be used on almost all lenses, but provide the most value with shorter, prime lenses. For instance, extending a 50mm lens a further 50mm is reasonable and will allow it to magnify at about 1x. Extending a 200mm lens by 200mm is possible, but not reasonable, however, a small amount of extension can still be beneficial with telephoto lenses, allowing them to focus closer.
Extension tubes are usually sold individually or in sets of three different sized tubes (my Kenko set for Nikon come in 12, 20 and 36mm, the Vivitar set for Canon comes in 13, 21 and 31mm) and can come in manual or automatic varieties. Manual tubes have no electrical contacts and will not transmit information to the camera, so apertures will have to be stopped down manually. Note that most modern lenses do not have this ability, so opting for automatic tubes will be best for those who do not want use older lenses that still have an aperture ring. Extension tubes can be stacked and used together, and if it is intended to use them in this way with a heavy lens, it is important the tubes have sturdy metal bodies and mounting plates.
Teleconverters (sometimes also called tele-extenders or multipliers) are more expensive than extension tubes because they include a number of corrected glass lens elements. Adding more lenses to the optical pathway will slightly decrease image quality, but the convenience, size and light weight may make that slight loss justifiable. Teleconverters multiply the existing magnification capabilities of a lens by a calculated amount, usually 1.4x or 2x. As with extension tubes, there is a price to pay for this convenience. Besides the marginal loss in image quality, which increases by the amount of magnification, there is also a decrease in the native aperture of the lens. However, unlike extension tubes, multipliers will still allow you to focus to infinity. The benefits are that they are a light-weight and affordable option to expanding the focal length of lenses. Note that not all lenses can be used with teleconverters–be sure to check the lens compatibility charts for each brand before purchasing. Note also that some converters are designed to be matched with certain lenses in order to provide the best quality.
Before we look at some extension tube/multiplier combinations as a substitute for a macro lens, we should understand what to look for a macro lens.We are interested in overall lens quality, how much it can magnify and what the lens to subject distance is. Lens quality is a big subject and won’t be entered upon here, except that to say that for the most part, all the major brands produce very good quality macro lenses. As for magnification, it should reach 1x (1:1, subject size= size on the sensor). Lens-to-subject distance is a lesser known factor in lens choice. Having too small a distance between the end of the lens and the subject can make it difficult for lighting and, if photographing insects or spiders, can increase the chance of frightening off your subject. I’ll talk more about how this affects macro lens choice in another post, but let me clarify some of the confusion that can arise around lens-to-subject distance and minimum focusing distance.
Lens specifications usually include either Minimum Focus Distance or Closest Focus Distance. This distance is not measured from the end of the lens, but from the sensor plane of the camera. Most cameras have a ‘Φ’symbol on the top surface of the camera that indicates the film or sensor plane. With macro lenses, the general rule is that, given equal magnfication, the longer the lens, the longer the lens to subject distance.
To give an idea of how various combinations affect magnification and lens-to-subject distance, I did some tests with a combination of a Nikon 50mm f1.4 G, a Kenko 1.4X multiplier and 48mm (32mm+12mm) of Kenko extension tubes.The results would be similar with other 50mm lenses, but not exactly the same, as it varies according to the lens aperture and the native magnification of the lens. (A helpful calculator for different focal length lenses and extension tube combinations can be found at Cambridge in Colour)
Looking at the chart above, we can see some combinations that are more useful than others…
- With just the multiplier (#2), the 50mm becomes a 70mm, and it can still focus to infinity. This would now make an acceptable portrait lens, but it only magnifies to 1/4 life size, so is not equivalent to a macro lens.
- Extension tube between the camera and the “70mm” (#3). This gives almost 10cm of lens-to-subject distance at 1x magnification, but focusing to infinity is no longer possible.
- Just 48mm of extension (#4) gives 1.2x magnification, but the lens-to-subject distance is only about 5½ cm. Focusing to infinity is not possible.
- Extension tubes between the 1.4x and 50mm (#5) give the highest magnification, but with a meagre lens-to-subject distance of only 1.8″. Focusing to infinity is not possible.
One thing I have not yet talked about is how these combinations effect aperture. Aperture is dependent on focal length, so whenever you add extension tubes or multipliers, the aperture decreases (i.e the f-stop increases) and your viewfinder will appear darker. If you have purchased automatic tubes or multipliers this change is not an issue, the camera will compensate for exposure, but you may have to raise the ISO to maintain a suitable shutter speed.
Conclusion: Extension tubes and multipliers can be an affordable and lightweight way to begin playing with macro photography. If you later decide you want to invest in a macro lens, these accessories do not become obsolete but can still be used with your macro lens or other lenses to gain magnification, to allow closer focusing, or to increase lens-to-subject distance.
Coming (relatively) soon: Reversing Lenses for Macro.