In tips # 8(Warbler photography) and in again in tips #40(Sammy's Secret), we briefly discussed the use of extension tubes in bird photography. This month we decided to dedicate a full tips article to the use of tubes. Extension tubes are quite essential to photographing small birds. Most lenses at the minimum focus distance give a rectangle about 12 inches on the horizontal. If you imagine a 4-inch long bird in this rectangle, you can see that there is a lot of wasted space. To increase the size of the bird in the frame you must either use extension tubes or resort to teleconverters. Teleconverters unfortunately, degrade the image quality and reduce your maximum f-stop (aperture) by one stop with a 1.4x and by two stops with a 2x converter.
So what exactly are extension tubes? They are very unremarkable devices in that they are simply hollow cylinders or spacers that are placed between the lens and the camera. They can be purchased in different lengths from 8mm to 50mm depending upon camera manufacturer. Third party manufacturers also make tubes at less cost than the camera manufacturers. A cautionary note is needed here, as some third-party extension tubes will cause vignetting (dark corners due to the light path being blocked) with some lenses. For most bird photography, a tube as small as 12 mm will be sufficient.
Extension tubes work by moving the film plane further away from the lens resulting in magnification of your subject. As mentioned in tips #40, this can be likened to moving a slide projector further from the screen causing the projected image to be larger. Their main purpose is to increase the magnification of the subject by decreasing the lenses minimum focusing distance, and hence allowing you to get closer to your subject. As mentioned in tips #40, they also magnify the subject without moving closer due to the "slide projector" effect discussed above.
There are a few disadvantages of using extension tubes. First, the addition of the tube does reduce the amount of light reaching the film, so like teleconverters; they do cost you some light. The amount of light lost however, is much less than when using TC's. The amount can be approximated by the following formula. Extension tube length / focal length of the lens used. Therefore 25 mm of extension divided by a 400mm focal length lens gives 1/16 of an f-stop of light loss.............relatively insignificant compared to a full stop lost with a 1.4x TC.
The second disadvantage concerns electronic coupling of the light meter and autofocus systems with the extension tubes. Depending upon your camera manufacturer, you may lose some light metering modes such as matrix/multi-segment metering. The autofocus capability may also be lost.
Another disadvantage of the tubes concerns the magnification itself and the mechanical connection of the tubes. Since the magnification has been increased, any vibrations will also be magnified necessitating the need for good technique and higher shutter speeds. Aggravating this vibration problem is the mechanical coupling of another device. There is always some "play" or looseness when a camera and lens are mounted together. The addition of an extension tube adds two more connections to create instability/movement to the system. The end result is increased vibration and blurry photographs.
The last disadvantage of using extension tubes is that you lose the ability to focus at infinity. The lens is limited to a much smaller zone that it can be focused upon. For bird photography this is not really an issue since a bird 50 feet away will be too small in the frame to photograph anyway. How far away the lens will focus with a given extension tube is related to the focal length of the lens. The shorter the focal length of the lens, the greater the loss of the lens's far focusing ability.
For the ultimate in magnification, consider using the extension tubes stacked in combination with your teleconverter. By placing the extension tube between the lens and teleconverter, you will be able to focus even closer than when placing the tube between the TC and the camera. There will however, be a significant loss of light with this combination, so fast film, fast lenses, bright days and slow birds may be required (see tips # 4 on pushing your film).
In summary, extension tubes provide a means of increasing the magnification without the image degradation and light loss associated with teleconverters. On narrow trails the ability to focus closer is appreciated, as you cannot always back up and remain on the trail or boardwalk.
