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What are Extension Tubes?
All lenses have a minimum focusing distance. When shooting Macro (or very close up photography) it requires that we get really, really, close to the subject. The problem is, in order to get close enough for a good shot, we are TOO close (outside the minimum focusing range) and therefore, cannot get a good tight focus.
There are many lenses designed specifically for Macro photography, like the Canon MP-65 ($800) which I tested (not practical unless you have something called a focusing rack and TONS of light). The Canon 100mm 2.8 is a good macro lens, but it is also pricey at about $450.
Extension tubes were the best solution I found for beginning photographers who dont want to spend a ton of money on a good macro lens, but still want to experiment. Extension tubes add space between the lens and the camera body and allow for shorter focusing distances. A set of three Kenko (Canon Compatible) are about $170, and will work on any EOS mount. A few rules to remember with Macro photography:
1. You will need a TON of light if you want a depth of field thicker than a tenth of a millimeter. Remember, the closer you are to your subject, the shallower the depth of field. The result is that you will have to shoot at say f22 if you want to get a deeper depth of field. If you are going for thicker depths of field.....
2. You will need a tripod because the in order to compensate for the small aperture, you will need a slow shutter speed.
3. You will need a lens release cable, infared transmitter or use the timer (the camera can shake if you push the shutter release with your hand).
There are some "macro" type lens filters. They are expensive, give about the same results as the tube extensions, and can only be used on specific size lenses.
If you are beginning Macro photography, try the extension tubes first. This will give you a good taste of what its all about before you invest in an expensive lens. Besides, they may do the trick you need anyway.