Since I enjoy macro photography, I wanted to create an easily referenced list of which of my lens and accessory combinations will provide what magnification factor and at what focusing distance. Not all combinations have the same image quality; I'll try to post example photos when possible.

In the table below, the magnification factor indicates the ratio of an object's actual size to the size at which it is rendered on the imagine sensor (which is 22.2mm or 7/8" wide on a Canon XTi/400D). Numbers marked in green are greater than 1:1 (1.0x), or larger than life size. At 1:1, a 10mm object will be rendered at 10mm on the image sensor, filling nearly half the frame. This is what you need for portraits of bugs or similar objects. Numbers marked in yellow are greater than 1:2 (0.5x), or half life size. At 1:2, a 10mm object will be rendered at 5mm on the image sensor, filling about a quarter of the frame. This is still a respectable magnification for flowers or other small objects. The listed magnification is always achieved at the minimum focusing distance, of course. Working distances here are always listed from the front end of the lens.

One useful method for improving magnification is to mount a wide angle lens in reverse, either directly to the body or to the end of another lens. Special adapters are available to make these attachments. To reverse mount a lens directly to the body, the adapter has an EOS mount on one side and filter threads on the other side. You simply screw the adapter onto the lens' filter threads, and then mount the assembly in reverse on your camera body. If you want to stop down the lens below the maximum aperture (and you will), you'll need to use an older lens with a manual aperture control ring. Wider angle lenses give larger magnifications. My 50mm lens provides about 1:1.3 at about 4". The range of working distance is extremely small--only about 1/2" from the min to max distance.

Mounting a second lens to the end of your main lens works similarly. The adapter simply has filter threads on both sides, and both lenses screw to it. The second lens is always used wide open, so manual aperture rings are not necessary here. However, the wider the aperture, the more light will be available to your primary lens. The magnification achieved is the focal length of the first (forward) lens divided by the focal length of the second (reversed) lens. A 50mm lens mounted to the end of a 100mm lens will give 2x magnification. A 50mm mounted on a 300mm lens will give 6x magnification, which will completely fill the frame with the head of a butterfly. Depth of field is razor thin at these magnifications (a fraction of a millimeter at f/32), so you'll have to stop down the primary lens quite a ways to show any detail. Using a flash is nearly mandatory.

Extension tubes are just spacers placed between your camera body and the lens. Longer tubes allow a lens to focus more closely than it otherwise could. Both the minimum and maximum distance are affected, so you'll lose infinity focus when using tubes. They also reduce the amount of light that makes it to the sensor. Moving the lens closer to the subject naturally makes the object appear bigger. This affect is more pronounced on wider-angle lenses, as evidenced by the change in magnification values for the 50mm and 135mm lenses in the table below.

Diopters are essentially just magnifying glasses attached to the filter threads on the end of your lens. I'm a little fuzzy on how they do their thing, but they also reduce the focusing distance. Diopters are more effective on longer focal length lenses. Again, compare the results for the 50mm and 135mm lenses in the table below. Diopters can be stacked on top of each other to combine their strength, so a +2 stacked on a +3 will yield the same magnification as a +5 (and relatively low image quality).

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