Extension Tubes and Amount of Macro Magnification
Extension tubes can increase the magnification by about 20-50% depending on the focal length of the lens. The longer the lens, the less effective they are. For example, with a 50mm lens and a 25mm extension the magnification would be about .7X or about 30% more magnification, but with a 100mm lens it would only increase the magnification by about 20% or 1.2X for a 100mm macro lens.
Most of us get the 100mm macro lens and then "macro photography lust" takes over and there is the desire for smaller objects with greater detail. You do lose a little light with extension tubes, about 1 to 2 stops, but can get even more magnification with your macro lens. The distance is also reduced by about 20%. The tube has no glass in it; its sole purpose is to move the lens farther from the digital sensor.
I use Canon brand extension tubes. They are about 30% more then the Kenko brand but Canon extension tubes are made better.
Canon makes extension tubes in 12mm, and 25mm lengths.
Nikon makes extension tubes in 8mm, 14mm, and 27.5mm lengths. The Nikon PN-11 Extension Tube provides a reproduction ratio of approximately 1:1 with both the 105mm f/2.8 Micro-Nikkor and the 105mm f/4 UV-Nikkor lenses.
Kenko is a cheaper popular brand that can be used with Canon and Nikon. They can be purchased in a set of 12mm, 25mm, and 36mm lengths.
Extension tubes can be used together. For example, with the Kenko brand you could get a total 73mm extension tube by combining them all together. However, this would be an awkward set up because the whole set up would be too long, and produce too much camera shake.
Using a 12mm extension tube will increase magnification for a 24mm lens to about 0.60X. For a 70mm lens magnification is increased to about 0.50X, and for a 200mm lens the magnification is increased to 0.25X.
Using a 25mm extension tube will increase magnification for a 70mm lens magnification to about 0.80X, and for a 200mm lens the magnification is increased to 0.30X.