Extension tubes are commonly used for Macro shots...they decrease the lens' minimum focusing distance and are usually sold in 5mm, 10mm, 25mm, and 50mm. The longer the extension tube, the closer the lens can focus. It goes both ways, however, the longer the tube, the shorter the maximum (infinity) focus becomes. Not a whole lot of light loss unless you start stacking them.
Doublers of course, double a lens' focal length. You normally have a light loss in f stops comparable to the multiplier on the doubler. i.e. for a 1.5X doubler, you would increase a 300 f/2.8 to a 450 f/4.2 or f/4.5 depending on your camera. So, you decrease your maximum aperature...
Hope this helps,
Charles Roberts
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