Doublers or teleconverters increase the focal length of your lens (simultaneously decreasing your aperture and image quality at a factor of how good or bad the glass is), in other words, it "zooms" you in. Extension tubes allow you to focus on an object that is closer to your lens, allowing you to get closer. The primary difference is one of visual perspective (compressed space vs exaggerated space). Another is optical quality, where a teleconverter will definitely diminish your sharpness, perhaps to a minimal degree depending on how good the glass is, whereas an extension tube will probably decrease the quality only slightly if at all, depending on just how optically well your lens resolves objects at a very short focusing distance (if it's a macro lens you probably won't be hurting your image at all). In either case, critical focusing becomes essential, and a fast shutter doesn't hurt a bit, either.
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