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I know of a filmer who uses an HC21 and he has a 2X "doubler" lens that he screws onto the camera's lens to get 40X optical. I've got doubler lens for a couple of my cams also. They can really be handy. HOWEVER, they also tend to really accentuate even the tiniest shaking, so you've got to have a very steady hand - or better yet - use them only with a tripod.
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I bought a cheap Sima 2x telephoto lens and it works fine with my Sony, yes the telephoto lens will magnify everything including shakes but as long as its not handheld or not on something like a ledge that vibrates from the bass its usually fine. Telephoto lens do not fully double your zoom, I used to think that I'd get 20x with the lens but it only magnifies it so in my case my cams zoom is 10x, with the 2x lens attached I compared it to my cam with 20x zoom with both cameras zoomed in on the same subject and the telephoto lens brings a 10x zoom to about 15x. So the HC21 or any cam with 20x zoom plus a telephoto lens probably brings you to whats equal to a 25-27x zoom.