First, it's not going to give 10x magnification. I can't find what Opteka means by 10x. A front-loading optic like this is measured in dioptres, so maybe it's+10dpt. No matter what host lens is used, your working distance will be about 10cm / 4in. Magnification depends on focal length. For a +10dpt optic, M= FL/100, so on a 55-100mm lens, you range from 0.5x to 3x.
I haven't used the Opteka. I have a few no-name +10dpt optics that are fair enough and that cost US$10 each, shipped, readily available on eBay. The Opteka might be better; I have no idea how it compares with the weaker and more expensive Raynox closeup optics. If you want to blow US$20 on it, go ahead. It probably won't be terrible.
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