Your link was removed (not permitted on this forum)... but I have a pretty good idea what you were referencing.
I bet it's a "wide angle lens adapter" that screws into the front of another lens you already have.
While there are some relatively high quality ones, there also are a lot of cheap junk adapter lenses that will make for awful image quality. Those screw-in adapter lenses also are difficult to shade well and can be a bit clumsy to use. You'd have to get one that's the correct diameter for whatever lens you want to fit it to, also.
IMO, you would be much better off just getting an actual wide angle lens.
The good news is that since you have Canon 70D, there's a very good bargain-priced lens available to you: Canon EF-S 10-18mm IS STM. It sells for under $300 and has surprisingly good image quality, considering its price. It's also the most compact (67mm filters) and lightest weight of lenses in this category. And it's the only ultrawide from anyone with image stabilization. The EF-S 10-18mm is a little plasticky (and the lens hood for it, which I'd recommend getting, is sold separately), but most other lenses of this type cost at least $200 more and some are far more expensive than that. (The cheapest Nikon ultrawide is 3X as much, $900... and probably doesn't have as good image quality!)
Canon also offers an older model EF-S 10-22mm with a bit higher build quality, excellent image quality, and higher performance USM focus drive... but without IS and about costing about twice the price of the 10-18mm.
There are some other good ultrawides... but you really don't have to look much farther than the two Canon offers, for some of the best of all that sell for reasonable prices.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
