Excerpt: the standard G15 has a zoom ratio of 5x, from 28mm to 140 mm focal length (35mm film equivalent values). I don't have much use for the 28mm wide angle but would like a bit more telephoto. A 1.4x teleconverter sounded ideal - it should give a range of 39mm - 196mm. Sorry, it doesn't. You'd think that an expensive lens made by Canon specifically for this camera would be useable across the whole zoom range. It isn't.
Summary: The standard G15 has a 5:1 zoom range - 6.1mm to 30.5mm, or a 35mm film equivalent of 28mm-140mm. In this review below I give all focal lengths as 35mm film equivalent. A zoom ratio of 5:1 is about the lowest I can live with. For basketball pictures, for instance, that's about the range you need to zoom between someone on the near side of the court and on the far side.
Excerpt: I was less than happy with this lens. I ended up returning it and buying a Raynox DCR-1541PRO. The Raynox has 67mm filter threads in front, the TC-DC58D does not. Plus the Raynox is a 1.54x vs. Canon's 1.4x. The glass is not bad and it works just fine there are just better options.
Excerpt: I found this product did not add more than marginal advantage as a telephoto extender for the G12. The quality was fine, but it was only slightly more telephoto than the built in lens. Maybe it was more useful on the G10.