7x and 10x power makes no sense for a magnifying lens. The only proper measures would be either focal length, or diopters. Focal length would specify how far from the tinder you would have to put the lens.

The measurement that's important for starting fires is area. The bigger the lens, the more light it collects, and thus the higher temperature it will get. It doesn't even have to be very optically pure, which makes those thin fresnel lens sheets ideal for firestarting. I got one about 8x10, it will strip paint on a hazy day in the fall in MN. A glass lens of that area would probably weigh a pound or two, the fresnel lens is just an ounce or two.