Hi,

I bought 2 credit card size Fresnel Lens (a clear one and a green tinted one) last weekend and they have 2 different magnifications built into them. About 2/3rds of the unit is the regular rectangular 2x lens with a small (about the size of a dime) circular 4x lens in the corner.

I just now tried them and the larger 2x lens is much quicker at producing heat than the smaller 4x lens (the size difference is about 10:1). The clear lens also seems slightly better at developing an ember than the green tinted one.

Thats the results of my totally non-scientific test on the back porch this morning (my neighbours must think I am crazy).

Mike