I just thought about this a little, and my last post wasn't entirely correct. Focal length does matter for firestarting a little bit. A shorter focal length, which would correspond to a higher magnification or a thicker lens (higher diopters) will shine a smaller image of the sun on the tinder. That will make the image hotter. However, total light gather capability is much more important. I had a hard time even making smoke with a credit card sized fresnel lens.