I am going to put either a fresnel lens or a magnifying glass in my survival kit. I know that it is easier to get a flame with a large (11"x7") fresnel lens than a credit card sized one. But, does magnification power matter? Is 2x better than 4x or vise versa? Same question with regards to a glass.

The magnifying power does matter. It's actually stands for the focal distance parameter of a particular lens. So, the higher the magnification - the shorter the focus distance.

The shorter focus means two things for our consideration: 1. It makes the image of the sun smaller, so the solar power will be concentrated on a smaller spot, producing higher temperature compared to a longer focus lens. 2. The lens with shorter focus must be placed closer to the tinder. So if that's a plastic Fresnel lens it's easy to damage it with the heat.

Also the cheap short focus lenses (magnifying glass kind) are usually have a lot of aberrations (spherical, astigmatism, chromatism, etc), which may reduce the fire-starting effectiveness, simply diffusing the sunlight. So, I'd recommend to go for something not more than 5x power lenses (2 inch focus minimum).

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