FRESNEL MINI-LENS (Mini C.C. size) Measuring only 3.5'' x 2.25''.   These are little gems!

Our Field guide: "ACW pocket survival/life guide" can come with a lens already.  These are great and compact back-up resources for reading small things, Fire starting with the sun, signaling , working on small things (tick removal, or tinkering in the field).  The larger "full sized" lens can be found here. 

The way they work is rather interesting, a Fresnel lens is a type of composite compact lens...

For a video/ field demo of starting a fire with the Fresnel mini lens, check this one out.