I made a collimator today following directions from a pamphlet by Edmunds Scientific called Fun With Optics. I can use it to check the focal lengths of all of the odd lenses I have. I'm using it with my fathers high school optical bench. Basically its a meter ruler with lens and projection screen holders. You can assemble multi element lenses in front of the collimator to test the resulting focal lengths. My snow globe has a focal length of 40mm and the jelly jar measured 210mm.