For Haz Baz: The mm (millimeter) of a lens is it's focal length. Examples in 35mm format would be
135, 300 or 100-300. The mm size is measured from the rear glass to the front glass.
The more the front glass is curved the greater the angle. An example would be 24mm wide angle.
The powerful telephoto lenses or zooms magnify the most. In 35mm format to figure out the magnification divide
the lens size by 50. 300mm would magnify 6x. Not very powerful if you compare it to a 10x40 pair
of binoculars which magnifies 10x . So there are programs like Photoshop that allow you to digitally
manipulate or magnify even greater. Since I still use 35mm I tend to use 300-500mm lenses or a 300mm
with a 2x extender which brings it to 600mm. Confusing? Not!