But in the world of optics, all things are rarely equal, and here's how to perform a quick and easy test to determine if your optic has an objective lens that's as advertised:
• Take the shade off a standard incandescent lamp, on a coffee table or a bedside table. You need a naked light bulb for this experiment.
• Grab a clean white sheet of printer paper.
• Get a pencil, and then grab your buddy. You'll need an extra set of hands.
• Now, take your optic--this works with riflescopes, binoculars and spotting scopes--and hold its eyepiece right up against the lightbulb. Light should be streaming through the optic.
• Now hold your paper up against the objective lens of the optic. You should see a stark black circle surrounding a bright disk of light. Have your buddy mark the edges of that circle of light as precisely as possible.
• Take the optic away from the light and measure the marks on the paper with a ruler marked in millimeters. The marks should be close to the dimensions of the optic. For instance, if you tested a 40mm riflescope, the marks should be in the 38mm to 39mm range.