Interesting test you have there, Forgive me for asking, but what is the practical purpose of this test? I see alot of talk on these forums about true aperture, and binoculars being undersized. I am just wondering what action I should take if my binoculars fail to measure up?
It's useful but not the be-all and end-all.
To answer your question directly, if you have binoculars that are called 10x50, and you measure them and find they're 10x40, and you're also considering upgrading anyway, that measurement might provide an extra nudge.
But here are a few things to put this all in perspective. First, it's hard to measure to a millimeter or better, and the light source is *not* genuinely cylindrical. Between those two facts, expect an error of a few percent -- and don't get bent out of shape about it.
Second, a difference of 5% in aperture would probably be completely undetectable in practice, and would in any case matter less than other things that affect light throughput, such as the quality of the coatings.
Even 10% isn't vast. You can detect it comparing two instruments side by side, but it's fairly subtle. (I've done this with two identical telescopes, one masked, one not.)
When the error gets up to 15%, you have good grounds to get grumpy about false advertising.
a difference in good coatings vs avg coatings might mean .25% to .5% per surface or 3% to 6% overall when compounded over 12 surfaces. A differe4nce in 5% of the diameter of the objective lens is a loss of 19% the light gathering area, 3x to 6x the effect of a difference in coatings. So a difference in aperure has far greater implications to total light.
And, no it is not hard to measure to a millimeter at all. I've prerformed three different tests on some binoculars and get all three results to agree within a millimeter.