Telescope Reviews: True aperture? A Quick Way to Measure!

Glenn, Ron

I tried this simple test this morning and found it to be quite easly performed and also quite accurate. I took out a 15x70 that I know is 63mm. I used a small bright LED white light. Actually I found there is quite a bit of leeway in the angle of the light entering and I could still easily see 63-64mm on a scale placed directly over the barrel.

One caution is the instruction to keep the light a reasonable distance back from the eyepieces. That is important. With the light right up near the eyepieces, a mistake someone might make is to tilt the light from side to side slightly. This allows you to shine the light right out to the objective edges on both sides, showing a false apparent aperture of 70mm. With the light perhaps 6" to 8" behind the objectives, I could tilt the light a little to each side and the effect was to "move" the 63mm projected circle across my scale.

BTW, the reduction in aperture in this binocular is a "too small" baffle opening, in front of the prism aperture, but still deep down inside the barrel and not near the objective. So it seems as long as the light source is a reasonable distance back from the eyepieces, a slight tilt will not affect the outcome of the test.

Of the several tests that have been documented to test aperture (briefly outlined below), this seems the simplest and most readily available to nearly every user.

Measuring exit pupil is magnification dependant, so requires two exacting tests, one (measuring magnification) which most users often fail to even recognize needs to be performed and furthermore most will never perform. The other, actually measuring the exit pupil, requires precise instrumentation and exacting placement, also which many users may not have or may not achieve. Measuring exit pupil is probably the least accurate way of predicting effective aperture.

Reading aperture through the eyepiece using a loupe requires the posession of a good loupe, a tool many people do not have, and care to not look around, but to look directly on-axis.

Using progressively smaller aperture masks and taking repeated readings of exit pupil will give an indication of both aperture and magnification, however, to get accurate results, requires accurate mask sizing, several masks, precise measuring of exit pupil and skill with mathematics, not so easy for everyone to achieve.

Measuring aperture using a target laser by projecting thru the aperture and out through the eyepieces requires a proper (newtonian collimation target type) laser, glass platten, and tripod. The fewest people would ever have the tools and patience to do this test.

This flashlight test is simple, straight forward and with reasonable care, accurate.

Thanks Glenn.

edz