This may have been talked about in the past but not that I had seen. One of the most basic descriptions of binocular performance seems to be brightness. Example, all things equal, porro is brighter/dimmer/whatever than roof prisms. With smaller bins, it's especially critical to get every possible photon of light through to the viewer. Different coatings, etc. claim to improve the brightness of bins. Exit pupil is supposed to be a very good indicator, but with so many other factors that can affect brightness, I am not satisfied with exit pupil alone. But how can one check brightness without resorting to an advanced test lab?
I've toyed with the idea of using a camera spotmeter with a standard gray card at a given brightness (noon/100w lamp/whatever), but I figured out another way to check it that is easy and fast. Unfortunately you need two bins to compare. But that is often the case when one is shopping for bins.
Method:
* Find a white, or preferably off white or grey surface that is sufficiently lit with a reasonably soft light source. A very powerful light source may bias the results. This can be a white card or piece of paper on a table.
* Take two binoculars to compare and hold them equal distance from the surface, in the same direction, with the objective lenses pointed at the surface. Surface to objective lens distance of about 3-4 inches is fine.
* Observe the exit pupil coming out of each and note the relative brightness of the exit pupil. Do this a minimum of 10" away from the eyepiece of the bin, as this will allow your eye to maintain an equal pupil and make the result more accurate!
* First test a very small bin compared to a very large bin (like an 8x21 vs 8x42). This will be very easy to calibrate to, making appreciable differences more detectable.
Cons:
* The exit pupil itself can be different sizes which also affect end result. I'm only discussing observing what's in the exit pupil.
* I am guessing that younger people are less affected by brightness as their eyes dilate and contract more in response to the view.
I compared 3 bins using this method and it was quite telling. I hope that if anyone has any suggestions or objections to the method, they will post. Otherwise, I hope it's useful to those shopping or comparing!
Matt
Intriguing. What three bins did you test and what was revealed?
You can also try two at the same time, hold one half to each eye. Close one eye to go quickly back and forth.