A number of Cloudy Nights binocular forum regulars of yesteryear used to send me surprise little gifts in the post all the way from the USA.
One of these little gifts was a very simple but ever so useful device which could be used both for measuring ( or at least estimating to within 0.5mm, exit-pupils and eye pupil dilation.
It was basically just a rectangular piece of plastic measuring approximately 5 inches x 3 inches, with 20 perfectly round holes drilled through it in every size from 0.5mm to 10.0mm. with the size indicators neatly printed below each one.
By simply holding binoculars up to a light source with one hand ( or mounted ) to see the exit - pupils, it was just a case of holding up the device and carefully moving it around until the small discs of light were matched in size.
It's very surprising how easy it is to spot a difference of only 0.5mm -- so easy I reckon it would be virtually impossible for anyone with reasonable eyesight to make a mistake.
I use the past tense because somewhere along the line I appear to have misplaced ( or lost ) it, probably during a slightly crazy period which ended up with using four or five different hard padded cases for carrying around eyepieces, barlow lenses, erecting prisms, tripod adaptors, red dot finders, lens caps, objective masks, spare lanyards, test cards and all sorts of paraphernalia every time I went away to our static caravan with binoculars, spotting scopes and tripods, which for around 6 years in the "naughties" was almost every weekend.
Estimating magnification is a little trickier but by no means rocket science.
I found a couple of spare sheets of plywood, each about 1.5 metres square ( 5x5 feet ).
On the first sheet, I simply drew a series of 10 circles in a vertical line from top to bottom down the left hand side of the plywood sheet, each 1cm in diameter.
Then to the right of those, another vertical line of circles, this one ranging from 1cm to 10cm in diameter.
Similarly on the second sheet, but with the larger circles ranging from 11cm to 20cm in diameter (due to their larger size, so as to fit onto the panel, arranged in two separate vertical rows, spaced apart from each other ).
Using a black sharpie pen to draw the circles, I then marked the inner circle in numbers from 1 to 10, and 11 to 20 respectively, so that at a glance I could easily see what size they all were ( and what magnification they very accurately represent )
It's just a case of placing the board a far enough distance away from the binoculars to enable perfect close focus, then looking through the magnified image of the 1cm circle through one eyepiece of the binoculars, whilst at the same time looking at the unmagnified image of the larger circles naked eye, slowly moving the binoculars in a downward direction until the two sizes match up.
As most of us may know, with the majority of binoculars, the magnification at closest focus is slightly larger than it is at infinity, and can even differ slightly from what is seen through the right hand eyepiece compared to through the left.
However, this is just a very simple, quick method that most people should be able to have fun with, in order to establish a basic "ballpark" estimation of magnification, and would certainly very clearly show the difference between a binocular advertised as being 20x magnification, when in reality it is only 10x.
Related reports of a far more precise and technical nature, the likes of which I used to enjoy many hours reading back in my more enthusiastic days ( when I tended to contribute more posts of this nature to this forum ) can be found here: