7x image is bigger than 8x and 9x ?! (1 Viewer)

Having just rediscovered the endearing qualities of an old Nikon 7x35A porro, which I've been enjoying anew over the last few days, it seemed a good idea to compare it with a couple of other similar-dimensioned but more recent high quality porros, namely Opticron Minerva 9x35 and Opticron SRGA 8x32, for a bit of fun. The SRGA seems the sharpest, which it should be since it's newer than the Minerva and Nikon, these latter two appearing to be quite close in image quality yet, surprisingly, I preferred the 'reach out and touch' view of the Nikon. The SRGA is 8.25', while the other two are both marked 7.3'. But here's the strange thing: the Nikon image looked 'bigger', but it should have been slightly smaller (shouldn't it?). So I directly compared the 7x Nikon with the 8x SRGA, looking at a stained glass window with square panes, and there was no doubt the Nikon's image was significantly bigger. I even swapped over from one eye to the other, in case there was some kind of optical illusion, but the result was the same: bigger Nikon image. I then tried the 9x Minerva; no doubt this would be a noticeably larger image than the 7x Nikon... No! There wasn't much in it, but the 7x Nikon's image was just a bit bigger than the 9x Minerva. How can this be? Does it make nonsense of specifications published by manufacturers, or have I totally lost the plot somewhere along the line?
Help!

James,

I am sure one of the more knowledgable folks on here has a better explanation for it. Could it be something like a perception of a larger image because of the field of view or possibly the type of edge distortion displayed. I seem to remember comments stating that the latter can have an effect on how large an image appears.

James,
It doesn't make a lick of sense to me. It could be that the explanation is simply that you are delusional. I am. A way to test this hypothesis, which for the sake of science I trust you will not find too offensive, is to measure the diameter of the exit pupils, which will be the diameter of the objective divided by the true magnification. Focus the binocular at a far distance, point at a bright sky or other broad light source, and measure the bright circle in the middle of the eyepiece.

A true 7x35 would have an exit pupil of 5.0mm, 9x35 a hair under 4.0mm, 8x32 would be 4.0mm exactly. Just holding it up and visually comparing the Nikon to the Minerva should do it. Of course if you're going to question the magnification, you might as well question the aperture while you're at it, so first measure the "open" objective diameter, looking to see if there's a limiting ring hidden just behind the entrance.

I'm sorry to be so teachy, but I know some of you real birders who have seen 4000 species have never thought about how binoculars work. How could you, there's no time for that! Me, I've checked about three boxes in my life list, and am obsessed with optics.

It's funny, the frightening precision that MUST be maintained to make optics at all, and the lousy standards in the precision of stated specifcations.
Ron

James,

You say that the image with the Nikon was "bigger". Do you mean that it looked as though you were closer to the object you were viewing, or that the object that you were viewing took up more space in your view? If it is that latter that would make sense to me.

The Nikon and the Opticron Minerva have similar angles of view of 7.3 degrees. The Opticron SRGA has a wider angle of view of 8.25 degrees. Since the SRGA has a wider viewing angle you will see more of the view around the subject object. Consequently the subject object appears smaller compared to the overall view. If you look at an object that just fills the field of the Nikon or the Minerva and then you switch to the SRGA you would see additional area around the object.

Try looking through two of you binoculars at once, one with each eye. It isn't easy to get everything lined up this way. Look at the stained glass windows, or something simple and contrasty. Now you will see the difference in the size of the object due to different magnification.

Mike

Our experience of magnified images likely can be deceived by differences in apparant field of view. Also the eye will be deceived when using a binocular at all. Study an object with naked eye and then instantly put the glass to your eyes. Using a power of for example, 10x will not result in a perceived magnification of 10x, but maybe 5x! Try it and you will se.

The only way to measure true magnification is by measuring the eye relief and comparing to the objective lens. If aperture is 50mm, exit pupil is 5mm the magnification is 10x PROVIDING YOU CAN MAKE USE OF THE ENTIRE APERTURE.
Try it by put for, example, a pen in front of the objecive lens. If you see the pen in the exit pupil instantly when it protrudes the objective lens edge you can make use of the full aperture. If you don't the aperture is stopped down. In that case it's more difficult to measure true magnification...

Mike, I'd already tried doing just as you say, looking through two binoculars at the same time, one with each eye, then swapped each to the other, carefully lining up the images of the easily-compared stained glass window squares, thereby seeing the Nikon 7x was noticeably bigger than the SRGA 8x and just slightly larger than the Minerva 9x image... Not only was the Nikon's 7x image bigger, directly lined up alongside the others, eye to eye, but it also seemed to have greater depth. I tried to verify this by changing my view to a fir tree in the garden, much closer than the stained glass window: all the binoculars gave a nice crisp image, but the Nikon rendered a superior "reach out & touch" effect; I expected the 9x Minerva to be best at this 'closer view', but the Nikon was a more three dimensional result. I'll try some other bin's of different magnifications, just for fun...

Nikon made a 9x35A. Maybe that's what you have although it's hard to see how a 7x35 prism cover plate would have wound up on a 9x35 (a repair using what was available, maybe?). All of the parts are interchangeable. I made up a 9x35 Nikon E by combining the objectives from a 7x35 E with the eyepieces from a 8x30 E.

A couple of other checks might help. One would be to compare the size of the field circles in the Nikon and Minerva by placing one to each eye and lining up the fields. The 9x35 Nikon apparent field circle would be about the same size as the Minera and the 7x35 substantially smaller. Did you try Ron's suggestion of comparing the exit pupils side by side by examining them from some distance behind the eyepiece? A 7x35 5mm exit pupil should look obviously bigger than a 9x35 3.9mm exit pupil.