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