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post #27 of Old 07-03-2013 Thread Starter
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Re: Binocular Conundrum: Field of View?

Nice link, johnny, but it then goes on to say:
"Exit Pupil
The exit pupil is the image of the objective that is formed by the eyepiece. It's where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope's focal ratio"

In other words, whatever image the binocs create, if they have the same exit pupil size the final image on your retina is going to be the same size.

Now ask yourself this: If Binoc#1 has a FOV if 200 feet at 1000 yards, and Binoc#2 has a FOV of 300 feet at 1000 yards (i.e. 50% wider) but they both create a 4.2mm wide image on the retina, isn't that physically impossible unless the apparent magnification is also 50% different?

If I take a 200 foot wide image and scrunch it into 4.2mm, that's a reduction of 0.00006889763. If I take a 300 foot wide image and scrunch it down into 4.2mm, that's a reduction of 0.00004593175. The wider FOV must result in a lower effective magnification.
Supposedly the human eyes combine to provide about 120d of binocular vision, which would be a FOV of some 10,380 feet at the standard thousand yards. This can't be the number that binoc makers are using, since a 10x magnification of a 10,000 foot FOV would reduce the FOV to 1000 feet--not 200-300.

But ignoring the numbers and sticking to the matter how you slice it, you can't have the FOV change without the magnification changing, or the exit pupil changing.

And fryewe, you're right, I got my triangles crossed. Don't let that distract you, it doesn't affect the facts at all. You still can't change one element of a fixed equation, without changing another one.
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