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Re: Binocular Conundrum: Field of View?

pappy, I understand what you are saying but I think you are missing something.

The binoc makers all claim that a larger exit pupil size is crucial, because the size of the exit pupil (or the iris, whichever is smaller) will "crop" the size of the image on the retina.

So, given that the image is being cropped by the same size exit pupil, which in theory means the same size image is falling on the retina, if the smaller FOV already produces an image that fills the retina, how can the larger FOV be presented onto the same retinal area? Unless the image size is reduced, which would mean a smaller magnification.

You see where I'm finding a contradiction in their logic? If you can fit a camel through the eye of a needle...don't you need smaller camels if you want to fit them through two at a time? (Do I get an award for most mangled analogy on that one?)
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Old 07-02-2013
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Re: Binocular Conundrum: Field of View?

From a birding forum - in answer to the question "What determines field of view?"

"The short answer is the field stop in the eyepieces. The field stop is roughly the narrowest diameter of eyepiece, the widest piece of unobstructed glass. If you compare a narrow field binocular with a wide field binocular, other things being equal, the latter has the wider field stop.
To get a wide field with a sharp image across the greater part of the field requires a sophisticated design of the eyepiece."

Make sure you are comparing the same measures of FoV - there is real FoV; and there is apparent FoV which adjusts for magnification; and then there is the FoV at 1000 meters. FoV and apparent FoV are angular measurements and FoV at 1000 meters is a distance. The first two are the angles of the cone of light into the instrument and the last is the diameter of the base of the cone at 1000 meters.

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Re: Binocular Conundrum: Field of View?

"FoV and apparent FoV are angular measurements and FoV at 1000 meters is a distance."
Those are two different ways to say the same thing. The angular measurement will always translate into the same FOV at the same distance, 1000 yards or otherwise.

Anyone who would try to convince you otherwise, is either a charlatan or failed high school geometry. Included angle, distance, all basic fixed numbers and this is how we get cosines and tangents and all those other painful numbers that used to come from a book of tables before pocket calculators replaced slide rules. A 3-4-5 triangle will always be a 30-60-90 degree triangle. Same same.

And whether it is the field stop or the exit pupil, as long as the image is being cropped the same way at the same point, those will be the same as well, all else being equal.
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Re: Binocular Conundrum: Field of View?

Quote:
Originally Posted by hellosailor View Post
"FoV and apparent FoV are angular measurements and FoV at 1000 meters is a distance."
Those are two different ways to say the same thing. The angular measurement will always translate into the same FOV at the same distance, 1000 yards or otherwise.

Anyone who would try to convince you otherwise, is either a charlatan or failed high school geometry. Included angle, distance, all basic fixed numbers and this is how we get cosines and tangents and all those other painful numbers that used to come from a book of tables before pocket calculators replaced slide rules. A 3-4-5 triangle will always be a 30-60-90 degree triangle. Same same.

And whether it is the field stop or the exit pupil, as long as the image is being cropped the same way at the same point, those will be the same as well, all else being equal.
Wow. Wouldn't ever try to convince you otherwise. Wouldn't want to be thought a charlatan. And I can't even remember high school geometry.

But you may want to check your tables or slide rule. A 3-4-5 triangle is a 37-ish/53-ish/90 triangle.

And the relationship between FoV and apparent FoV is determined by the following:

tan ω' = Γ x tan ω
Apparent field of view:2ω'
Real field of view:2ω
Magnification:Γ

The differing ways of saying the same thing are important to the consumer because they are the way products are labeled. If you understand the differences you can compare the products properly...if you have your slide rule with you when you are shopping.

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Re: Binocular Conundrum: Field of View?

Quote:
Originally Posted by hellosailor View Post
OK, I'm familiar with optics in general and the issues of binoc quality and choices to be made. But there's one variable that really is a conundrum to me, since no one bothers to explain it but everyone swears they aren't just making up numbers.

Field of View.

Take two sets of binocs, 7x50 or 10x42 or whatever you choose. Same size objective lens, same magnification, and same size exit pupil as well. Now look at the FOV and you may find one is rated for a 50% wider FOV than the other, despite the fact that the exit pupil and the objective lens size are the same, so in theory the "cone" of light has to be the same angle, and with the same magnification...I don't see how it is possible for the specs to be the same, the laws of physics to be the same, and yet somehow one pair of binocs is "seeing" a 50% wider angle on the FOV.

What's the hocus-pocus here? What aren't they saying that allows two "identical" optical systems to have such widely different cones of vision?
Hey Hello,

THis thread has nothing to do with kids, so I am moving it into general discussion.

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Old 07-03-2013
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Re: Binocular Conundrum: Field of View?

This difference between your hypothetical binos is the field stop of the eyepieces.

True Field of View

Eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will see (in degrees), divide the eyepiece field stop diameter by the telescope's focal length and multiply the result by 57.3:

True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3

The Field Stop and Apparent Field of View

The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plössl eyepieces have an AFOV of 50°, Radians have 60°, Panoptics have 68°, Delos have 72°, Naglers have 82° and Ethos eyepieces have 100° or 110°.

Tele Vue Optics: Choosing Eyepieces

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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 theories...no 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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Re: Binocular Conundrum: Field of View?

Brian-
I don't mind the move at all, but "kids" has got nothing to do with anything. I posted the opening question in the General Interest / General Discussion area, unless the mouse slipped and dropped it elsewhere?
Although, it should show kids why they don't want to cut class when all those pointy geometry and trig things are being taught. (G) Personally I thought spherical trig was a special form of abuse, until years later when it made a whole lot of sense for celestial nav.
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Re: Binocular Conundrum: Field of View?

Quote:
Originally Posted by hellosailor View Post
pappy, I understand what you are saying but I think you are missing something.

The binoc makers all claim that a larger exit pupil size is crucial, because the size of the exit pupil (or the iris, whichever is smaller) will "crop" the size of the image on the retina.

So, given that the image is being cropped by the same size exit pupil, which in theory means the same size image is falling on the retina, if the smaller FOV already produces an image that fills the retina, how can the larger FOV be presented onto the same retinal area? Unless the image size is reduced, which would mean a smaller magnification.

You see where I'm finding a contradiction in their logic? If you can fit a camel through the eye of a needle...don't you need smaller camels if you want to fit them through two at a time? (Do I get an award for most mangled analogy on that one?)
Its a good question Hellosailor
One fundamental mistake you are making is with the understanding of the pupil. The pupil does not effect the field of view.

All the feild of view passes through each point of the pupil.

As our pupil gets larger and smaller our field of view does not change.

This is true when viewing without binoculars (your field of view is not smaller in bright sunshine when your pupil is smaller) or with binoculars (once again the FOV with the binoculars is not worse in bright sunshine)

Another way of understanding the problem is to think of camera. As we stop the lens down we don't take a picture with a narrow FOV. We take the same FOV but some of the light captured by the camera lens does not get thought to the film. Despite blocking of some of the light we still get the whole FOV. We can make the iris bigger, smaller round or a funny shape and we get the same photograph just brighter or duller.

I hope this helps at least part of the puzzle.

Last edited by noelex77; 07-03-2013 at 02:41 PM.
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Re: Binocular Conundrum: Field of View?

Yes, same exit pupil diameter = same size image on your retina. What information that image contains is a function of the field of view. Larger FOV, you see more of the world. Smaller FOV, you see less of the world, but the cone of light on your eye is the same size.

It doesn't get any simplier than the formual provided. True FOV a direct function of field stop diameter in your case of otherwise identical binoculars.

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