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-   -   Binocular Conundrum: Field of View? (http://www.sailnet.com/forums/general-discussion-sailing-related/100702-binocular-conundrum-field-view.html)

hellosailor 06-22-2013 07:25 PM

Binocular Conundrum: Field of View?
 
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?

timangiel 06-23-2013 10:35 AM

Re: Binocular Conundrum: Field of View?
 
there is an angular field of view and a linear field of view, as well as true and apparent field of views. I don't really understand all of the differences, but I wonder if manufactures are using different terms in their specs to try to make their products appear better on paper.

SVAuspicious 06-23-2013 11:42 AM

Re: Binocular Conundrum: Field of View?
 
Quote:

Originally Posted by hellosailor (Post 1048033)
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.

Not at all. The first product has a magnification of 7x v. 10x in the second. Not the same. Lens on the first is 50mm v. 42mm on the second. Exit pupil on the first is a bit over 7mm while the second is only 4.2mm. Huge difference, particularly in low-light.

So if you're looking for good low-light performance on a bouncing boat 7x50s are a good choice. If you are on a very stable platform and have lots of light and are interested in more magnification than 10x42s are a good choice.

MarkofSeaLife 06-23-2013 12:07 PM

Re: Binocular Conundrum: Field of View?
 
I have been led to understand that 7 times magnification is best for sail boats but now with image stabilization higher magnifications are better.
As Auspicious says, then one needs the wider lens to let in more light.

If you are thinking about a new pair you might want to look at the extra $$ for stabilization and see if you think its worth it.

svHyLyte 06-23-2013 02:05 PM

Re: Binocular Conundrum: Field of View?
 
Quote:

Originally Posted by hellosailor (Post 1048033)
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?

The field of view of the instrument is determined by the optical distance between the objective (front) and eyepiece lenses and has nothing to do with magnification. The farther apart the lenses are, the more narrow the field of view (think of the angle in a short fat cone verses a tall narrow cone). Generally a somewhat wider field of view is desirable for enabling one to pick up targets with somewhat less sweep or traverse of the field of view. I find that the field of view of the Fuginon Polaris binoculars is just about right.

FWIW,,,

hellosailor 06-23-2013 02:16 PM

Re: Binocular Conundrum: Field of View?
 
tim-
I'm talking about ONE manufacturer, giving a 50% wider FOV for two different "same numbers" binocs they make. So however they define FOV, it is THE SAME definition, from the same maker, for both binocs.

Auspicious-
Same thing. I'm not comparing apples to oranges, I'm saying that it doesn't matter which glasses you compare. 7x50 to 7x50, or 8x42 to 8x42, pick ANY SIZE you want and then compare two different bincos IN THAT SAME NOMINAL SIZE. Whether they are made by the same maker or two different makers, but the SAME NUMBERS on both glasses. As I said, I'm familiar with optics, I know pretty well that a 7x and a 10x will--or at least should--have very different FOVs under most conditions.

HyLyte-
I'm still not understanding this. First off, I'll assume when you say the length between the front and rear lens you mean the length of the optical path, since that can be folded differently. But even so, if the magnification is the same, and the size of the image coming out the back is the same (as measured by the exit pupil), then the distance shouldn't matter. Same is same, if the apparent view is 10x "bare eye" sized, you can only blow things up so far UNLESS you change the size of the image. So in theory a wider FOV would require a wider exit pupil image. That exit pupil image "crops" whatever the apparent FOV is going to be, doesn't it?

Doesn't make sense, unless the exit pupil becomes a meaningless number.

SVAuspicious 06-23-2013 03:47 PM

Re: Binocular Conundrum: Field of View?
 
Quote:

Originally Posted by hellosailor (Post 1048286)
Auspicious-
Same thing. I'm not comparing apples to oranges, I'm saying that it doesn't matter which glasses you compare. 7x50 to 7x50, or 8x42 to 8x42, pick ANY SIZE you want and then compare two different bincos IN THAT SAME NOMINAL SIZE. Whether they are made by the same maker or two different makers, but the SAME NUMBERS on both glasses. As I said, I'm familiar with optics, I know pretty well that a 7x and a 10x will--or at least should--have very different FOVs under most conditions.

Then I'm confused about your confusion. It's all about the angles. For a fixed lens diameter, the longer the optical path the narrower the field of view. Physics is physics. The variables are optical path length, the two lens diameters, and the magnification of each lens. For field of view to change one of those parameters must change also.

hellosailor 06-23-2013 04:17 PM

Re: Binocular Conundrum: Field of View?
 
I understand what you are saying, but from what all the binoc makers say, the size of the exit pupil effectively "crops" the size of the image coming out of the binocs and into your eye. So assuming an exit pupil size fo 5mm, the focused image in your eye, on your retina, will be constrained by the 5mm circular image of the exit pupil size of the binocs.

To use some arbitrary numbers, let's say binoc #1 has a FOV of 500 feet, while binco #2 has a FOV of 250 feet, while both have the same 5mm exit pupil size.

Shouldn't that mean that binoc #2 has twice the magnification as binoc #1, since it is showing half as much FOV, but putting the same size (5mm cropped) image into your eye?

If the image that is projected into your eye is the same size, the FOV and magnification MUST be the same for both binocs, mustn't they? If you increase the magnification, the FOV has to decrease, if the image at the exit pupil is the same size. And if the FOV gets wider, then the magnification has to proportionately decrease, doesn't it?

svHyLyte 06-23-2013 07:14 PM

Re: Binocular Conundrum: Field of View?
 
Quote:

Originally Posted by hellosailor (Post 1048286)
...

I'm still not understanding this. First off, I'll assume when you say the length between the front and rear lens you mean the length of the optical path, since that can be folded differently. But even so, if the magnification is the same, and the size of the image coming out the back is the same (as measured by the exit pupil), then the distance shouldn't matter.

It is the included angle of the cone of light between the objective lens and the pupil lens that dictates the width of the field of view. Magnification has nothing to do with it at all. If you disbelieve me, touch your index finger to your thumb and hold the resulting loop against one of your eyse and close the other. Then notice how much you can see through the loop of your fingers. Then slowly extend your hand/the loop of your fingers away from your eye and continue to observe what you can see through the loop of your fingers as you move your hand. The further your hand is from your (open) eye, the less you will see through the loop of your fingers as your "field of vision" narrows as the distance between your eye and hand increases--i.e., your "cone of vision" becomes more narrow.

Here Homer Nods...

hellosailor 06-24-2013 05:04 PM

Re: Binocular Conundrum: Field of View?
 
Homer, the finger loop analogy falls short to me.

If the exit pupil size is supposed to correspond to the final size of the image on the retina, we have a fixed size image, let's say 5mm. That's at one end.

On the other end, we have a fixed life-size image. If we are viewing that image at "life" size, magnification 1, the FOV and the image on the retina are always the same, regardless of the finger loop blocking it. The finger loop effectively becomes the apparent exit pupil size, and a smaller FOV only happens with a smaller apparent exit pupil.

Since the eye relief and the distance from the retina to the exit pupil image are pretty much the same on all the binocs...I just don't buy the finger loop moving around as being analogous to what happens with binocs. Maybe I'm too dense to see it.

From what the binoc makers would have us see, everything behind the back lenses of the binocs is the same. Everything from the front lenses to the image is the same. And you're saying, if the only change is the length of the internal light path, that can account for a 50% change in the width of the FOV?

This must be why I wasn't a math major. Here, have another flagon of wine and try again. Please.


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