SailNet Community banner
  • SailNet is a forum community dedicated to Sailing enthusiasts. Come join the discussion about sailing, modifications, classifieds, troubleshooting, repairs, reviews, maintenance, and more!

Binocular Conundrum: Field of View?

4K views 36 replies 15 participants last post by  Noelex 
#1 ·
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?
 
#2 ·
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.
 
#3 ·
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.
 
#4 ·
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.
 
#16 ·
I realise neither of us are really addressing the issue at hand but methinks you are making a good point.

We recently purchased a pair of Canon 10x30 stabilised and am quite convinced of their advantages. Went 10x30 as they were a good entry point dollar wise to the world of stabilised but almost certainly will go up a size or two when/if we buy a second pair. I loath almost every Canon product I have ever bought except for their cameras, thankfuly the binos seem to meet the camera standard and not match their lousy printers. ymmv. I'd think that something around 10x50 would be exceptional.

Cheers

Andrew B
 
#5 · (Edited)
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,,,
 
  • Like
Reactions: groggy
#6 ·
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.
 
#7 ·
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.
 
#8 ·
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?
 
#10 ·
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.
 
#11 ·
I do not believe I can make the principal any easier to understand and so, shall pass. I suggest you just pick whatever instrument seems to meet your requirements and forget details you can't/won't understand. One could also argue the principal of Gravity which some do not understand and view as irrational. Maybe, but, if you pick up both your feet at the same time in defiance, I can assure you, that you will end up on your butt.
 
#12 ·
Oh, gravity I canunderstand. In Newton's memoirs he states it was a story he made up to hide the real truth, which is that dogs like table scraps, and the only way to guarantee table scraps will fall down, is by creating gravity. So when you see dogs "sleeping" all day, that is not sleep, that is the hard work of ensuring that everything falls DOWN to where the dogs can get it.

And if you think that is any less likely than "physics" you go explain Bose-Einstein Condensates or why one particle can be split and exist in two discrete places at the one same time. Brookhaven Labs proved that one about 15-20 years ago, spin 'this' particle and "that" one moves with it--despite the absence of any physical connection that we can observe.

So...FOVs...Yeah, when the numbers don't add up, there must be a politician in the room.
 
#14 ·
Not really groggy. I'm still stuck on the exit pupil being the same size, the eye relief being the same or very similar, and as a result the geometry, the size, of the image being put on the retina is going to be the same size.

If the "reality" is 100 feet wide, and the image on the retina is, let's say 10mm wide, then the apparent magnification is fixed and the only way to change the FOV is going to be by changing something, which will either result in a different magnification, or a reduction in the size of the image on the retina.

If all these binocs have the same "night factor", the same apparent size image as fixed by the exit pupil size, the iris, the retina, then they cannot have different FOVs. In order for the FOV to change, the magnification has to change (and they all say it doesn't) or the size of the image on the retina has to change. And they all say that's limited by the exit pupil size.

I'm no math ace, but either the binoc makers are tlaking in tongues, or there's something they just don't talk about which is in fact changing, along with the FOV. And for the FOV to change by 50%, the image on my retina has to become 50% larger as well, and that would mean some significant change in the back of the binocs. Exit pupil, eye relief, something has to show a similar huge change.

Or, they're all cooking the books with the numbers. You know, like "this boat sleeps eight!" Sure, as long as that's Snow White and her little friends, two to a berth. (G)
 
#17 ·
"simpleist the higher the magnification {first #) the narrower the field of view."

As a sweeping generality, yes. Now take a Nikon Monarch 5 10x42 and a Nikon Monarch 7 10x42, both with 4.2mm exit pupils, both 10x with a 42mm objective lens. The FOV for the M7 is stated as fully 50% wider than it is for the M5.

Perhaps the magnification for one of them has been grossly mis-stated? No, there's something else "wrong" here. (Not to pick on Nikon, it isn't just them.)
 
#20 · (Edited)
I kinda skipped the last few posts so, I don't know if this was mentioned or asked but look at it as though it was a zoom lens, same glass both ends but the difference in length between both ends determines the FOV; shorten the lens the wider the FOV, extend the lens the narrower the FOV, same optics but different effect so, the question is, are the dimension on both binoc barrels the same. I know it takes very little movement on one of my lenses to change the FOV


Another thought, all things being the same, maybe the objective lens has a more pronounced fisheye cut to it, giving it a wide FOV
 
#21 ·
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?)
 
#29 · (Edited by Moderator)
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.
 
#22 ·
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.
 
#23 ·
"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.
 
#24 ·
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.
 
#26 ·
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
 
#27 ·
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.
 
#28 ·
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.
 
#30 ·
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.
 
#32 ·
Brian-
Well, considering the eight or ten seconds it took while my page moved around and reloaded THREE TIMES just now, thanks to all the crapware from advertisers who need to be housebroken (or horsewhipped, please?)...thanks for the cleanup.

And now back to our main program.

noelex, you totally misread me. I never mentioned the pupil, as in the pupil of the eye, as being a factor in this. What I said was the the EXIT PUPIL of the binocs is all the same, and THAT is stated to be a limiting factor by all the binoc makers. That is, the field of light coming out of the binocs is constrained to a circle 4.2mm in size at the point where is focuses on the retina. (At least, that's how some of them define it.)

Just got off the phone with perhaps the most highly respected brand name of German glass and their answer was "well we use extra-dispersion glass instead of...and..." in other words, they hadn't got a clue as to what FOV even was.

So I fired up the inverse tanget finagler, which says the difference between some of the FOV numbers (like 268/336/375 which seem to represent the spread for 10x42 glasses from multiple sources) correspond to a difference of 5.11d/6.21d/7.32d in terms of the degrees of the actual conve of vision in the binocs. A plus-or-minus one degree change would account for the different FOV, but that also would change the magnification from roughly 9x to 12x as well. Given, again, that the size of the image on the retina is a fixed and limiting factor for all of these.

Now, if they are full of FUD, it is also possible that the narrower FOV glasses will simply present a black ring around the image, rather than filling the retina. That would explain it very simply, allowing for the "exit pupil" to remain the size size, if they are defining "exit pupil" incorrectly and somewhat misleadingly.
Looking at my own 7x50's, which claim a 366 foot FOV, there's a black outer ring, so maybe the ones with the narrower FOV simply have bigger black rings? And the gentle manufacturers are trying to ignore mentioning their bonics often have tinier images?
 
#33 · (Edited by Moderator)
noelex, you totally misread me. I never mentioned the pupil, as in the pupil of the eye, as being a factor in this. What I said was the the EXIT PUPIL of the binocs is all the same, and THAT is stated to be a limiting factor by all the binoc makers. That is, the field of light coming out of the binocs is constrained to a circle 4.2mm in size at the point where is focuses on the retina. (At least, that's how some of them define it.)
?
The same comments apply to the exit pupil.( not surprisingly as this should coincide with the eyes pupil). All the image goes through all parts of the exit pupil. If we chop off part of the exit pupil we do not loose part of the image, the image just becomes less bright.

The exit pupil is not on the retina. It has nothing to do with the retina. It is is the same position as the eyes pupil (technically slightly incorect, but close enough without confusing the issue)

The exit pupil can be the same size with a very different FOV. We can reduce the exit pupil without effecting the FOV.

If we chop of some of the light in front or behind the exit pupil we will loose some image. This is why its important the position of the binoculars exit pupil ( the eye relief) at the the same position as eyes pupil, otherwise if the eyes pupil is smaller than the light circle parts of the view will be obscured so the FOV will be reduced.
This is why people using glasses need binoculars with a long eye relief. The glasses move the eyes pupil further back and the exit pupil of the binoculars must be also be moved back or portions of the image will be cut off.
The eye relief is the position of the exit pupil of the binoculars and this should match the position of the eyes pupil.
 
#34 ·
I'm a bit late on this thread, perhaps you haven't purchased a pair yet?
I broke down and paid the "big" bucks on the stabilized binos and am very happy I did. You can get more powerful ones (10 or 12 X magnification verses 8, the standard for small craft) as the stabilizing really does work and field of vision is not a factor when you can stay on target.
Again, this is another (of few) new items that greatly exceeds the hype, in performance.
My $650.00 8X50 rubber coated "marine" binos w/ compass never get used any more.
 
#35 ·
"The exit pupil is not on the retina. It has nothing to do with the retina. It is is the same position as the eyes pupil (technically slightly incorect, but close enough without confusing the issue)"
I understand that. But in this context, all eyeballs are the same size and all retinas are the same size, and that puts them all in the same position/geometry in relation to the exit pupil of the binocs. If the binocs had a 50mm wide exit pupil--it wouldn't matter because the iris would cut off the image at ~6mm and the retina can only accomodate a certain image size, no matter what angle the image entering the eye is "expanding" to.
The size of the retina is a limiting factor. The length of the eyeball is a limiting factor. The size of the iris is a limiting factor. The exit pupil size is another limiting factor--and the only relevant one that can be changed by the binoc maker, within limits.

Capta, I have purchased bincos. Many years ago. I'm looking for something a little different now, and I don't want image stabilized. There's moving parts and electronics and stuff to break in there. I was po'd when an internal lens (prism?) shattered in my binocs one winter, while they were stored in the closet. Apparently it was mounted ever so slightly too tightly and after enough years--the tension shattered it. Nice suprise, huh? So, I'll stick to the ones with no moving parts and just borrow or oggle yours.(G)
 
#37 ·
"The exit pupil is not on the retina. It has nothing to do with the retina. It is is the same position as the eyes pupil (technically slightly incorect, but close enough without confusing the issue)"
I understand that. But in this context, all eyeballs are the same size and all retinas are the same size, and that puts them all in the same position/geometry in relation to the exit pupil of the binocs. If the binocs had a 50mm wide exit pupil--it wouldn't matter because the iris would cut off the image at ~6mm and the retina can only accomodate a certain image size, no matter what angle the image entering the eye is "expanding" to.
The size of the retina is a limiting factor. The length of the eyeball is a limiting factor. The size of the iris is a limiting factor. The exit pupil size is another limiting factor--and the only relevant one that can be changed by the binoc maker, within limits.
)
The exit pupil does not limit the field of view.

Nor does the size of the retina, or the size of the iris.

The whole of the FOV passes through each point in the exit pupil. The exit pupil and/or the iris could be made minutely small and the FOV would be unchanged (but the image would be very dull)

The binocular manufacturer can modify the FOV without altering the exit pupil size.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top