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A quick question about FOV and exit pupil size (1 Viewer)

When looking through binoculars you only get a narrow circle surrounded by blackness to look through. What I want is for this circle to be as large as possible for a less claustrophobic viewing experience.

Should I be looking at FOV (i.e Meters at 1000M) for this or the exit pupil size (in mm)?

i.e Would a 143M @ 1000M have a bigger circle than a 119M @ 1000M

or should I be looking at the exit pupil size and is 4mm be a bigger viewing circle than 2mm?

Many thanks
 
Welcome to the forum.

Assuming you have the eye relief correctly adjusted it's the FoV @1000m figure that's important. The exit pupil size makes no difference.

David
 
You can ignore exit pupil size. It means nothing when determining the width of the FOV from specs.

There two kinds of FOV specs - real and apparent. Meters at 1000m is an old convention in binocular specs for expressing the real field. It tells you how wide a swath of real estate is contained in the binocular FOV at 1000 meters (or it may be expressed in feet at 1000 yards to add to the confusion). The real field may also be expressed in degrees formed by the pie slice of real area contained in the FOV.

I expect that what you want to know is the apparent or subjective field. That's the angle of the pie slice formed inside your eyeball when the binocular FOV is projected onto your retina. It determines how "claustrophobic" the view appears to be. It's advantage as a spec is that it is independent of magnification. A 60º apparent field always fills the same area of of the retina whether the binocular is 6x or 10x, so one number is all you need to know to predict whether a binocular should be considered a wide or a narrow field instrument. FOV expressed in real field requires adjusting for each magnification. For instance, your example of 143M @ 1000M would be an extremely wide angle binocular at 10x with an apparent FOV of about 82º, but it would be a somewhat claustrophobic 49º at 6x.

Unfortunately, apparent field is not always consistent in specs. It is often approximated using two different methods that give different results and occasionally actually measured. As always it's best to try for yourself, but as a general rule of thumb AFOV below about 55-57º may appear claustrophobic while AFOV above 60º probably won't.
 
When looking through binoculars you only get a narrow circle surrounded by blackness to look through. What I want is for this circle to be as large as possible for a less claustrophobic viewing experience.

Should I be looking at FOV (i.e Meters at 1000M) for this or the exit pupil size (in mm)?

i.e Would a 143M @ 1000M have a bigger circle than a 119M @ 1000M

or should I be looking at the exit pupil size and is 4mm be a bigger viewing circle than 2mm?

Many thanks



Hi White Dragon,

I see this is your first post so welcome to Bird Forum:hi:

You can easily find out what the exit pupil of a binocular will be by dividing its power into the diameter of its objectives. So a 7x42 binocular will have a 6mm exit pupil; an 8x42 will have a 5.2mm exit pupil and a 10x42 will have a 4.2mm exit pupil. And a 8x32 binocular will have a 4mm exit pupil. (EP)

Their field of view is determined by the design of their eye pieces and the ones that have wide FOVs will be more expensive. But generally speaking lower power binoculars will have wider FOVs. (ie 8x42s will have wider fovs than 10x42s.)

You also should understand that 8x42 or 10x42 binoculars will be larger and heavier than an 8x32 or a 10x32.

What you should do at this point is decide how much you want to spend on a binocular and then try them out if you can.

Keep asking questions, it is the only way to learn:t:

Bob
 
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To get a quick indication of the apparent or subjective field you could simply multiply the real field of view with the magnification. 8x bins with a 143 field of view yield 1144. 6x bins with a 143 fov give 858. Anything under 1050 I find narrowish.

George
 
You can ignore exit pupil size. It means nothing when determining the width of the FOV from specs.

There two kinds of FOV specs - real and apparent. Meters at 1000m is an old convention in binocular specs for expressing the real field. It tells you how wide a swath of real estate is contained in the binocular FOV at 1000 meters (or it may be expressed in feet at 1000 yards to add to the confusion). The real field may also be expressed in degrees formed by the pie slice of real area contained in the FOV.

I expect that what you want to know is the apparent or subjective field. That's the angle of the pie slice formed inside your eyeball when the binocular FOV is projected onto your retina. It determines how "claustrophobic" the view appears to be. It's advantage as a spec is that it is independent of magnification. A 60º apparent field always fills the same area of of the retina whether the binocular is 6x or 10x, so one number is all you need to know to predict whether a binocular should be considered a wide or a narrow field instrument. FOV expressed in real field requires adjusting for each magnification. For instance, your example of 143M @ 1000M would be an extremely wide angle binocular at 10x with an apparent FOV of about 82º, but it would be a somewhat claustrophobic 49º at 6x.

Unfortunately, apparent field is not always consistent in specs. It is often approximated using two different methods that give different results and occasionally actually measured. As always it's best to try for yourself, but as a general rule of thumb AFOV below about 55-57º may appear claustrophobic while AFOV above 60º probably won't.

Thats very useful thank you.

Its a shame apparent field of view can't be calculated reliably.
Do you know of a simple formula to calculate the apparent field of view, just to use as a guide?
I'm making a spreadsheet of specs will be handy to add that number to use as a guide choosing which binoculars to get.

I'm going to a shop tomorrow to get a handle on the great 8x vs 10x debate and what they look like to my eyes. I'm thinking compact 32mm objectives or less so from what I have read (but not experienced in person) it seems 8x will be better.
 
To get a quick indication of the apparent or subjective field you could simply multiply the real field of view with the magnification. 8x bins with a 143 field of view yield 1144. 6x bins with a 143 fov give 858. Anything under 1050 I find narrowish.

George

Thanks.

Eeek, that does seem to be a high bar. That puts all 8x binoculars below
132M @ 1000M in the narrow category. That seems to be most of them.
 
Whitedragon 101,

That number above is subjective. I use a Leica Ultravid 8X42 130m/1000m, that FOV is fine for me and many others.
As stated above by Henry Link regarding the apparent field of view, check that value on any glass you are looking at to see if it is above 57. To me if it is below 55, it appears like tunnel vision.

Andy W.
 
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Hi,

the apparent field can be easily calculated like this:

real field in degrees = real field in m@1000m / 17.45

apparent field in deg is approximately real field in deg times magnification

This is not correct to a degree as there can be distortion in the EP - either added by the designer for a reason or by accident.

Joachim
 
Thanks.

Eeek, that does seem to be a high bar. That puts all 8x binoculars below
132M @ 1000M in the narrow category. That seems to be most of them.



Most 8x32s are wider than 132M @ 1000M.

Allbinos website can help you here. It ranks binoculars based on their sizes.

Allbinos does not discuss apparent FOV in their rankings. They give the angular (or real) FOV in meters @ 1000meters and in degrees.

The link below shows the rankings of thirty one 8x32 binoculars: (The #1 ranked Nikon 8x32 EDG has an angular FOV of "136/1000 7.8º.")

https://www.allbinos.com/allbinos_ranking-binoculars_ranking-8x32.html

If you click on the review at the end of each ranked binocular you will find its angular FOV in meters and degrees. 31 are ranked. Most of them have FOVs larger than 132M @ 1000M.

Bob
 
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Whitedragon,

If you're looking at Swarovski, Zeiss or Leica binoculars their AFOV specs appear to be actually measured now, so presumably are accurate. Nikon uses the ISO method of AFOV calculation, which usually understates AFOV for most binoculars. The Nikon 8x30 EII, for instance. would be 70.4º using the simple method of muliiplying real field (8.8º) by the 8x magnification, but Nikon's spec is only 63.2º using the ISO method. The EII's actual measured AFOV is 66.5º. In general, you will be pretty close to the true measured AFOV including distortions if you multiply the real field by magnification and then subtract about 5%.
 
Supposedly its calculated like this (ISO 14132-1:2015 standard)

tan(w') = tan(w).Γ
where Γ is the magnification of the binocular, w' is the apparent FOV and w is the real FOV.

I am not 100% sure this is right though. It implies that the wider the FOV, the wider the apparent field, as long as the magnification remains constant.
 
One, relatively simple, specification check to can do is see if the linear FoV, manification, eye relief and diameter of the eyepiece all add up.

Looking through an eyepiece is like looking out the window. The angle of view you see changes with how far you stand from the window. Longer ERs result in narrower views than shorter ones for a given eyepiece lens diameter. The angle from the ER point to the width of the lens gives the maximum possible AFoV. There a several online triangle calculators that can help. The Maximum AFoV divided by the magnification gives the AFoV which can be converted back into the linear AFoV using the same calculators.

The values for most binoculars I've checked have agreed closely with those published by their companies, but I have found points of difference. This check only tells you the maximum possible from the lens diameter, but using less than the full lens width will resulting in narrower angles. Values that exceeds the maximum don't immediately tell you which parameter is at fault.

Most binoculars I've checked have matched the published specifications, but there were a few exceptions. One over estimated the linear FoV and a couple overestimated the ER. So far, I've not found an error in magnification.

David
 
I have found many errors in published specifications particularly with lower priced binoculars.

The magnifications are wrong, say a Celestron 8x30 that is actually about 6.7x27.

20x70 with 3.5 degree field, actually about 2.6 degrees. This a good Japanese binocular.

Also even top quality binocular specs are sometimes wrong, especially when they convert from metres at 1000 metres to feet at 1000 yards.

The 8x42 figures are often lowish because the eyepieces have a longer focal length than a 8x32 or 10x42.
This is related to field stop size.

There is only one way to know if a binocular is suitable for the observer and that is to try it preferably for a fair amount of time.

Then there are numerous binoculars, particularly Chinese low priced binoculars advertised on the internet, where the specs., claims and most everything else is fiction, often fraudulent.
 
The question of magnification accuracy gets regularly mentioned. Indeed, if you look through a number of allbinos reviews, you might be led to believe that magnification inaccuracies were common, even amongst fairly expensive models. This seemed peculiar to me in this age of computerised design and machining. However, if you look closely at their methodology they do not measure magnification at all, only the ratio of the objective diameter to exit pupil which is quite incorrect and can produce significant errors.

Earlier this year I checked out for myself 8 binoculars I had to hand.

The oldest binocular was an 80+ Year old REL 6x30. Entrance and exit pupils and magnification were were spot on. A 60 year low priced Japanese 8x30 and had an moderately undersized entrance pupil, but the exit pupil was reduced proportionally, and again the magnification was correct. A low price Chinese 12x50 had a substantially reduced entrance pupil but once again the magnification was correct. Another 5 mid priced modern roofs were also accurate, at least to within my measurement capabilities. However, I did find some ER and FoV errors as I mentioned above.

I know there are some rediculous claims for both magnification and objective size by some unscrupulous vendors, on that auction site in particular, but these days the major brands are obliged to comply by the ISO14133 standards. A magnification error of up to 1.5% is permitted for the higher standard, and 2% for the lower one.

Davd
 
The exit pupil (objective diameter divided by magnification) tells you the size of light patch coming it the back that will go into your eye. 3-5mm is common, larger are better for low light situations. As mentioned it won’t affect the “width” of the view.
If you don’t wear glasses then you could “experiment” with old second hand Porro binoculars (off eBay) that can provide very wide apparent fields of view (with varying degrees of quality), but that have limited eye relief (you sometimes need to rest your eyeballs on the eyepieces). The benefit is that the “dark circle” of the field of view is so big that you notice it less and you feel more immersed in the view. Personally I aim for at least 70degree field of view, but can tolerate a bit smaller if necessary ;-)

Welcome to the forum

Peter
 
When looking through binoculars you only get a narrow circle surrounded by blackness to look through. What I want is for this circle to be as large as possible for a less claustrophobic viewing experience.

Should I be looking at FOV (i.e Meters at 1000M) for this or the exit pupil size (in mm)?

Many thanks

You will probably have noticed this already while trying out the binoculars you mentioned in your other post, but although the circle of light may seem small when your binoculars are held say 2 feet away from your eyes, once you get to the right distance in front of your eyes they quite suddenly "open up" and you then see much more. The term for that distance is known as "eye relief" and is the reason why binoculars with longer eye relief (say 15mm+) can be used with glasses, which act to hold the binocular further away from your eyes. Binoculars with short eye relief, as willspd says, require you to put them much closer to your eyes. This won't be an issue if your vision is good enough that you don't require glasses, or wear contact lenses; or if what you are observing is so far away that it requires binoculars to see. But if you do require glasses in daily life, and remove them to use your binoculars, they will need to focus sufficiently "beyond infinity" to compensate for your glasses.

Field of view has already been well explained. Wider fields of view really shine when searching for targets over large areas of sky or similar situations (I don't do much birding in woodland but a wide field of view would be useful there for similar reasons), but aren't as necessary for certain situations eg. looking at birds at a feeder or already spotted by eye. Wider fields of view tend to be more desired, but at lower price points you may have to choose between wider field of view and image quality (cleaner, sharper across the field of view).

Exit pupil size: Binoculars with a large exit pupil have two main advantages - a large exit pupil admits more light, which is an advantage in twilight or low light, and also makes the binocular less sensitive to "eye placement" (ie. it is less necessary to have the binocular eyepieces placed in just the right location in front of your eyes). My own experience is that the larger the exit pupil the more comfortable (for want of a better word) the view becomes - but recent reviews of eg. the Swarovski 8x30 CL and Zeiss Victory Pocket 8x25 suggest that the latest eyepiece designs have made binoculars with smaller exit pupils very much less finicky. Unfortunately most binoculars with large exit pupils (eg. 7x50, 8x56) are rather large and heavy. 7x42 gives you a large exit pupil in a reasonably sized package and has a lot of fans, but you may find you require more magnification when observing over longer distances.

I strongly recommend looking through a number of binoculars in your price range and taking careful notes - all the jargon will become much more relatable once you have spent some time looking through and comparing 8x32 with 8x42 and/or 7x42 with 10x42.
 
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