# Field of view

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At an astronomy meeting last night someone said that binoculars have a set field of view. So, for example, all 7x50s will have the same field of view, all 8x42s will have the same ... Etc.

Should this be true? I know exit pupil will be the same within a size but if FOV just mathematical too?

In my bino shopping experience binos of the same size have different field of views advertised. Is this because comers have been cut? Or is it normal for binos that are the same size to have a different FOV?

Any opinions appreciated.

Thank you

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No, it is not true. The field of view of, say a 7x50 binocular, will depend on the type of eyepiece it uses. You may find differant makes of binocular all have different fields of view.

Eyepieces used in the binocular may have different designs, therefore different fields of view, for example: an eyepiece with a field of view of 60° at x7 will have a field of view of 8.5°, whereas any eyepiece with a 50° fov would give 7.1°

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Hi Victoria,

Field of view is determined by a number of things. The magnification used, the apparent field of view of the eyepieces, and any internal restrictions such as limited apertures on the eyepiece side.

I think most probably use 50 degree afov eyepieces, so will have similar true fields of view, but in theory it is possible to use wider afov eyepieces, although this may introduce distortions into the view.

So, a 7x50 pair with 50 degree afov eyepieces should have an fov of 50/7 = 7.14 degrees.

I think the differences are more likely to be because of internal aperture restrictions on cheaper units versus more expensive pairs. If this internal field stop reduces the fov then the calculation above is inaccurate. In telescopes, you then use the calculation eyepiece field stop/telescope focal length x 57.3. The same calculation should apply for binos, but it's not always easy to find this information out.

http://www.monkoptics.co.uk/General/binocularterms.html

http://www.televue.com/mobile/TV5_page.asp?id=212&plain=TRUE

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Thank you both!

So if 50' lenses are used in a 7x this would get me 7.1 degrees FOV and in a 10x it would get me 5 degrees FOV?

So 60' lenses result in 7x having a 8.6' FOV and 10x have a 6' FOV?

So, to try and simplify things a little, this means that any less than these figures then there is internal field stop which is bad?  And any more than these figures means that they are using wider angle lenses which is likely to distort the view which is also bad?

Sorry if I've over simplified but just trying to get my head around how the maths actually affect real life.

Thanks again for your help.

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Thank you both!

So if 50' lenses are used in a 7x this would get me 7.1 degrees FOV and in a 10x it would get me 5 degrees FOV?

So 60' lenses result in 7x having a 8.6' FOV and 10x have a 6' FOV?

So, to try and simplify things a little, this means that any less than these figures then there is internal field stop which is bad? And any more than these figures means that they are using wider angle lenses which is likely to distort the view which is also bad?

Sorry if I've over simplified but just trying to get my head around how the maths actually affect real life.

Thanks again for your help.

Yes, those calculations look right.

If the fov is less than 7.1 degrees then it can be for two reasons as far as I'm aware. Firstly the afov of the eyepieces could be less than 50 degrees, 45 or 40 perhaps. This is not necessarily a bad thing, depends what you are looking for. Second possibility is that there is an internal field stop on the eyepiece which is reducing the fov. This may be there to mask poor optical performance on the edges, or for other reasons I'm not sure of.

Regarding wider afov eyepieces, I guess you just have to be careful about the quality. The wider you go with afov, the more demanding it is on eyepieces to control the different optical issues towards the edge of the fov. Binoculars are by definition reasonably fast optically, I think around f4 or f5 so this is quite demanding on eyepieces. Cheaper binos with wider fields are unlikely to be good in the outer zones. Testing them out or checking reviews is the best way really.

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Yes, those calculations look right.

If the fov is less than 7.1 degrees then it can be for two reasons as far as I'm aware. Firstly the afov of the eyepieces could be less than 50 degrees, 45 or 40 perhaps. This is not necessarily a bad thing, depends what you are looking for. Second possibility is that there is an internal field stop on the eyepiece which is reducing the fov. This may be there to mask poor optical performance on the edges, or for other reasons I'm not sure of.

Regarding wider afov eyepieces, I guess you just have to be careful about the quality. The wider you go with afov, the more demanding it is on eyepieces to control the different optical issues towards the edge of the fov. Binoculars are by definition reasonably fast optically, I think around f4 or f5 so this is quite demanding on eyepieces. Cheaper binos with wider fields are unlikely to be good in the outer zones. Testing them out or checking reviews is the best way really.

Thank you!

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So 50'/60' in a 7x should get me 7.1' to 8.6' FOV; and

so 50'/60' in a 8x should get me 6.3' to 7.5' FOV

Following on from this, and in your opinion please, I've found the Bushnell Legacy WP 8x42s that have a FOV of 8.3 (but now this seems too wide).  Someone said that 42s might be a little dim so to go for 50s and recommended Opticron Imagic TGA WP 7x50s with a 6 FOV which seems way too restricted!

I'm now in a kerfuffle!  Do you have any opinions please?

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Even the Opticron Imagic TGA WP 8x42s seem a little restrictive at 6.5 degrees FOV...

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What's your budget Victoria?

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So far the ones I have been finding are in the £100 to £200 bracket.  I don't want to buy something that I'll want to replace in a few months so happier to go higher if it saves me buying twice

Although, as nice as they are, IS is too high.

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Ok, does this help?

You seem concerned about the fov being restrictive, but even at six degrees I don't think that will be the case. I've overlaid circles of 6, 7.1 and 8.3 degrees on various large objects so you can see the comparisons.

M42/Orion's sword

Orion's Belt

The Double Cluster and Stock 2

The Coathanger Cluster

M31, the Andromeda Galaxy

So, I would not get too hung up on fov, but 7.1 does look pretty good. I don't think you need wider personally.

For what it's worth, I have a pair of Barr and Stroud 8x56 EDs. They have an exit pupil of 7mm and give lovely bright images. Whilst they are probably a bit heavy for you, my point was to confirm that I think you are right aiming for a large exit pupil as I think it does help with Astro use.

http://www.bestbinocularsreviews.com/Barr%20&%20Stroud8x56SavannahED-60.htm

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Ok, does this help?

You seem concerned about the fov being restrictive, but even at six degrees I don't think that will be the case. I've overlaid circles of 6, 7.1 and 8.3 degrees on various large objects so you can see the comparison

...

So, I would not get too hung up on fov, but 7.1 does look pretty good. I don't think you need wider personally.

For what it's worth, I have a pair of Barr and Stroud 8x56 EDs. They have an exit pupil of 7mm and give lovely bright images. Whilst they are probably a bit heavy for you, my point was to confirm that I think you are right aiming for a large exit pupil as I think it does help with Astro use.

http://www.bestbinocularsreviews.com/Barr%20&%20Stroud8x56SavannahED-60.htm

Wow!!!  Thank you so much

Your advice is amazing and the pictures on the app thing are really helpful

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I would not get concerned too much about the field of view.

The reality is that about the biggest object you will try to view and expect to see anything is M31, Andromeda, and that is 3 degrees. There are objects above that in size but they tend to be too dim for realistic use of binoculars.

I have Bushnell H2O's and Bushnell Natureviews - roof prism variety in both. The Natureviews are better optically then the H2O's, slightly odd then that I tend to use the H2O's about 80% of the time for looking at the sky. The H2O's live in the car and the car takes me to darker locations, is the reason.

One aspect of 50mm objectives is that in some they are stopped down internally to reduce aberrations, so although 50mm they act like 40-45mm objectives. This I suspect occurs on the real budget binoculars as they can advertise big and still sell cheap, but a recognised brand would not I hope be doing this.

A nice set of 7x50 should be brighter then 8x42's, however 50mm lens wil be about 40% heavier at the front.

A higher magnification will make whatever you are looking at bounce round.

You are likely going to have to flip a coin for a pair and see/hope they come out OK (sorry). I went to the Birdwatch Fair at Rutland and looked through about 100 pairs, then chose the Bushnells based on budget and performance. Happy with them, however I did get 2 sets in a moment of semi-maddness.

As was previously said 8x42's will allow you to see the Jovian moons as small points, a small 70mm scope will show that each point is a different colour. Jupiter itself remains a small bright dosk with no real features. To see bands in Jupiter I have used a 70mm scope at about 40x to 50x. To see something like Saturn's rings, even just make them out means a scope and around 80x or more.

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I would not get concerned too much about the field of view.

The reality is that about the biggest object you will try to view and expect to see anything is M31, Andromeda, and that is 3 degrees. There are objects above that in size but they tend to be too dim for realistic use of binoculars.

I have Bushnell H2O's and Bushnell Natureviews - roof prism variety in both. The Natureviews are better optically then the H2O's, slightly odd then that I tend to use the H2O's about 80% of the time for looking at the sky. The H2O's live in the car and the car takes me to darker locations, is the reason.

One aspect of 50mm objectives is that in some they are stopped down internally to reduce aberrations, so although 50mm they act like 40-45mm objectives. This I suspect occurs on the real budget binoculars as they can advertise big and still sell cheap, but a recognised brand would not I hope be doing this.

A nice set of 7x50 should be brighter then 8x42's, however 50mm lens wil be about 40% heavier at the front.

A higher magnification will make whatever you are looking at bounce round.

You are likely going to have to flip a coin for a pair and see/hope they come out OK (sorry). I went to the Birdwatch Fair at Rutland and looked through about 100 pairs, then chose the Bushnells based on budget and performance. Happy with them, however I did get 2 sets in a moment of semi-maddness.

As was previously said 8x42's will allow you to see the Jovian moons as small points, a small 70mm scope will show that each point is a different colour. Jupiter itself remains a small bright dosk with no real features. To see bands in Jupiter I have used a 70mm scope at about 40x to 50x. To see something like Saturn's rings, even just make them out means a scope and around 80x or more.

Thank you so much for your help!!!

Your last paragraph is awesome, I hadn't been able to find that information anywhere!  I'm keeping it for when my bins aren't enough and I need a scope

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A warning is that the numbers given are sort of minimal ones, and come from using a 70mm refractor.

Obviously a good image at 120x will show more then a good image at 50x.

Equally a poor image at 120x may show less ultimately then a good one at 50x.

I use refractors and they tend to produce a fairly good final image so I am happy to lose some magnification in favour of a little better image.

All "3" scope types produce slightly different overall images to your eye.

I just find that refractors suit me, I tend to follow a post from some time back on the CN site where it was reckoned that a good 80mm refractor was about the best all round scope, many people had sold theirs when upgrading then regretted it - it manages most things well, although it may seem small to many people.

I use a 90mm ED refractor. Good optics and nice size. Does everything I want it to very well, and I can nail a DSLR onto it if I want to.

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Let's not forget that this thread relates to binoculars, not scopes.

7x50 binos (or similar) are not bought for planetary viewing as the magnification is just not high enough. Binos give lovely widefield views of open clusters and also other brighter DSO's, some galaxies etc. With a 7 degree fov you can take in significant parts of constellations such as Orion's Belt or sword to see everything in context.

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Hi Victoria,

Some good advice in this thread! (I covered similar points in your other thread on this) and, to reiterate what Mr Spock said, the advice you were given about a particular size of binocular having a set FoV is just plain wrong!

I think you are now suffering from "paralysis by analysis". There is no definite "best binocular" from your options; I would suggest now just going for the best quality glass you can afford, probably 7x or 8x  40 or 42mm - you'll be able to hand-hold it steadily, you'll be able to see hundreds of objects that you now cannot see and, if you get good kit now, it'll be a while before you'll have a burning urge to replace it, by which time you'll have the experience to know what you need to upgrade to.

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Hey Victoria,

BinocularSky (aka Steve Tonkin) is pretty much the Go To Guy for all things binocular - good advice from him there.

I've got a little pair of Helios Nirvana ED 8x42's that I absolutely love. Essential part of my kit now. Great crisp views, light and comfy to hold, minimal false colour on moon and planets thanks to the ED low dispersion glass. They're great for starhopping and planning when I've got the telescope out, or just having a quick gander in between clouds. Great for enjoying learning the sky. Youre not going to see galaxies and double stars in them (get a scope) but they'd be genuinely useful for any and all astronomy you do, however advanced. I wouldnt go so far as to say they choose their owners like Harry Potter wands, but binoculars are quite a personal choice so I suggest you try some before settling. Your budget should get you a pair of keepers. Good luck and enjoy whatever you choose.

Regards, Simon

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Thank you so much for your help guys!

I've bit the bullet and ordered Bushnell Legacy WP 8x42s and Opticrons Imagic TGA 8x42s, both with big FoVs!  Shall spend the next week or so testing them and then will keep the best pair for me

Thanks again everyone, for all your help

ps. Big Sumarian, what app were you using for your FoV demonstrations above please?

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