What determines the field of view for the same magnification and objective lens? (1 Viewer)

[Barbican1987]

I am puzzled. I thought all 8x42 would have identical fov but no!!

Can anyone who has more knowledge tell me why the the field of view differs even though the magnification and objective lens are the same!!

For example a friend of mine has a 10 year old Opticron 8x42 with only 6 degrees FOV but my Vikings 8x42 ED with field flatteners has 8 deg!!.

My guess is the answer is in the specification of the prisms but any more details would be appreciated.

 

[Mono]

Differences in field of view are largely down to differences in eyepiece design. Other factors do play a part but it is mostly down to eyepiece size.

 

[Binocollector]

The prisms also play a role. Smaller prisms can't accommodate the wider light cone necessary for a larger FoV.
That's the main reason why the vintage super wide 7x35s are so chunky.

 

[Barbican1987]

Differences in field of view are largely down to differences in eyepiece design. Other factors do play a part but it is mostly down to eyepiece size.

Thanks Mono that's interesting, I wasn't thinking about eyepieces having an effect

 

[Barbican1987]

The prisms also play a role. Smaller prisms can't accommodate the wider light cone necessary for a larger FoV.
That's the main reason why the vintage super wide 7x35s are so chunky.

Thanks Binocollector, I had a lovely pair of Minolta 7x35 wide angle. They were very clear. I think I gave to a charity shop, wish I hadn't now but wanted to clear out some of my collection. Hopefully someone else is getting some use out of them now. I guess the popularity of roof prism binoculars and the more compact size is a factor in a narrower fov of these binoculars.

But the models I referred to were both roof prism models albeit 10 years between their manufacture

 

[tenex]

But the models I referred to were both roof prism models albeit 10 years between their manufacture

Larger and more complex eyepieces will be more expensive, so some differences are just due to the price point. (How much of the field is sharp is a further issue; combine them and you get Swarovski NL at $3500.)

 

[has530]

There are two kinds of field of view, true and apparent (tFOV & aFOV). tFOV is what angle of the real world the binocular shows you while aFOV is how wide the image circle looks when you look through the binoculars. While there are some intricacies in converting between aFOV and tFOV (simple vs tangent formula, distortion etc) for simplicity here we will just say that two binoculars with the same magnification will have the same aFOV for a tFOV. Now here is where it gets a bit complicated.

Both the tFOV and aFOV are set by the field stop diameter of the eyepiece. Inside the binoculars in the focal plane of the eyepiece is a circular aperture that sets how much of the image from the objective lens is projected through the eyepiece to your eye. This aperture alone sets the field of view. If it is wider the FOV is wider. The tFOV is set by the field stop diameter and objective focal length (notice it is the focal length of the objective, not the diameter! Although larger diameter objectives tend to have longer focal lengths) given by the equation:
tFOV=2arctan(r/fo)
where r is the radius of the field stop and fo is the focal length of the objective.
The aFOV is set by the field stop diameter and eyepiece focal length given by the equation:
aFOV=2arctan(r/fe).
You may notice that the field of view does not depend on objective diameter, prism size, or anything else, only objective and eyepiece focal length and the field stop diameter. The magnification is also set by the ratio of the objective and eyepiece focal lengths so hypothetically for two binoculars with the same objective and eyepiece focal lengths they would have the same magnification but could have different field of views if they had different field stop diameters (I am typing this in bed while very tired so let me know if this doesn’t make sense).

As for the aforementioned mention of prisms being related to the field of view, wider fields of view require a larger image to be formed at the field stop. In order to transmit this larger image from the objective lens to the field stop (at the appropriate orientation) larger prisms must be used or else you will clip out some of the light cone resulting in vignetting.

 

[Barbican1987]

The prisms also play a role. Smaller prisms can't accommodate the wider light cone necessary for a larger FoV.
That's the main reason why the vintage super wide 7x35s are so chunky.

Larger and more complex eyepieces will be more expensive, so some differences are just due to the price point. (How much of the field is sharp is a further issue; combine them and you get Swarovski NL at $3500.)

Thanks TENEX I find this whole subject interesting.

 

[Barbican1987]

There are two kinds of field of view, true and apparent (tFOV & aFOV). tFOV is what angle of the real world the binocular shows you while aFOV is how wide the image circle looks when you look through the binoculars. While there are some intricacies in converting between aFOV and tFOV (simple vs tangent formula, distortion etc) for simplicity here we will just say that two binoculars with the same magnification will have the same aFOV for a tFOV. Now here is where it gets a bit complicated.

Both the tFOV and aFOV are set by the field stop diameter of the eyepiece. Inside the binoculars in the focal plane of the eyepiece is a circular aperture that sets how much of the image from the objective lens is projected through the eyepiece to your eye. This aperture alone sets the field of view. If it is wider the FOV is wider. The tFOV is set by the field stop diameter and objective focal length (notice it is the focal length of the objective, not the diameter! Although larger diameter objectives tend to have longer focal lengths) given by the equation:
tFOV=2arctan(r/fo)
where r is the radius of the field stop and fo is the field stop diameter.
The aFOV is set by the field stop diameter and eyepiece focal length given by the equation:
aFOV=2arctan(r/fe).
You may notice that the field of view does not depend on objective diameter, prism size, or anything else, only objective and eyepiece focal length and the field stop diameter. The magnification is also set by the ratio of the objective and eyepiece focal lengths so hypothetically for two binoculars with the same objective and eyepiece focal lengths they would have the same magnification but could have different field of views if they had different field stop diameters (I am typing this in bed while very tired so let me know if this doesn’t make sense).

As for the aforementioned mention of prisms being related to the field of view, wider fields of view require a larger image to be formed at the field stop. In order to transmit this larger image from the objective lens to the field stop (at the appropriate orientation) larger prisms must be used or else you will clip out some of the image resulting in vignetting.

Blimey, I knew someone with more knowledge would answer my question, that's very comprehensive reply. Thanks for taking the time and trouble answering in such a detailed way. Fascinating!! I have always had an interest in science and engineering so thanks for the relevant equations,

PS you quoted -'where r is the radius of the field stop and fo is the field stop diameter.' Is there a typo here? The radius is always half the diameter why the variable? Does fo refer to objective lens I wonder?

 

[wllmspd]

Might be the visible field width you see when you look in. The actual field stop is a circular hole somewhere inside or at the bottom of the eyepiece that is what you are seeing as the edge of the field when you look through the eyepiece. Wide angles with long eye relief need to have huge eye lenses that can be hard to use and are heavy as they have large lenses in. The Nikon WX shows what is possible, magically shrinking it is unlikely.

Peter

 

[has530]

PS you quoted -'where r is the radius of the field stop and fo is the field stop diameter.' Is there a typo here? The radius is always half the diameter why the variable? Does fo refer to objective lens I wonder?

Whoops yes I edited it now. fo is the focal length of the objective.

 

[Barbican1987]

Might be the visible field width you see when you look in. The actual field stop is a circular hole somewhere inside or at the bottom of the eyepiece that is what you are seeing as the edge of the field when you look through the eyepiece. Wide angles with long eye relief need to have huge eye lenses that can be hard to use and are heavy as they have large lenses in. The Nikon WX shows what is possible, magically shrinking it is unlikely.

Peter

OK thanks, it's interesting that some older Porro designs like the Minolta range had a much wider fov than some modern roof prism types but of course they were heavier and less compact.

 

[Barbican1987]

Whoops yes I edited it now. fo is the focal length of the objective.

Thanks for getting back to me

 

[Barbican1987]

There are two kinds of field of view, true and apparent (tFOV & aFOV). tFOV is what angle of the real world the binocular shows you while aFOV is how wide the image circle looks when you look through the binoculars. While there are some intricacies in converting between aFOV and tFOV (simple vs tangent formula, distortion etc) for simplicity here we will just say that two binoculars with the same magnification will have the same aFOV for a tFOV. Now here is where it gets a bit complicated.

Both the tFOV and aFOV are set by the field stop diameter of the eyepiece. Inside the binoculars in the focal plane of the eyepiece is a circular aperture that sets how much of the image from the objective lens is projected through the eyepiece to your eye. This aperture alone sets the field of view. If it is wider the FOV is wider. The tFOV is set by the field stop diameter and objective focal length (notice it is the focal length of the objective, not the diameter! Although larger diameter objectives tend to have longer focal lengths) given by the equation:
tFOV=2arctan(r/fo)
where r is the radius of the field stop and fo is the focal length of the objective.
The aFOV is set by the field stop diameter and eyepiece focal length given by the equation:
aFOV=2arctan(r/fe).
You may notice that the field of view does not depend on objective diameter, prism size, or anything else, only objective and eyepiece focal length and the field stop diameter. The magnification is also set by the ratio of the objective and eyepiece focal lengths so hypothetically for two binoculars with the same objective and eyepiece focal lengths they would have the same magnification but could have different field of views if they had different field stop diameters (I am typing this in bed while very tired so let me know if this doesn’t make sense).

As for the aforementioned mention of prisms being related to the field of view, wider fields of view require a larger image to be formed at the field stop. In order to transmit this larger image from the objective lens to the field stop (at the appropriate orientation) larger prisms must be used or else you will clip out some of the light cone resulting in vignetting.

The aFOV is set by the field stop diameter and eyepiece focal length given by the equation:
aFOV=2arctan(r/fe).
Should' r ' read field stop radius rather than diameter?

 

[Maljunulo]

What is the answer to OP’s question if we remove the field stop?

 

[wllmspd]

The field stop may be a physical aperture or the physical edge of the tube/a lens, you can’t remove it as something else will then act as the field stop. Something has to define the edge of the field…. The only case where there isn’t is when you are not looking though any optics! Sometimes the field may be reduced slightly in order to remove distracting edge aberrations, straylight or other reason. We can then compare what the manufacturer has deemed best given the tradeoffs they have made.

Peter

 

[Maljunulo]

Okay, let me rephrase.

What is the answer to OP’s question if the manufacturer has not installed a field stop in the focal plane of the eyepiece?

We can assume that the physical diameter of the optical tube limits the field of view, and I would guess that OP is aware of that limitation.

 

[Binastro]

The eyepiece determines the field.

If the field stop is removed there is a blurry edge, which has limits and becomes fainter.

There are small field, medium field, wide field, extra wide field and extreme widefield eyepieces.

From say 15 degrees with a monocentric eyepiece to 120 degrees with a Koehler eyepiece from c.1944.
The 120 degree eyepiece might reach 130 degrees without a field stop.

As to lenses, from a few degrees to at least 220 degrees. (Nikon about $50,000 plus).

The problem with binoculars is that at medium magnification the eyepieces get so large that two are wider than a person's IPD.

With high magnification binoculars one can use very wide angle eyepieces.

The very rare Soviet 8x30 binocular has a 13 degree field.
The Besser 7x32 over 13 degree measured field but awful aberrations using mirror prisms.
There are older Porroprism binoculars 7x or 8x with claimed 13 degree fields, but when measured probably one degree less.

With a telescope no prism is needed if one accepts inverted images, which is how I use my astro scopes.

Regards,
B.

 

[Maljunulo]

The Fujinon 16X70 is designed for folks with short noses.

My beak, combined with close-set eyes, proved to be quite a challenge. I ended up going it to my daughter and son-in-law, whose IPD may be greater. It was quite remarkable for astronomy.

 

[Maljunulo]

With a telescope no prism is needed if one accepts inverted images, which is how I use my astro scopes.

Regards,
B.

Stars look pretty much the same irrespective of orientation, and you quickly realize that it is pretty much irrelevant.

It can become a challenge when locating an object, using paper charts. Inverted and/or reversed charts are available, or can be generated, but it can still be a nuisance.

Maybe I just never got used to it.