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Field of view in relation to brightness/clarity?

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Old Thursday 18th January 2018, 18:54   #1
chris6
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Field of view in relation to brightness/clarity?

May I ask if, at the same magnification and all else being equal, a wider view might be less bright?
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Old Thursday 18th January 2018, 19:14   #2
Gijs van Ginkel
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chris6, post 1,
The FOV does not have any effect at all on image brightness, this has been discussed a little while ago on this forum.
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Old Thursday 18th January 2018, 20:38   #3
chris6
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Gijs van Ginkel, thank you.

Right, got it.

Does eye relief have to go down with a wider field of view?
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Old Thursday 18th January 2018, 21:05   #4
Binastro
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Chris,

It depends on the eyepiece design, but usually yes.
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Old Friday 19th January 2018, 08:16   #5
chris6
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Binastro, thank you for your reply.

The qualification about eyepiece design is appreciated and I see from specifications and illustrations that an eyepiece alone can have 5 or more elements. Compared to prescription 'spectacles', and along with the prisms and other lenses, this makes prices for binoculars seem very reasonable.
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Old Friday 19th January 2018, 14:22   #6
Binastro
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Eyepieces for binoculars commonly have 3, 4 or 5 elements.
Some have 6.
The Russian 7x30, 10x42 has 7 elements, earlier 8.
Russian 100 degree eyepieces for their rare large binoculars had about 9 or 10?
Maybe less for the Russian 90 degree 15x110 binocular.

Not sure about the Nikon WX.

In early uncoated binoculars, the wider fields would indeed have less bright images because of more lens elements, but with modern multicoating this won't normally happen.

Higher magnification binoculars can more easily have wider fields as the eyepieces are shorter focal lengths.

My best astro eyepiece may be the Edmunds RKE 8mm (probably 7.3mm). This has 3 elements and is noticeably brighter and sharper than other eyepieces.

My Tolles eyepiece, maybe by H. Dall, has 1 element. Very narrow field.

William Herschel, I think, used small spherical eyepieces, one element very short focal length, very high power. Tiny FOV.
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Old Friday 19th January 2018, 14:48   #7
typo
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Quote:
Originally Posted by chris6 View Post
Does eye relief have to go down with a wider field of view?
Chris,

You can get an estimate of the potential AFoV by a bit of geometry. Use the diameter of the eyepiece lens as 'size' and the ER as the 'distance' in an online calculator like this one.
http://www.1728.org/angsize.htm

The FoV is the AFoV divided by the magnification. Although optical distortion can cause errors it's worked well for for most binoculars I've checked out. The main exception I've found is where the prism is undersized and the FoV has been reduced accordingly.

David

PS. Note that ERs in published specifications are not always reliable.

Last edited by typo : Friday 19th January 2018 at 15:04.
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