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milt
professor emeritus

Reged: 09/13/04
Posts: 603
Loc: Arizona
Binocular Exit Pupil Primer
#874625 - 03/17/06 05:11 PM

I occasionally note terms used in a way indicating that the writer does not fully understand what is going on at the eyepieces of a binocular. I was there once, and while I am still no expert would like to attempt a very basic primer on some terms associated with the exit pupil. These are intended as conceptual descriptions only. With thanks to Rick Shaffer:

Exit Pupil is the round (hopefully) disk of light that you see coming out of each eyepiece as you back away from the binocular. The exit pupil diameter is equal to the objective diameter divided by the magnification. The size of the exit pupil is not related to the apparent field of view (AFOV). An 8x42 with 40° AFOV will have exactly the same exit pupil diameter as an 8x42 with 80° AFOV. The difference between the two will be in the angle at which light rays are allowed to leave the exit pupil (more on this later).

The exit pupil can be thought of as a tiny image of the objective. There is an exact correspondence between where any given light ray leaves the exit pupil with where it entered the objective. If a light ray enters at the center of the objective, it will leave at the center of the exit pupil and if a ray enters at the edge of the objective it will leave at the edge of the exit pupil.

When your eye's pupil is smaller than the exit pupil, not all of the rays leaving the exit pupil will enter your eye. This is equivalent to placing a mask with a circular opening over the objective that blocks rays destined to leave the exit pupil outside of your eye's pupil.

Field Position is the position that your brain perceives an object to be in the field. This was a tough concept for me to grasp. Every small feature in a field generates light rays that enter every portion of the objective and leave every portion of the exit pupil. The positions that light rays leave the exit pupil are completely unrelated to the positions of objects in the field. It is the angle that the rays leave the the exit pupil that determines the perceived position of the object.

Apparent Field of View (AFOV) is the perceived angular size of the field. Apparent field of view is independent of magnification. Thus, a 7x50 could have a 50° AFOV and a 25x100 could have a 75° AFOV, or vice versa. Binoculars with a larger AFOV allow light rays of greater angles to leave the exit pupil, which we then perceive as objects farther from the the field center.

Apparent field of view is normally calculated using the eyepiece focal length and field stop, but neither specification is available for most binoculars. So, we use the short cut of multiplying the binocular's true field diameter by its magnification. A 10x binocular with a 6.3° true field would have a 63° AFOV.

Eye Relief is the distance behind the eyepiece lens at which your eye's pupil completely captures all the light rays leaving the exit pupil at all angles allowed by the AFOV. If you move your eye a few millimeters inside or outside of the eye relief distance, rays leaving the exit pupil at the steepest angle will miss your eye's pupil completely and you will perceive a smaller field of view.

Binoculars that have generous (15-20mm) eye relief and adjustable eyecups are the most versatile. Non-eyeglass users can adjust the eyecups outward to position their eyes at the eye relief distance while eyeglass wearers can adjust the eyecups inward to allow room for their glasses.

Exit Pupil Illumination is a measure of what percentage of the total light rays entering the objective actually make it out the exit pupil as a function of distance from field center. Distance from field center is determined by the angle that light rays enter the objective. If a distant object is centered in the field it generates rays that come straight into the objective. These are called on-axis rays because they are parallel to the optical axis. Objects away from the field center generate rays that enter the objective on an angle and are called off-axis rays.

Most binoculars pass 100% of on-axis rays through to the exit pupil and are said to be 100% illuminated at field center. When you move the binocular well away from your eyes to examine the exit pupils, you are seeing only rays entering the objectives very nearly on-axis, because off-axis rays are flying by your head to the left and right. Thus an exit pupil that appears to be chopped off indicates that the binocular will not 100% illuminate even the field center.

Quality binoculars will continue to pass 100% of mildly angled rays from the objective to the exit pupil. However before rays reach the angle corresponding to the field edge, almost all binoculars fall below 100% illumination. For example, a binocular with 6° true field should be able to transmit all rays entering the objective up to an angle of plus/minus 3°. If 2.4° degree rays falling on 30% of the objective area do not make it to the exit pupil, then the binocular would be 70% illuminated at the 80% (2.4/3) field radius.

Fortunately, our brains have a difficult time distinguishing a 30% drop in brightness, so most would not notice the difference between the center and edge of the field.

I hope this helped more than it confused.

--------------------
Clear skies, Milt

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KennyJ
The British Flash

Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
Re: Binocular Exit Pupil Primer [Re: milt]
#874730 - 03/17/06 06:12 PM

Milt ,

That is such a masterful piece of explanatory writing that I hope it proceeds to " PASS GO , without collecting £200 " and is sent straight , not to JAIL , but to BOTH the BEST OF section AND to the newly created " BEGINNER'S GUIDE " .

Regards , Kenny

--------------------

Milton Wilcox R.I.P

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Carpal Tunnel

Reged: 01/17/05
Posts: 2447
Loc: Connecticut
Re: Binocular Exit Pupil Primer [Re: KennyJ]
#874764 - 03/17/06 06:34 PM

milt,

Nicely done. Are you referring to true FOV when you refer to the focal length and the ep field stop?

Peter

--------------------
Peter

Telescopes 25 - 318 mm
Binoculars 12 - 100 mm
Microscope 50x - 1000x

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EdZ
Professor EdZ

Reged: 02/15/02
Posts: 18806
Loc: Cumberland, R I , USA42N71.4W
Re: Binocular Exit Pupil Primer [Re: pcad]
#874800 - 03/17/06 07:05 PM

This is a better primer than my exit pupil link that I pasted in the beginners' link. I'll leave mine to the advanced researcher and replace it with a link to this one.

edz

--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21

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btschumy
Vendor - Southern Stars

Reged: 04/13/04
Posts: 1708
Loc: Longmont, CO, USA
Re: Binocular Exit Pupil Primer [Re: EdZ]
#874863 - 03/17/06 08:23 PM

Very nice job Milt! The "Field Position" concept is indeed a tough one. Even though I think I've fully understand it, I still find myself occasionally making errors when thinking about it.

--------------------
Bill Tschumy
Southern Stars

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edcannon
professor emeritus

Reged: 11/19/03
Posts: 693
Loc: Austin, Texas
Re: Binocular Exit Pupil Primer [Re: btschumy]
#875185 - 03/18/06 12:49 AM

Nice article. One very minor point -- Oberwerk (any other?) calls eye relief "exit pupil distance". (I've also seen "long eye relief" called "high eye point".)

Possibly might mention the diamond-shaped exit pupils that indicate BK-7 prisms. There used to be a very good photo of this online, but I can't find it anymore.

Possibly also might consider mentioning that an (the only?) advantage of exit pupils larger than the eye pupils is that it's easier to keep the exit pupils in place -- which can help when the observer or the target is moving. But for astronomy, of course, that's not much of an issue (maybe on a windy night, or with extra shaky hands).

Ed Cannon - Austin, Texas, USA

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KennyJ
The British Flash

Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
Re: Binocular Exit Pupil Primer [Re: edcannon]
#875348 - 03/18/06 05:22 AM

Since LIGHT is a form of energy , and so much of it is being WASTED when we use any binocular with an exit - pupil larger than our pupil size , I wonder if THAT is why binoculars with exit - pupils of 4mm and above seem to COST so much ?

Perhaps it is due to " energy tax " which governments impose on manufacturers , but which manufacturers keep quite about ?

Kenny

--------------------

Milton Wilcox R.I.P

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milt
professor emeritus

Reged: 09/13/04
Posts: 603
Loc: Arizona
Re: Binocular Exit Pupil Primer [Re: pcad]
#875826 - 03/18/06 02:04 PM

Quote:

Are you referring to true FOV when you refer to the focal length and the ep field stop?

Hi Peter,

No, I was referring to apparent field of view. In the equation:

FOV = (57.3 x field stop diameter) / focal length

You get TFOV if you use the objective focal length and AFOV if you use the eyepiece focal length. I was using the latter.

Thanks,
Milt

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milt
professor emeritus

Reged: 09/13/04
Posts: 603
Loc: Arizona
Re: Binocular Exit Pupil Primer [Re: EdZ]
#875837 - 03/18/06 02:10 PM

Quote:

This is a better primer than my exit pupil link that I pasted in the beginners' link. I'll leave mine to the advanced researcher and replace it with a link to this one.

Hi Ed,

Thanks much for this compliment. And thanks also to Kenny and others for your kind words and comments. Just writing this down helped me to cement it in my own mind.

Best regards,
Milt

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KennyJ
The British Flash

Reged: 04/27/03
Posts: 20139
Loc: Lancashire UK
Re: Binocular Exit Pupil Primer [Re: milt]
#876068 - 03/18/06 05:26 PM

< Just writing this down helped me to cement it in my own mind. >

Indeed , that's a very useful thing to do , Milt .

RE - READING these things from time to time helps , too !

Kenny

--------------------

Milton Wilcox R.I.P

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Anonymous
Unregistered

Re: Binocular Exit Pupil Primer [Re: KennyJ]
#1130707 - 09/03/06 10:06 PM

I just read this and want you to know it was very good, imo.

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milt
professor emeritus

Reged: 09/13/04
Posts: 603
Loc: Arizona
Re: Binocular Exit Pupil Primer [Re: ]
#1130747 - 09/03/06 10:39 PM

Thank you for your kind words, Gabby. Thanks to Ed it is still "findable" after 6 months.

--------------------
Clear skies, Milt

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jdonner
member

Reged: 08/20/08
Posts: 28
Re: Binocular Exit Pupil Primer [Re: milt]
#2665775 - 09/27/08 04:15 AM

Your post is 2 years old Milt, but I see that you're still posting here this month, so allow me say "thank you", very useful!

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