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I'm not sure I get this exit pupil thing

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#1 Photon

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Posted 27 June 2006 - 09:02 AM

Yes, I know that the large the exit pupil the brighter the view. 7x50s would have an exit pupil of 7+mm. But, I'm 62 and probably have 4 or 5mm pupils. So, does that mean that there is NO POINT in me buying binocs with more than a 5mm exit pupil? Will a larger exit pupil look no different to me than a 5mm one? If so, an 8x42 binoc would be just fine wouldn't it?

Pete
PS -- How can I measure my pupil size when dark adapted?

#2 btschumy

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Posted 27 June 2006 - 09:20 AM

First off, you're right that you should measure your exit pupil rather than assuming it is a certain size based upon your age. Check out this thread for info on measuring your exit pupil.

Generally, you're right that if your eye pupil only expands to 5 mm then it makes little sense to buy a binocular with an exit pupil larger than 5 mm. You are just paying for wasted aperture and weight.

However, some members on this list are big fans of oversized exit pupils. They claim it makes for a more relaxed eye placement and easier viewing. There is probably some truth to this. In addition, since most binoculars tend to vignette light entering along the periphery of the objectives, having an oversized exit pupil will help ensure that the entrance pupil is fully illuminated (pretty sure that last statement is correct. right, Edz?).

#3 Pinewood

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Posted 27 June 2006 - 09:22 AM

A larger exit pupil will differ from a smaller one, even if your eye pupil is smaller than the exit pupil. You will have an easier time lining up the objectives, eyepieces and your own eyes. There will be a "picture window" effect which allows the observer to find objects, a little more easily.
If you were observing from the deck of a ship, this could be useful. That is why the 7x50 is generally considered a good mariner's binocular.

Incidentally, in full daylight, the eye pupil is generally smaller than 5 mm.

Clear skies,
Arthur

#4 BluewaterObserva

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Posted 27 June 2006 - 09:41 AM

My favorite hand held binos are my 11x80's...

Exit pupil and comfortable observing is somewhat just individual preference.

#5 Joad

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Posted 27 June 2006 - 09:41 AM

According to Alan MacRobert in Sky and Telescope (July 2000, p.63) a larger exit pupil produces more "surface brightness," but "has nothing to do with how bright any particular" object will appear. It does not "brighten individual objects," and also "squeezes more light pollution into your view."

I've been wondering what my exit pupil was, so I asked my ophthamologist to measure it a few weeks ago when I got my new reading glasses. I expected that it would be about 4mm (I'm over 50), but it was 6.5mm. A nice surprise.

EDIT: NO DILATING DROPS WERE INVOLVED IN MY PUPIL MEASUREMENT

#6 EdZ

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Posted 27 June 2006 - 11:30 AM

However, some members on this list are big fans of oversized exit pupils. They claim it makes for a more relaxed eye placement and easier viewing. There is probably some truth to this. In addition, since most binoculars tend to vignette light entering along the periphery of the objectives, having an oversized exit pupil will help ensure that the entrance pupil is fully illuminated (pretty sure that last statement is correct. right, Edz?).
Bill T


A larger exit pupil will differ from a smaller one, even if your eye pupil is smaller than the exit pupil. You will have an easier time lining up the objectives, eyepieces and your own eyes. There will be a "picture window" effect which allows the observer to find objects, a little more easily.
If you were observing from the deck of a ship, this could be useful. That is why the 7x50 is generally considered a good mariner's binocular.

Incidentally, in full daylight, the eye pupil is generally smaller than 5 mm.

Clear skies,
Arthur



My favorite hand held binos are my 11x80's...

Exit pupil and comfortable observing is somewhat just individual preference.
BWO



According to Alan MacRobert in Sky and Telescope (July 2000, p.63) a larger exit pupil produces more "surface brightness," but "has nothing to do with how bright any particular" object will appear. It does not "brighten individual objects," and also "squeezes more light pollution into your view."

Joad



This topic always draws out some intersting comments and perspectives. I'd like to comment on each of these remarks above, as they are all related. Rather than refering to each person who commented above, you can see those comments, I will just reply.

Larger exit pupil does make eye placement easier. You would have two 7mm exit pupils to adjust to your two 4-5mm eye pupils, giving you a leeway of a few mm in IPD. However, if you don't center the exit pupils to your eye pupils IPD, you will be using the outer edge of the exit pupil to transmit light into either one eye or possibly both eyes and this outer portion of the exit pupil is vignetted in almost all binoculars. Therefore, it would be true that using a larger exit pupil, only if centered to your IPD, could provide you with an unvignetted view. Fact is though, if your eyes are only 5mm, this is an unvignetted view from a smaller effective aperture.

If you have 5mm eyes and use a binocular with a 7mm exit pupil, then your 5mm eye pupil is reducing the effective aperture of the binocular. A 10x70 binocular being used by a person with 5mm eye pupils becomes a 10x50 binocular, no ifs, ands or buts about it. If being used by a person with 4mm eye pupils, then it becomes a 10x40 binocular. Advantages: you may be getting a wider field of view and you could possibly take advantage of the greater light delivered by the unvignetted central 5mm of the exit pupil, especially in darker skies. Disadvantages: it is almost certainly a heavier binocular and you may be able to get that wider filed of view with a lighter weight binocular.

A larger exit pupil does indeed produce a brighter image. That is true for a 7x50 vs a 10x50 and also true for a 10x50 vs a 10x70. However, for an individual to see the difference in brightness between a 5mm exit pupil and a 7mm exit pupil, they must have EYE pupils larger than 5mm. Otherwise the effective apperture rule above is in control. It does not increase the brightness of any particular object. You could however increase the image brightness with additional aperture.

In the case of the 7x50 vs 10x50, no additional light is gathered, but a slight increase in magnification delivers a slightly deeper maximum limit to the eye as a result of increased contrast with background. This increase in magnification could work in your favor to darken the background and make images stand out more.

Assuming pupils large enough to see all, In the case of the 10x50 vs the 10x70, the 10x70 gathers more light but also makes the extended sky background appear brighter due to the additional light gathered with no increase in magnification. In bright skies, this will work against you and result in washed out views. In dark skies it works for you, since the sky background is darker to start with and the additional light gathered does not show a brighter background. Therefore you gather more light but the images don't get washed out.

edz

#7 Photon

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Posted 27 June 2006 - 03:57 PM

Thanks, everyone. I did follow the thread about measuring exit pupils and determined that I'm at about 5mm. So, I guess my 10x50 Steiners are just about right for me. My only complaint now is that when hand-held the shake is a bit annoying. That might argue for a 7x or 8x instead. But, then, I'd not have the sky-darkening effect of the 10x... can't have everything. As for having more room to make eye centering easier, I can't say that I've noticed a problem with that. It seems I'm talking myself out of needing new binocs.

Oh, about getting the ophthamologist to measure it... If he's measuring it after putting in those dilating drops, I wonder if you get a bigger pupil that way than simply in the dark.

Pete

#8 EdZ

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Posted 27 June 2006 - 04:10 PM

Don't measure after drops, you'll get eye pupils up around 10-11mm.

If your Steiners are to shaky at 10x, you could try a 9x or 8x, but that won't always get rid of the shake. But it does sound lie the 5mm exit pupil is right for you.

edz

#9 patter1

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Posted 27 June 2006 - 04:50 PM

Yes, I know that the large the exit pupil the brighter the view.

The word 'brightness' is a tricky word.... it depends on which kind of brightness you're dealing with; surface brigtness or total brightness (light output).

Compare the views through a 7x50 and a 12x60. Despite having a smaller exit pupil (and thus lower surface brightness on non-stellar objects), stars will be brighter in the 12x60 due to the larger aperture. The darkened background sky due to the higher magnification will also help.

As for hand-held shakyness, don't overlook weight; in my experience it's as important a factor as magnification is.

#10 KennyJ

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Posted 27 June 2006 - 04:52 PM

Pete ,

Don't be TOO easily put off LOWER magnification than 10x if 10x proves too SHAKY .

Star gazing can still be a VERY enjoyable pastime through 7x binoculars , which have a wider true field of view as well .

I'm not one for naming stars , or even counting them , or measuring apparant distances between them in arc seconds .

In fact , very high on the list amongst the LAST things I want to think about when stargazing includes arithmetic , geometry , trigonometry and physics !

I've often spent an hour or two swapping between and comparing views of the night sky between 10x and 7x binoculars , and although MY personal impressions go somewhat " against the grain " amongst the majority of heavy hitters in this forum , for what it's worth , I happen to think the following :

The steadier , wider views , and often less pronounced optical defects seen at 7x , often outweigh the so called advantages of the 10x magnification .

If and when I'm REALLY concerned about seeing MORE DETAIL , I switch to either 15 x 70s or my 85mm spotting scope , which shows a surprising amount of detail between 30x and 40x , with NO collimation problems , plenty of eye - relief and and very few other optical unpleasantries , come to think of it .

Regards , Kenny

#11 Photon

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Posted 27 June 2006 - 10:04 PM

I do agree, Kenny, that lower magnification is not a bad thing. Sometimes I use my Nikon roof prism 8x32s and have been pleasantly surprised at how close it comes (for me) to what I see in the Steiners. To be sure, I don't see as much in part because the exit pupil is 4mm I suppose. But the steadier view compensates to some degree. Indeed, when I'm mainly using the telescopes, I'd rather hang the smaller binocular around my neck just for quick looks. But when I'm doing a more ambitious tour of the sky I'd rather use the Steiners.

Pete


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