Binoculars: Exit Pupil & Field of View?

Question

Good day!

I want to buy a binoculars for stargazing and have been making research as I am totally new in the field. I understood that Exit Pupil plays an important role in the quality of image. So... 
Is Exit Pupil fixed in Binoculars? The Exit pupil is the result of dividing the aperture by the magnification. For example: 10x70 and 20x100 exit pupil are 7.0 and 5.0mm. Using 20x100 if I use magnification up to 16 only, will it produce 6.25mm Exit Pupil? Or is it always fixed to 5.0?

Sorry for my totally amateur question;-)

Thanks!

Answer 1

Generally, the only time you need to worry about the exit pupil size is if your age is such were your pupil dilation is small.

https://info.synergeyes.com/hs-fs/hubfs/Images_Library/Landing_Page_Images/Duette%20Progressive%20Hubspot%20Pupil%20Size%20Chart.png?width=968&name=Duette%20Progressive%20Hubspot%20Pupil%20Size%20Chart.pngSo unless you're over the age of 50, it's not that much to worry about.Other factors that are worth considering about binoculars -1)  Get, or make binocular holders. You'll enjoy the views much better if you're sitting down and not holding them. This is even more true if you get large aperture binos that have some weight behind them. There are lots of different plans on how to make binocular mounts, and most are fairly easy to do (minimal handtools, and skill level).http://binocularsky.com/binoc_mount.phpSo make sure the binos have a tripod mount hole (usually 1/4" x 20 thread).2) Generally speaking, aperture is everything and low magnification is better than high magnification. A large aperture allows for more light to reach your retina, allowing you to see dimmer objects. And a larger aperture allows for better separation of closely spaced objects (such as double stars). A lower magnification can mean a wider field of view, which can be important when looking at open clusters or other large objects such as the Orion Nebula or Andromeda Galaxy.3) Do Not Get Zoom Binoculars! The vast majority are a waste of money. Narrower field of view, usually mediocre optics, and just more hassle than they are worth.The same goes for exotic-sounding optical coatings like red or gold tinting. Those are marketing gimmicks. Stick with clear optical coatings that really do help.4) if you wear glasses, pay attention to eye relief. This is the distance between the bino eyepiece to your eye. Longer eye relief is better us eyeglass wearers.Or, if you choose not to wear glasses while observing, get binos that have individual focusable eyepieces. I'm nearsighted and have a full diopter difference between my eyes.For more practical matters -1) Get away from the city lights. Light pollution simply destroys your view of the sky.Get out where it is really dark.2) Fun and comfort. Amateur astronomy is all about Fun and Comfort. If you're not comfortable, you won't have fun. And if you're not having fun, you will not have any fun.



A) Sit down when looking at stuff. You can get fancy and get an observing chair, or look into getting a drummer's throne or guitarist chair.

B) Dress warmly. May sound odd for summer observing, but you are not moving much so your body is going to lose heat. You might need a light jacket to ward off the late-night chill.



Also, bring snacks and water. Caffeine does constrict the pupils a little bit but otherwise is ok. That's your personal preference.

C) Take frequent breaks. Get up and move around. Blink your eyes - lots. The film over your eyes dries out if you don't blink. Take eye drops if you have something like dry eye.



3) if at all possible, join a local astronomy club. A club can be your single best resource for all sorts of astronomy information. Outings, speakers, learning from others, looking through other people's equipment, and just helps to make the hobby fun.

https://www.go-astronomy.com/astro-club-search.htm



4) Guide books, software, and observing programs. 

Arguably one of the best astronomy general-purpose guidebooks is the 'Giant Pocket Sky Atlas'

https://www.barnesandnoble.com/w/sky-telescopes-pocket-sky-atlas-jumbo-edition-roger-sinnott/1122726462



But there are other guides as well.

http://skymaps.com/store/cat01.html



For software, seems there are all sorts of apps to get. And quite a few of them are free, or low cost. The trick here is to set the thing up for your location, date, and time. Then you can make observing charts for the stuff you want to look at. 

https://astrobackyard.com/astronomy-apps-for-stargazing/



The Astronomical League has a wide range of observing programs for all sorts of skill levels and observing equipment. You do not need to be a member to work the lists either.

https://www.astroleague.org/observing.html



And there's so much more - learning to read the weather, keeping a logbook, trying your hand at sketching, and so on. but this missive has gone on far too long.



So good luck, and clear skies!

Answer 2

 For any binocular of fixed magnification, the exit pupil is fixed in size.

More important than that exit pupil size is the overall construction of the binoculars:

Wide field view depends on eye lens design.  More costly ones have wider view.  Better eye lenses have flatter fields (edge and center at same focus).

Coating of all lens and prism surfaces should be multi-layered to pass as much light as possible.

Prism size should be large enough to pass all the light from objective to eye lens.

Much more expense for stabilized system of view.  Tiny electromagnets swing the light path to compensate for the motion of the binoculars.  Jiggling of the view is stopped.  But you can instead spend money on a mount system and chair.

While viewing faint objects, dark adaptation is crucial.  At least 20 minutes in a blackout will get your visual purple active in your retinas.  Longer is better.  Eat some carrots.

No white lights around at all.  Too bright a red flashlight is not recommended, dim orange one is much better.   Get filters over all computer and phone screens.

Monthly column of binocular astronomy on Sky & Telescope can be seen on line.

Answer 3

What you want is the biggest front lens that you can get.   It is that lens that determines how much light you can pull in.   The bigger the lens the more faint stars you will be able to see.   You do NOT want much more than 15 power as you will be holding these in your hand and the more magnification you have the harder it is to stay on the one object you want to look at.   If you really want to look at the stars,  get a good Newtonian telescope instead of binoculars.