Exit Pupil Vs. Aperture

[Refractordude]

Is this true? Click image and read the underlined. Thanks to you all.

[AntennaGuy]

Yes, I believe that is correct. But... the 25x100 binoculars will provide more magnification. And if you want 25x magnification, it is certainly better to have a 100mm objective than a 70mm objective! Caveat: I'm not an expert. Others on this forum with far more knowledge and experience may wish to chime in. Best regards.

[Bigzmey]

Yes, assuming your eye pupil can dilate to 7mm. Also, I find views with exit pupil above 5mm too washed out and lacking contrast and prefer 4mm exit pupil.

[notFritzArgelander]

Yeah but "brighter" is not the same as "more contrast". Smaller exit pupils will provide more contrast since the sky background gets darkened.

[DeanD]

Yeah, what they said.

It depends what you want the binos for: 7x50 with 7mm exit pupil will give you a bright image for sure, but a lot of objects will be next to invisible because the background sky will be too bright and the objects are too small. Contrast, surface brightness and object size will determine how well you see a lot of DSO's: so higher power with the same objective diameter will often enable you to easier see (say) galaxies or planetary nebula which are effectively invisible in the brighter, but lower power view.

Most of us don't have 7mm pupils any more either: so some light is wasted and you might find that 7mm exit pupils give you no "brighter" view than 5mm, especially in suburban skies when your eyes can't fully dark adapt.

A lot of expert bino observers will tell you that an easy way to determine a binocular's over-all suitability for astro viewing is to multiply the magnification by the objective lens diameter. Usually, the higher the number the better. 7x50's give you 350, while 11x70's give 770, 25x100's give 2500, etc. (Mind you, the naked eye view gives a number of 1x7=7 (maximum): so any binocular will give you amazing views compared to nothing at all!!!)

As for telescopes and eyepieces there are lots of factors involved, and no one instrument is suitable for everything...

Many people enjoy the portability of binoculars and hand-holding, but of course the higher the power and the larger the binocular the harder this becomes.

There are lots of articles, books, and discussions out there about all this, so that is enough from me for now!

All the best,

Dean

[russmax]

Is the brightness proportional to the objective's diameter or area? That would change my answer about the correctness of the underlined text.
--Russmax