An old discussion : 7x50 v 10x50

[NGC 1502]

Hi all. As many will know, years ago the "standard" astronomy binocular was 7x50. But for a number of years the 10x50 has been the most often recommended, especially for older observers when their dark adapted eye pupil may not open large enough to take advantage of the 7x50s exit pupil of around 7mm. "Wasted" light was said to be the result, so best buy 10x50s where all (or most) of the 5mm exit pupil will enter the eye.

And that was what I fully accepted.

However, this thread http://www.cloudynights.com/topic/522401-7x50-binoculars-forgotten-favorite/has got me reconsidering, especially the part that mentions that the outer portions of the exit pupil may not be fully illuminated.

You need to read well down that thread to get a handle on the discussion. Not saying this is definitely correct, but worth a read I think.

Regards, Ed.

[Alien 13]

I could never understand why the change happened it is quite common for gun sights and military spotting scopes to have exit pupils above 8mm and even 12mm or more and the view is very relaxed and clear in fact the best ever astro view I ever had was with a 6x70 scope.

I have also found it odd that an ep/scope combo that gives an exit pupil of say 2mm is considered fine but not one that has 12mm exit pupil.

Alan

[BinocularSky]

Lots of theory doing the rounds on CN   but, when it comes down to it, the only true test is to put them to your eyes under your skies and compare. In my (6mm eye pupils) experience, the 7x50 wins in good, dark skies, but the 10x50 shows me more in the skies that I routinely observe under (but NGC 7000 & M33 are slightly easier in 7x50).

In short, there is no universally "right" answer: just choose what works for you, and enjoy it.

[Dinosour23]

10x50 first... 7x50 later, nothing wrong with buying a few pairs over time, they are different .. But every bino observer should have a nice 10x.

My fav combo when I am driving to a site is a 10x50 and a wide 7x35.

7x50.. It is useful but at 7x I better have a nice field and in modern ones you will not find anything worthwhile.

I think the astro equivalent of a 7x50 is a 11x70.

7x50 is a night only binocular, and dark site only.

If you are already taking the time to go to a dark site with a large exit pupil then grab a 10x70... 7x50 was never "made" with astro in mind, they are all sold as Mariners for use on boats out in the sea, even by companies with Astro product lines

[jetstream]

Just read some of the CN ideas...I'm not exactly convinced that when I use a 2mm exit pupil for instance that my effective aperture is reduced... it does work the other way round though, if the exit pupil is bigger than the eye can open, this does reduce effective aperture.IMHO.

[NGC 1502]

Thanks all for the feedback, more food for thought.........

Regards, Ed.

[Alfian]

I came close to buying some Strathspey binos a while back and its interesting that John comments about the 7x50 Marine that though they don't sell so well on line (compared to 10x50 I assume) when people actually try them in the flesh they like them. The 7x50s are almost certainly less than 50mm with probably nearer a 6mm exit than a 7mm but it makes you think and certainly supports Steve's comments about there not being a universal "right" answer.

Regarding the 2mm exit, I think this is often seen as an eyepiece "sweetspot" that gives just a nice useful balance of magnification, contrast, and resoultion.

[jetstream]

I thought that the 7mm exit pupil deal was the best thing going for twilight hunting. My riflescope is a 3-12x 50mm objective and when tested in near darkness it is surprising that the 7mm exit pupil did not show as bright as using a higher mag... my 1.5-6x 42mm will not show as bright as the 3-12x cranked up to higher mag. These are both VG scopes from the same manufacturer, same coatings etc. With respect to these scopes and also binoculars Swarovski and others use a thing called "twightlight factor" to describe this.

Unfortunately binoculars don't zoom, so much trial and error might be needed and also might differ if filters are used. The twighlight factor mentioned might be worth investigating for astro bino use.

Gerry

[paulastro]

The best thing to do is try both side by side on the things you like to use them for most, it will probably then be clear which is best for you.

For what it's worth, I see far more with 10x50s.  They give greater contrast and make better use of the resolving power of the lenses.  I didn't decide this by  considering any theoretical issues, I just tried them out side by side to see which gave me the best view. 

Anyway, if you compare them for yourself side by side I think you'll find it's obvious which you prefer.