# Thread: 8x40 vs 7x50

1. The (EP^2)*M (or EP*D) formula is interesting, but should not be taken at face value.

Let's take two binos that yield a value of 320:
What would you prefer for a night under the stars: 20x80 astronomy binoculars or 5x40 opera glasses...?
Last edited by Johannes; 05-13-2011 at 04:33 AM.

2. Glad I could be of some assistance.

3. Hi Johannes, its true what you say, and I think nearly all people would opt for the 20x80's. However, if some were wanting to familiarise themselves with the constellations they might find hand-holding these mothers and their approx 3 degrees FOV a bit limiting! (initially atleast!) I've no idea what the FOV's are for opera glasses (Probably not much) I think the equation can be a useful guide though, for example where I gave the 20x50's a hard time earlier with my review. They only yield 125 in the EP squared*M equation, giving a pretty good indication of how bad they are for astronomical use! (In my opinion). As always, we need to use common sense.

4. ## Re: 8x40 vs 7x50

Originally Posted by Arny Moots
What a fascinating bit of research! The only real way to tell if a binocular is better than another is, of course, to get out there and test the things!

The fact that for you the 7x50s be the brightest means that you are better with a 7mm exit pupil.

So I wonder how true it is about the iris shrinking to 5mm by middle age?
The issue of pupil dilation comes up all the time in forums reviewing binoculars. Let me state that it isn't a given. For most people, the pupils will lose a couple of mm by late middle age, but some only lose a little and some lucky people in their 60s can still achieve very close t0 7mm. The idea that once you reach middle age (when is that today?) you can no longer use the advantage of a 7mm exit pupil needs to be treated with a bit of caution. Most healthy people in middle age will achieve a bit more than 5mm, but it may take a little longer.

What I hope people will realise is that you need to try the binoculars that suit your use, and if you prefer the 7x50s and you're over 40 then trust yourself, rather than what is repeated without correct analysis of the evidence. Check out the evidence in the charts in this site Pupil size, large pupils and LASIK: WARNING!!!
If the 10x50s work better for you, then it may well be that you are in the majority of people in your age group that suits them more.

5. ## Re: 8x40 vs 7x50

Great great report - i am looking into something on the order of the 8 x 40's and would love to look through them, want them for wide field constellation work

6. ## Re: 8x40 vs 7x50

Originally Posted by tony884
Hi JG,
Both are excellent replies, thanks. I now own the 7x50 Action EX
Originally Posted by admin
I recently got the 7X50 and I think they are a great balance of wide-field views, steady views, and price (for me in Australia at least as other binoculars are outrageously priced and many are junk).
Congrats on your new binoculars. I prefer 7-power and a 7mm exit pupil for ease of holding, so I've been thinking about the Action EX 7x50. My first astro bino was the Action 7x35,and its 9.3° FOV spoiled me. For that reason, as well as optical quality, I'm also considering the Williams Optics 7x50 ED.

Originally Posted by David Bea
The issue of pupil dilation comes up all the time in forums reviewing binoculars. Let me state that it isn't a given. For most people, the pupils will lose a couple of mm by late middle age, but some only lose a little and some lucky people in their 60s can still achieve very close t0 7mm. The idea that once you reach middle age (when is that today?) you can no longer use the advantage of a 7mm exit pupil needs to be treated with a bit of caution. Most healthy people in middle age will achieve a bit more than 5mm, but it may take a little longer.

What I hope people will realise is that you need to try the binoculars that suit your use, and if you prefer the 7x50s and you're over 40 then trust yourself, rather than what is repeated without correct analysis of the evidence. Check out the evidence in the charts in this site Pupil size, large pupils and LASIK: WARNING!!!
If the 10x50s work better for you, then it may well be that you are in the majority of people in your age group that suits them more.
Thank you for posting this. I'm 45 and much prefer my 8x56's exit pupil to that of the 7x35 and 8x42 I used to have. It's more forgiving of shakiness, and the 8x56 gives nicer views, even of my orange-red zone skies.

Originally Posted by johnrfeeney
Great great report - i am looking into something on the order of the 8 x 40's and would love to look through them, want them for wide field constellation work
If you want a wide FOV for constellation work, consider the Action EX 7x35 too. I could fit entire small constellations and several stars of larger constellations in the view. The short barrels put all the weight in your hands, not hanging out front, which makes the bino easy to hold for long periods of time.
Last edited by Lou3; 08-24-2013 at 12:41 AM.

7. ## Re: 8x40 vs 7x50

Lou3,

I posted this in another thread, but I'll repost here as it's very relevant to decision making:

If you are wanting to know more to make the decision about the right magnification for you (the brand is another matter and definitely needs the buyer to try lots first), it's usually said that a pair of 10x50s will enable you to see around half a magnitude more than 7x50s. That's actually a significant improvement for Astronomical purposes. For daytime use I find that the increased magnification is much less beneficial i.e. the difference between the naked eye and 7x or 8x is huge, but jump from 7x or 8x to 10x and it is slight - but with tremor!

Another point is that a mount, or a truly, rock steady grip will add another half a degree to the magnitude of objects revealed. Again, quite significant for astronomers. So it's fair to say that a 7x50 will possibly nearly equal a 10x50 that isn't mounted i.e. the benefit being cancelled by the tremor for some people. However, a mounted 10x50 could be much, much better. In practice I find that too.

Being a bit lazy and spontaneous, and living in a place where the winter nights are bitterly cold - often - the thought of being out for a long time, setting up a tripod and mounting, takes some pleasure out of viewing when I could be sweeping the heavens with 7x50s or 8x40s for a quick fix. As I said earlier, if you really feel that you might be missing out on some fainter objects, etc, then you are generally better to get a telescope instead or as well - which is the ideal."

8. ## The Following User Says Thank You to David Bea For This Useful Post:

Lou3 (08-24-2013)

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