10x50 or 8x40?

[David55]

I 'need' to get some binoculars to use when I've only a few observing minutes available. I've been using some 6x21 bins which are fine for seeing fairly bright targets like Andromeda, but feel I need a bit more aperture. I don't want to spend much, so have decided on the Olympus DPS1. The question is whether I should get the 10x50 or 8x40? My preference is the smaller binocular because it is lighter, smaller and the lower magnification will help reduce the shakes. So my question is whether the smaller aperture would make much practical difference.

My simple calculation is that for one side the 50mm lens has an area of 1964sqmm, whereas the 40mm lens has an area of only 1257sqmm or 20% less. But -and here's the bit I might have wrong- the smaller binocular only magnifies by x8, 20% less than the x10, and since more magnification would reduce the brightness of the image, I draw the conclusion that whatever I'm looking at will be the same brightness in each binocular, but smaller in the 8x40.

Am I right?

Thanks for any advice. David

[Special K]

Tough questions, I can't answer unfortunately.  However, I'm in the market in the same range as you and have settled on 10x50's.  Held the Pentax 7x50's second-hand and they are comfy.  Not sure about 8x40's but they will likely be roof prism which is more money typically.  Definitely lighter though.  What's your budget?  I was hoping for £100 but the Pentax are £146 at FLO.   They do the best deal of that spec and brand incidentally. 

There are some great guides on Cloudy Nights-  the following is an excellent article because he tells you what you can expect to see with various specs:

http://www.cloudynights.com/item.php?item_id=1634

[Moonshane]

brightness will be governed by exit pupil which in the case of bins is aperture / magnification so the answer is the same in this case as yur own calculation - 5mm so I agree they will have an image as bright as each other. personally, I find 8x bins much easier to hand hold than 10x bins and on this basis and the wider field I'd go for the 8x.

the difference in magnitude seen will be minimal too.

[Ewan]

10 x 50's would be my bag, there is not a lot in it to be fair, i think the benefit of the extra light gathering V's the lower mag would be beneficial overall. 

Good luck with your selection though.

[BinocularSky]

Theoretically the image brightness will be the same. The advantage of the 8x40 is, as you surmised, that it is much easier to hold steadily. However, if you can hold a 10x50 reasonably steadily, the increased magnification will show you more and, except under pristine skies, the image contrast will be slightly better.

Sent from my GT-I9100 using Tapatalk 2

[ronin]

You would see no difference in brightness, they come out to a meaningful amount the same. Both will deliver an exit pupil of 5mm, which is generally considered to be good for night use.

Using the birding world as a reference they seem to use 8x42's as an almost standard set up and there must be a reason for this. The binocular market in the birding area way exceeds that of astronomy.

I suspect that the main reason is simply ease of use - weight, size, performance etc - compared to light gather, cost. Remember that the extra 4-5mm at the edges are where the aberrations come from, so a good bigger objective costs more.

I have 2 sets of binoculars I use and both are 8x42's and they are very good. Another aspect is there is a good range of 8x42's to choose from.

[ollypenrice]

I don't use binoculars because I want aperture. I have telescopes for that. I used 10x50 for years before trying some 8x42s and found I greatly preferred the smaller ones. The increased stability of the view more than made up for the drop in resolution. I'm now firmly of the 8x42 camp, though maybe when I was younger and stronger I could hold 10x50 better than I can now.

As stated, the highly experienced birding community favour 8x42 as well.

Olly

[edjuh]

My choice would be 10x50, but I am quite firm (both in size and grip)

There is much to be said though for a monopod to be added, or some other construction which helps the steadyness.

If your usage for the bino would be more allround, and portability is another factor, than 8x40 or 8x42 is your main choice.

Could I suggest you go for a new 8x40 and go for a second hand 10x50 or 12x50 which could probably deprive you some 20 pounds at most ?

I recently bought a secondhand Asahi Pentax 10x50 for a fiver  (actually 8 euro's)

[David55]

Thanks for the answers everyone. My mind is now made up and I'll be going for the 8x40 because lightness and portability are more important to me than magnification. Of course, I'm now rethinking my Olympus choice......

[Special K]

Well done David and sounds like it will now be down to how much you spend. I reckon I might get a second pair in the future but have settled on 10x50 and see how I go. Happy Skies!