Subject: Binoculars
Date: Wed, 28 Mar 2007 15:10:06 +1000
On Wed, Mar 28, 2007 at 11:02:46AM -0800, Lloyd Nielsen wrote:
> I think the first thing when purchasing binoculars is to decide where 
> you are going to be doing most of your birding. That is in high light or 
> low light situations etc. If in low light (e.g. rainforest) you need the 
> most light you can get through your binoculars i.e. a high light value. 
> To work this out, multiply the magnification by the exit lens and square 
> it e.g.  7x42s gives a light value of 36 which lets in alot of light; 
> 10x40s gives a light value of  16 - moderate light; 16x50 gives a light 
> value of 9 - difficult in dark situations. The higher the light value, 
> the better your binocs will be in low light situations - and they will 
> be excellent in other situations as well. I think this is far more 
> important than comparing brands - Leica against Swavarosky etc and also 
> more important than magnifications - 7, 8, 10 etc. I would sacrifice a 
> little bit of magnification for alot more light every time. Weight is 
> also another very important consideration - often weight goes up with 
> higher magnifiactions.

This sounds like the square of the exit pupil of the binoculars;
see "Image brightness and clarity" to see why this matters 
(regardless of brand, lens coatings etc.):

Note that as you get older, your eyes' maximum pupil dilation
(how wide they open) diminishes.  In dark conditions, you want
the binoculars' exit pupil to be as close to your eyes' as 
possible to avoid wasted light - bigger isn't necessarily better: 

   Paul Taylor                           Veni, vidi, tici -
                    I came, I saw, I ticked.

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