To: <>
Subject: Binoculars
From: "Peter Shute" <>
Date: Wed, 28 Mar 2007 12:55:43 +1000
Either way, I assume we're talking about twilight factor here?  From

"Optics Myth 9: Twilight factor is key to performance in dim light.
REALITY - The quality of the coatings is much more important.
You sometimes see "twilight factor" listed in a binocular description,
as a measure of the resolving power in dim light. This term was more
important years ago, before modern optical coatings were invented, than
it is today."

I'm thinking the situation is more complex these days.  It seems like
not only can 10x be designed to have a wider field of view than some 8x,
some low twilight factor binoculars can be brighter than some high
factor binoculars.  I guess that leaves us with trying them all out.
How you really compare brightness in a shop, I don't know.

I suspect that the "easiest" way out is to only test expensive ones.
Then if they seem ok they probably are.  In the lower end of the market,
where I'm looking, it seems like it's harder to work out what will be
satisfactory.  I'm thinking I would have to increase what I'm prepared
to spend to get into the "safe" range.

 wrote on Wednesday, 28 March 2007
12:32 PM:

> Good morning Lloyd.
> Shouldn't that be divide 42 by 7 = 6. Then square it for 36 ?
> And 40 divided by 10 = 4 which squares to 16 ?
> And 50 divided by 16 = 3.12 which squares to 9.75 ?
> Tony.
> -----Original Message-----
> From: 
>  On Behalf Of Lloyd Nielsen
> Sent: Thursday, March 29, 2007 4:33 AM
> To: 
> Subject: [Birding-Aus] Binoculars
> 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.

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