Binoculars low light performance?

From: Harno (nothanks_at_nospam.com)
Date: 08/10/04 

Date: Tue, 10 Aug 2004 20:52:04 +1000

Hi everyone,

I need some advice on a pair of binoculars that are handy enough to carry,
but also good low-light performers. I understand that the lower the
magnification and/or larger the objective lens (all other things being
equal) the brighter the image, but I need reasonable magnification (around
8x) without the extra weight and bulk of 50mm objective lenses.

The standard to which I judge all binoculars is a pair of circa late-1970's
Pentax Asashi 7x50 porro-prisms (apparently manufactured for the Shar of
Iran's military officers prior to his rule ending?). Unfortunately these
are a little bulky for my intended use and don't actually belong to me
anyway. These bino's are fine low-light performers, and I really want this
new set I intend to purchase to match or better them, but with less bulk.

I've read that as long as the exit pupil is larger than 5mm (the size
limitation of the human pupil at full dilation), any more brightness is lost
to the human eye's perception. Is this correct? If this theory is correct
I can safely purchase the Pentax DCF SP 8x43's (exit pupil 5.37mm) I've been
looking at online. Unfortunately I cannot test out these binoculars in
person or for that matter anything similar as only budget brands and models
are available in stores where I live.

So I'm asking for the advice of forum members, is 25 years of lense
manufacture and coating advancement and the general high quality of Pentax
DCF SP 8x43's likely to make up for the higher magnification and smaller
objective lenses compared to 25 year old 7x50's under low-light conditions?

Best regards,

PTCB4EP

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