Binoculars low light performance?

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

If your eye pupils are 6mm, then you are using 7x6mm = 42mm of
aperture on the 7x50's, and all 43mm on the 8x43 (since 8x6mm
exceeds 43mm,) hence the light gathering is essentially the same,
aperture-wise. If your eye pupils are less than 6mm---which they
probably are---you will have *more* aperture from the 8x43 than
from the 7x50 (40mm vs. 35mm if you have 5mm eye pupils, for
example.) In addition to that, presumably more modern coatings
and lighter weight.

Cheers -- Harald

"Harno" wrote in
[snip]
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

Harno wrote:

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


I have a pair of Pentax 10X42. I had the opportunity to try these as
well as Zeiss and Swarovski at the same time. I felt that the Pentax
were as clear and bright as the others at about half the price, but then
I'm not an expert on optics. I'm very happy with my purchase.

gls858

"Harno" wrote in message ...
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.

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?

Best regards,

PTCB4EP

The 5mm maximum eye pupil is an average. I'm 51, my pupils dilate to
6.5mm. I know younger guys with maximum pupils of 4mm.

If you have large eye pupils, larger (7mm) exit pupils in a binocular
can be more detrimental than advantageous if your use will be
primarily in light poluted skies. They take in so much background
light from the extended sky background, they tend to wash out images.
They will perform as intended under darker skies. Slightly higher
magnifications resulting in slightly smaller exit pupils can have the
effect of improving overall contrast.

However, if your pupils do not dilate to anything larger than 5mm,
then getting a binocular with a larger exit pupil is a waste. Your
maximum eye dilation will control and the binocular will act as if the
objective lens is smaller (an objective that would produce the exit
pupil determined by your maximum eye pupil size.

To make the right choice in binoculars you should know your own pupil
size.

edz

The 5mm maximum eye pupil is an average. I'm 51, my pupils dilate to
6.5mm. I know younger guys with maximum pupils of 4mm.


I'm 49 and have sub-4mm pupils. (I tell people it's from growing up
in the desert.) 10x42mm binoculars are the perfect handheld nighttime
binocular for me.

-Florian

OTOH, the decrease in effective aperture has no effect on resolution, which
is a function of actual aperture and the rules of optics. So, while objects
won't seem any more bright in the larger aperture, the sufficiently bright
will be better resolved.

-Stephen


"Harald Lang" wrote in message
...

If your eye pupils are 6mm, then you are using 7x6mm = 42mm of
aperture on the 7x50's, and all 43mm on the 8x43 (since 8x6mm
exceeds 43mm,) hence the light gathering is essentially the same,
aperture-wise. If your eye pupils are less than 6mm---which they
probably are---you will have *more* aperture from the 8x43 than
from the 7x50 (40mm vs. 35mm if you have 5mm eye pupils, for
example.) In addition to that, presumably more modern coatings
and lighter weight.

Cheers -- Harald

"Harno" wrote in
[snip]
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

Stephen Paul wrote:
OTOH, the decrease in effective aperture has no effect on resolution, which
is a function of actual aperture and the rules of optics.

Actually, it does--or would, if the magnification were high enough to
detect the loss in resolution. As it is, your eye is the limiting factor
on contrast transfer.

But if your retina had dense enough receptors, and your eye's optics
were well enough corrected, you would detect a loss in contrast due to
diffraction at your eye's pupil, and by an amount that corresponds to the
loss in effective aperture.

This was covered in a thread both here and on the Astromart birding optics
(w/o photos) forum a few months ago.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Stephen Paul wrote:

OTOH, the decrease in effective aperture has no effect on resolution, which
is a function of actual aperture and the rules of optics. So, while objects
won't seem any more bright in the larger aperture, the sufficiently bright
will be better resolved.

Yes, but rarely does the power in binoculars even approach the point where
their full optical resolution becomes visible (ie: a 50mm pair would have to
be used at in excess of 59x before you would even begin to see the diffraction
pattern). Thus, if the observer is using binoculars with exit pupils
significantly greater than the current eye pupil opening, they still may be
better off with a somewhat smaller aperture pair. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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OTOH, the decrease in effective aperture has no effect on resolution, which
is a function of actual aperture and the rules of optics. So, while objects
won't seem any more bright in the larger aperture, the sufficiently bright
will be better resolved.

-Stephen

Actually we went over this exact issue a while back and it is my understanding
that the when the eyes pupil is masking the exit pupil of the scope, it is
effectively the same as masking the aperture of the scope and that indeed the
resolution is reduced. I believe Brian Tung had a rather nice explanation which
I will not try to repeat.

jon

"edz" wrote in message
om...

The 5mm maximum eye pupil is an average. I'm 51, my pupils dilate to
6.5mm.

You better not say that too loud... did you see, "Minority Report?"

If you have large eye pupils, larger (7mm) exit pupils in a binocular
can be more detrimental than advantageous if your use will be
primarily in light poluted skies. They take in so much background
light from the extended sky background, they tend to wash out images.
They will perform as intended under darker skies. Slightly higher
magnifications resulting in slightly smaller exit pupils can have the
effect of improving overall contrast.

However, if your pupils do not dilate to anything larger than 5mm,
then getting a binocular with a larger exit pupil is a waste. Your
maximum eye dilation will control and the binocular will act as if the
objective lens is smaller (an objective that would produce the exit
pupil determined by your maximum eye pupil size.

To make the right choice in binoculars you should know your own pupil
size.

I remember "pupil gauges" used to be found in Sky & Telescope, but I
haven't seen one for a while.