Binoculars w/reduced effective apertures

In the thread "Bresser Spezial Jagd 9x63 binoculars" yet another case
of binoculars with reduced effective apertures has surfaced.

What's to be done? At some of the current prices it's tough to
complain too loudly. Most people still get very good performance
considering the money spent. Yet, it's disappointing to discover that
one's new binoculars are, for example, 9x56 instead of 9x63.

In some cases the reduced aperture can serve to sharpen the image --
by masking the outermost region of the objective. In other cases the
center of the reduced aperture is substantially removed from the
center of the actual objective lens. In all cases light is lost --
not a good thing when it comes to faint, astronomical targets.

When I was searching the Internet for binocular information I stumbled
across one seller (perhaps it was Burgess?) who was selling a
binocular that they clearly stated was stopped down. They gave the
diameter of the objectives as well as the (smaller) effective
apertures; but unless something is done to force all sellers to
clearly state effective apertures, with penalties for lying, it's the
honest seller that gets penalized!

It would be nice if 'S&T' or 'Astronomy' would purchase a fair number
of commonly used astronomical binoculars and address this specific
issue. They could do one article on binocular effective aperture and
use the same binoculars for a later article on actual magnification
and true field of view.

Meanwhile, if I can get myself motivated enough and take time out from
other projects, I could work on detailed, illustrated, test procedures
that *anyone* could use to test effective aperture, magnification and
true field of view -- and put it on my web site. Unfortunately, I can
be a pretty darn good procrastinator -- too many projects -- never
enough time!

Meanwhile, my home made binocular mount for a pair of 25x100
binoculars has been completed and passed all indoor tests (It took
longer to finalize the design than it did to build the mount). It's
an alt-az mount with analog altitude and azimuth circles. An "L"
bracket can be attached that allows the use of smaller binoculars on
the same mount. Cooperative weather is needed before I can complete
my review of the Orion 25x100s.

Sketcher
To sketch is to see.

Meanwhile, my home made binocular mount for a pair of 25x100
binoculars has been completed and passed all indoor tests (It took
longer to finalize the design than it did to build the mount). It's
an alt-az mount with analog altitude and azimuth circles. An "L"
bracket can be attached that allows the use of smaller binoculars on
the same mount.


Hi Sketcher,

The mount sounds interesting. Any way you can put a picture=20
online for us to see?

-Florian

Sketcher wrote:
In the thread "Bresser Spezial Jagd 9x63 binoculars" yet another case
of binoculars with reduced effective apertures has surfaced.
[snip]
Meanwhile, if I can get myself motivated enough and take time out from
other projects, I could work on detailed, illustrated, test procedures
that *anyone* could use to test effective aperture, magnification and
true field of view -- and put it on my web site. Unfortunately, I can
be a pretty darn good procrastinator -- too many projects -- never
enough time!
[snip]

Hi Sketcher,

check this page out. Some pretty good work on stopped down binos:

http://www.kwic.com/~amj/bino.html

Sketcher
To sketch is to see.
--
I. N. G. --- http://users.forthnet.gr/ath/jgal/

Anton's article attempts to show what happens to the light as it
travels through the prisms, where it is possible the prisms act as
stops and again at the eyepiece field stops. It does not address a n
aperture stop ring at the objective lens. Nor does it clearly show the
bending of the converging rays before they exit at the back of the
objective lens. The rays bend and converge considerably before they
even exit the back end of the doublet objective. However, his article
lends some very good graphics to a difficult subject (which we have
discussed at length).

As I replied in the Bresser thread, there may be little or no light
cut off due to the ring placed behind the objective.

edz

Sketcher wrote:
In the thread "Bresser Spezial Jagd 9x63 binoculars" yet another case
of binoculars with reduced effective apertures has surfaced.
, it's disappointing to discover that
one's new binoculars are, for example, 9x56 instead of 9x63.


Well you have assumed because someone sees a ring that it has resulted
in a reduced effective aperture. But no one had questioned if there
was any evidence to support a reduced aperture. Verification of
magnification and size of exit pupil hadn't been done. Until then,
it's not quite fair to say "another example" has been found.

Meanwhile, if I can get myself motivated enough and take time out
from
other projects, I could work on detailed, illustrated, test
procedures
that *anyone* could use to test effective aperture, magnification and
true field of view --

At the Cloudy Nights Binocular Forum we have published test procedures
for just about everything you can measure in a binocular. A visit
there will provide you with a wealth of information.

edz

On Thu, 20 Jan 2005 15:19:50 GMT, "Florian"
wrote:

The mount sounds interesting. Any way you can put a picture
online for us to see?

Yes, there is a way. Give me a bit of time. I can't just take a
picture and post it. The background has to be right! OTOH, are you
sure you wouldn't prefer a sketch? ;-)

Meanwhile the following will give some idea of what the mount looks
like and how it works:

The mount is for a single, standing observer. The cradle that the
binoculars mount on is attached (at the eyepiece end) to a pair of
fork arms that extend upward and out from an old (but still reliable)
Criterion Field Tripod (originally used with an equally old Criterion
Dynamax 8 SCT). The bottom of the base of the mount is covered with
Ebony Star Formica (I had to buy a full sheet years ago for a much
older mount). The formica glides on three teflon pads that are on an
old, home made, tripod base plate. The same teflon padded base plate
and tripod has been used with a number of other home made astronomical
mounts as well as a rocket launch pad. (What can I say? It was a
really good tripod.)

The front of the cradle has an opening that a curved, (36 inch long
before bending it) 3/8 inch threaded rod passes through. The front of
the cradle rests on a couple washers on a home made, 3 inch diameter
hand-nut. Elevation is adjusted by turning the hand-nut (6.1 turns
equal 1 degree). Maximum elevation is 57.5 degrees (Comfort was a
factor. Besides, the nearest hardware store didn't have rods longer
than 36 inches.) The bottom end of the rod is epoxied into a 'holder'
on the front end of the mount's base.

The altitude pivot point coincides with my head's natural pivot point
(where the holes are in either side of my head! -- You know, near my
ear canals.) when my eyes are at the binocular eyepieces. In other
words, as the elevation of the binoculars changes, the eyepieces move
upward and outward to match the way my eye positions change when I
tilt my head to different elevations -- In even different words, my
eyes remain correctly positioned at the eyepieces regardless of the
elevation of the binoculars.

Most of the mount is made out of 5/8 inch plywood. The basic design
is very similar to the design I used with my old, home made, 20x80
binocular mount (a picture of which once was on my web site -- before
I decided to ban photographs from that 'sketch' site.); but I was able
to incorporate a few improvements this time around.

On the cradle is a 22mm high post. When my 25x100s are closed to
match my interpupillary distance the bottom of the mounting post on
the binoculars is 22mm higher than the bottom of the objective
housings. IOW, when I mount the 25x100s onto the cradle the binocular
mounting post rests on the cradle's mounting post while at the same
time the objective housings rest on the front portion of the cradle.
The threaded rod passes just in front of and midway between the 100mm
objectives.

I 'wasted' a fair amount of time considering a couple of other mount
designs before finally deciding to go with this design. I drilled a
pair of holes in the cradle for bolts to hold an old "L" bracket. I
can attach either my 20x80 or my 8x42 binoculars to the "L" bracket.
When these smaller binoculars are thus mounted their eyepieces end up
in the same position as those of the larger binoculars. The "L"
bracket must be removed before the 25x100s can be mounted, but the
cradle's mounting post for the larger binoculars is not in the way of
the "L" bracket nor in the way of the smaller binoculars.

Maybe you don't you need a picture now? ;-)

Sketcher
To sketch is to see.

On 20 Jan 2005 14:04:32 -0800, wrote:

Well you have assumed because someone sees a ring that it has resulted
in a reduced effective aperture.

Close enough. Actually my assumption wasn't based on the ring. It
was based on the statement that the binoculars effective aperture was
56mm. Perhaps Andy would be willing to perform the following
experiment and inform us of the results:

Focus the binoculars for infinity. Suspend a 56mm bar (the bar could
be cut from posterboard, etc.) right in front of and as close as
possible to the objective. Then examine the exit pupil. Adjust the
bar so that it bisects the exit pupil. Make additional adjustments
until you can either see light or a gap (while examining the exit
pupil) at *both* ends of the bar or no light or gap at either end of
the bar. In the former case your effective aperture is greater than
56mm. In the latter case your effective aperture is no more than
56mm. Repeat the experiment with bars of larger or smaller lengths
until you find a length that just barely prevents light from entering
at either end.

I performed this experiment using bars in two different orientations
(at right angles to each other) for each side of my 25x100s. The
results can show how much of the objective is used to form the image
of an object at the field center -- the on-axis effective aperture.

This procedure is based on the fact that the exit pupil is an image of
the system's entrance pupil.

Regardless of the effective aperture of Andy's binoculars, I suspect
that many binoculars fail to utilize the full aperture of their
objectives.

At the Cloudy Nights Binocular Forum we have published test procedures
for just about everything you can measure in a binocular. A visit
there will provide you with a wealth of information.

I just visited Cloudy Nights as well as the site that Ioannis
suggested. I've yet to find procedures quite like those that I use.
(One of the Cloudy Nights links didn't work for me.); but you're
correct. There is a wealth of information on the Cloudy Nights site!

Sketcher
To sketch is to see.

Sketcher wrote:
On 20 Jan 2005 14:04:32 -0800, wrote:

Focus the binoculars for infinity. Suspend a 56mm bar (the bar could
be cut from posterboard, etc.) right in front of and as close as
possible to the objective. Then examine the exit pupil. Adjust the
bar so that it bisects the exit pupil. Make additional adjustments
until you can either see light or a gap (while examining the exit
pupil) at *both* ends of the bar or no light or gap at either end of
the bar. In the former case your effective aperture is greater than
56mm. In the latter case your effective aperture is no more than
56mm. Repeat the experiment with bars of larger or smaller lengths
until you find a length that just barely prevents light from entering
at either end.

If you happen to cut the right size bar this would give you the
effective apperture. With several trials, you should get it.

The procedure we have outlined to test for the several aspects involved
in this a

You cannot simply measure exit pupil, since that does not confirm
magnification. So first magnification must be verified.

Test magnification. Observe a scale mounted on the wall across the room
with one eye to the binocular and one eye unaided. Use white cards,
business cards and tape work fine, to tape off a one inch unit.
Compare this magnified one inch to what is seen unaided. Use a movable
card to mark off the extent of what is seen unaided to compare to the
magnified unit. This would give you close focus magnification. An
attempt should be made to compare that magnification to a test at
distance, since magnification varies with distance of focus. About 3
out of 10 binoculars that I've tested vary by as much as 3% to 7%.
I've never found a binocular that had a lower magnification than
stated. But binoculars with higher magnification create smaller exit
pupils.

Measure exit pupil with binocular at close focus and then focused at
infinity. For this you need a vernier caliper, or at the very least a
precise scale that has divisions finer than mm and a magnifying glass.
You need to record exit pupil as accurately as you can, preferably to
0.1mm. A quick observation of the exit pupil is the first clear sign
that something might be amiss. If you have a 9x63 binocular and you
can quickly eye it up and only see a 6mm exit pupil, you know you have
a problem. But if a quick look seems to show you a 7mm exit pupil, you
may not have any problem at all.

Do the math with exit pupil and magnification. You can determine
effective aperture from these. When you are done, you will have
recorded measurements for three important aspects of you binocular.
edz

Yes, there is a way. Give me a bit of time. I can't just take a
picture and post it. The background has to be right! OTOH, are you
sure you wouldn't prefer a sketch? ;-)


A sketch would be nice. ;-)

-Florian

Dear Ed,
I would just add that the vernier for measuring the diameter of the exit
pupil should be located at the eye relief position to obtain the correct
value. Too close to the eyepiece gives too big a diameter and too far from
the eyepiece gives too small an answer.
you can point the binoculars at a bright sky ( not the sun) and see where
the bright spot (seen on a piece of paper for example) behind the eyepiece
is the smallest. This is the eye relief position.

Best Wishes,
anton