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Incorrect apertures and magnifications on binos?!?

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#26 MartinPond

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Posted 14 October 2014 - 11:54 AM

Ah, I see.  A pinhole as your eye's aperture.   Sure.

I misunderstood you method.  Very nice (if not the flashlight method).

And you are looking at something that's bright all the way across the field? (and hopefully, distant).

I'm guessing the moon can be great for that.  You need a lot of light, yes?  I don't dare suggest the sun.

 

If it passes through the whole instrument that's great.  I'm not sure how I could tell it was the obstacle or the objective.

(if it was cleanly cut down).    Can you tell because you see something more 'perfectly circular'?)

 

Bear in mind:  I am not questioning your method, I'm questioning shining a flashlight from a foot away from the eyepiece

or looking into the objective from a few feet away.


Edited by MartinPond, 14 October 2014 - 11:57 AM.


#27 GlennLeDrew  Happy Birthday!

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Posted 14 October 2014 - 12:47 PM

Martin,

Just to be clear, Jon is discussing a pinhole placed at the focal surface, with eyepiece removed. The pinhole is merely to assure that a smaller-than-iris beam of light is sampled by the eye. Without the pinhole, any relatively nearby obstructors cannot be seen in sufficiently good focus simultaneously with the more distant objective. This reduces sensitivity in determining at just what point the aperture is being clipped. This analagous to defining an 'edge' on a blurred shadow.

 

The practical benefit of the pinhole is its very great extension to depth of focus. The now much more sharply defined obstructor edge can be confidently located with respect to the objective edge, both being seen at essentially equal definition.



#28 MartinPond

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Posted 14 October 2014 - 01:00 PM

Right...even clearer.

Sounds great, but for the purposes of this thread:

----it is a lot simpler with a telescope than binoculars, since you would have to pluck out the eyepiece.

---that helps drive home the main point: it is a bit of a red herring since it doesn't involve nearby flashlights and the entire system.

 

In double-checking a technique, I stumbled on an even simpler way to check aperature...

----sit binoculars facing forward with a lit whitish background (ie, my mantlepiece)

----go 20 ft away and look directly at the face of a barrel (parallel) with some ~8x-10x binocs.

You will see(for most binoculars) what looks like a wide open tube looking at you.

This is true for Yosemite 6x30s, 8x30s, Sans&Streiffe 7x35 Hunter (a fave), 

and others that seem restricted with the flashlight. 

The 7x35 Fujinon/Bushnell Custom looks more like 33mm,

and the Meopta MeoPro 6.5x32 looks more like 30mm.  meh.

It can take a while to line up for the fullest circle.

The 10x50 Selsis are about....48-49mm.

 

This also turns out to be a great way to look for smutz on lenses and prisms.

I need to go at the Custom prisms again on one side.

 

Makes cool giant blue-grey wedges with BK7 binoculars as well.
 

re: pinhole:

On the telescope, I see what you mean about the 'obstructor edge'

...there is detail all around it and the artifact can be determined pretty easily,

if there is a very bright wall to look at.  Cool.  A good way to check stops

and blacking too.


Edited by MartinPond, 14 October 2014 - 07:45 PM.


#29 Jon Isaacs

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Posted 15 October 2014 - 11:38 AM

 

 

Bear in mind:  I am not questioning your method, I'm questioning shining a flashlight from a foot away from the eyepiece
or looking into the objective from a few feet away.

 

Martin:

 

What is interesting is that my laser technique and shining a flashlight from a foot or so away seem to provide essentially the same result..  

 

Just for the heck of it, I attached an image of the rig I use to measure the effective aperture of a telescope.  It's a laser collimator that is aligned via a acrylic collar to a 4.8 mm Nagler.  You can see the wide divergent beam pattern.  

 

Jon

Attached Thumbnails

  • Eyepiece and laser on floor cropped.jpg


#30 GlennLeDrew  Happy Birthday!

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Posted 15 October 2014 - 08:02 PM

That divergence is *good*, in that it ensures filling the entrance pupil with light.

 

Ideally, the divergence should be just large enough to do the job; a wider divergence for faster systems under test. Too large a divergence only dilytes the light, making for a dimmer emergent circle to measure.

 

The light entering the eyepiece is nicely collimated, and so the laser can even be touching the eye lens; distance here is no issue.

 

The use of a short f.l. eyepiece here eliminates the need for a beam expander. Relative to the eyepiece f.l., the entrant laser beam width is large enough to form a 'fat' light cone. In other words, the laser fills the exit pupil of the system and by extension fills its entrance pupil.

 

Red laser pointers often have 3-4mm beam widths, and so an eyepiece which delivers an exit pupil of a bit less than this are fine. Green LPs often have ~1mm beam widths, and so a an ~1mm exit pupil is better for these.


Edited by GlennLeDrew, 15 October 2014 - 08:06 PM.


#31 MartinPond

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Posted 15 October 2014 - 09:59 PM

I shined a laser into a 10mm eyepiece, and I only got an 8-inch spread at 2 feet.

Shining flashlights, I got anywhere from a few inches to 2 feet.

So...the divergence of flashlights varies enormously, and I know the

divergence of the laser beam actually varies in proportion of the laser beam width.

So....this not a standard quality of either source.

 

The laser through an eyepiece spreads and continues to spread.

It is divergent and uncollimated.

 

In optical engineering, light is only considered 'nicely collimated' when it

diverges at a tiny fraction of a degree, on an optical table.

In a slide projector, maybe a degrre or two. Spreading out a foot at 2 feet

would definitely be considered very un-collimated. 

 

 

This is collimated light:

http://en.wikipedia....ollimated_light

It is parallel...extremely parallel, not convergent or divergent.

 

I'm going by that definition.

I am also stating that a light cone inbound will experience different barriers than a light beam.

 

I'm just doing the science from my optics courses. 

'nicely collimated' light from a cone coming from a flashlight or a laser and eyepiece makes no sense,

being an obvious violation of the very definition of 'collimated light'.

Is this lore vs. science?

 

The difference (in aparent aperture) doesn't matter much in telescopes, but

it can be serious in binoculars.  It can make you believe that dozens of models

have been mysteriously short-sheeted in aperture even though all the size

and expense have been poured into a bigger lens. It sort of doesn't make any

economic sense.


Edited by MartinPond, 15 October 2014 - 10:12 PM.


#32 GlennLeDrew  Happy Birthday!

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Posted 15 October 2014 - 10:46 PM

Martin,

One must understand what's important in the so-called 'flashlight test' for aperture. It is this:

 

Rule #1. A sufficiently small near-point image of a light source must be formed at the common focus of objective and eyepiece.

 

Rule #2. To ease measurement of the emerging light bundle, the light *should* fill the exit pupil, so that it fills the entrance pupil and thereby provides a full disk not requiring light source movement.

 

All we are striving for is the sending of light through the system just as it transits during normal use, but in reverse. If the light entering the eyepiece is uncollimated, the emergent bundle is uncollimated by an amount equalling this times the inverse of the magnification. For example, if the light source as 'seen' by the eyepiece subtends 5 degrees, for a 10X system the emerging bundle is spread out by 0.5 degree. If one measures the disk edge from near the objective, the distance between screen and obstructor in the instrument helps to keep blurring of the disk edge minimal. For example, suppose the obstructor (front prism aperture, usually) and screen is 200mm. For a 0.5 degree divergence, the shadow edge is blurred by 1.75mm. One is then forced to estimate the point where brightness is 1/2 in order to assign the 'edge'. If we assume an error on this estimate of 0.5mm, for a 50mm diameter disk of light the uncertaintly is 0.7mm (an uncertainty of 0.5mm on each side results in the final uncertainty equalling the square root of the sum of the squares, or 0.707mm).


Edited by GlennLeDrew, 15 October 2014 - 10:52 PM.


#33 GlennLeDrew  Happy Birthday!

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Posted 15 October 2014 - 11:06 PM

The pain-inducing limits on the length of a post accommodated by my phone's text editing window force breaking this up...

 

The other consideration imposed by miscollimated light entering the eyepiece is the difference in its focused position with respect to the focus of the objective. This is why one should locate any non-collimated light source at least 10 eyepiece focal lengths from the eyepiece. This ensures a small enough difference in correct focus position of your light. (And one has already set to at least near to infinity focus.)

 

The distance of the light from the eyepiece controls location of its focused image with respect to objective focus. Whether the light is compact or wide, its image is focused at the same place. The anglar width of the light source as subtended from the eyepiece controls the extent of obstructor shadow blurring.

 

As long as these limitations are understood, and one appreciates their impact in any given setup, a non-collimated light can do the job, sometimes to a surprising degree. Over-persnickitiness is not required. ;)



#34 Jon Isaacs

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Posted 16 October 2014 - 08:07 AM

 

 

The difference (in aparent aperture) doesn't matter much in telescopes, but
it can be serious in binoculars.  It can make you believe that dozens of models
have been mysteriously short-sheeted in aperture even though all the size
and expense have been poured into a bigger lens. It sort of doesn't make any
economic sense.

 

Fast refractors can suffer from a pinched off light cone.. Improperly place baffles, focusers with drawtubes that are too long..  And since telescopes are used at high magnifications where every mm of aperture is important, where resolution is dependent on the aperture rather than the eye, a 70mm scope operating at 63mm is a significant limitation..  If one is splitting a close double star, the 10% difference in aperture can be the difference between making the the split and not making it.  BInoculars are operating at low magnifications where it's only a question of light gathering. 

 

There is economic sense in building inexpensive binoculars with 70mm objectives and an effective aperture of 63mm.  They are sold as 70mm binoculars rather than 63mm binoculars, few buyers are alert enough to know so from a sales point of view, they get the full benefit of the 70mm objectives.  The prisms are a significant portion of the cost of the binoculars and these can be undersized to save money without significantly affecting the sales. 

 

Jon


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#35 MartinPond

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Posted 16 October 2014 - 08:33 AM

Thanks for your patience, guys.

Having an economic case and technical easing eases my un-ease ;-

I see how the flashlight psuedo-cone can be 'groomed' at the eyeiece/exit-pupil.

I still love the mantlepiece trick (not hard once you figure how to light the wall)

and the long-distance observation, especially since I can also screen for cleaning

auctioned items, but I think it's likely I would confirm those not-70-but-63s myself.

Not that I want to seek them out...

I was set off long ago when someone made measurements on Leupolds that turned out to be bogus.

The look-at-objective-from-2ft-with-ruler trick is wildly off.

 

I've been de-persnickitized and re-gruntled.

I will put down the reference book and step back from the window.


Edited by MartinPond, 16 October 2014 - 08:34 AM.


#36 GlennLeDrew  Happy Birthday!

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Posted 16 October 2014 - 06:49 PM

Regarding the peering into the objective...

 

This method is a valid qualitative assessment for aperture clipping because the objective is a collimator--of the eyepiece-produced image and field stop ONLY. Other things not on the focal surface are not collimated and hence suffer parallax. And so any visual sightings into the instrument MUST have at the focal surface a fiducial. If not the field stop, a feature in the eyepiece-produced image serves. Indeed, for gauging aperture clipping at any point in the FOV not at the field edge, one can ONLY use some feature in that reduced image produced by a motionless, mounted instrument.

 

For assessing the more critical on-axis clipping of the aperture, some easily identifiable feature in the reduced image--a light fixture is ideal--is positioned so as to lie at the center. Then one's sight line is gradually moved toward the objective edge until that feature is appearing to just become clipped. The point on the objective through which the sight line passes marks the radius of the edge of the effective aperture.

 

If the eyepiece is set to infinity focus, this is reliable, and is not inherently sensitive to viewing distance. As your eye is moved from nearby to far from the objective, and this point of just clipping the central eyepiece-produced image is maintained, the eye moves through space along the marginal ray for the effective aperture (for the on-axis condition), which parallels the optical axis.



#37 MartinPond

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Posted 16 October 2014 - 10:00 PM

Another way to speak about the objective stop distortion is this:  the objective is a magnifier (the blooming of the parallax you refer to).

 

Comparing my two anxieties, here is an interesting fact:

(say, in a pair of 10x binoculars): The objective would be 10 times as sensitive to dispersion as the eyepiece,

sort of a counter-intuitive relationship.  Proving it is fun:

---from far away (roughly) play a laser across the face of the objective:

    you will see the dot coming out of the eyepiece waving around much more than the beam going in. It's a huge sweep.

---then play the laser (from far, yes) over the eyepiece. The beam coming out of the objective will only move sluggishly.

 

This explains why I am still persnickety about parallel light going into the objective, and I can be 10 times

less persnickety  about the cone going into the eyepiece.  I can budge to a foot or two near eyepiece,

but to be truly equal in low error, I can only budge to 10-20 ft on the objective ...

But that's just my choice.  I love looking in from afar. You get to use 2 pairs of binoculars!

 

That also explains why Ed Z's  sun-shadow-frame test  for collimation is 10 times more awesome (sensitive)

   than one might think.

 

Here's a typical adjustable-dispersion flashlight:

 

http://www.amazon.co...light with zoom

 

You can fiddle the central circle to paint the eyepiece without too much dispersion.  LED fever is almost as bad as binocular fever.


Edited by MartinPond, 16 October 2014 - 10:11 PM.


#38 Ptkacik

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Posted 17 October 2014 - 11:35 PM

FWIW, I just measured the clear aperture of two Oberwerks Ultra 28 x 110 and found them to be 107mm.

#39 MartinPond

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Posted 19 October 2014 - 04:41 PM

Sounds pretty close.  Dare I ask...the method?  



#40 samovu

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Posted 25 October 2014 - 02:13 AM

.......
 
Are the specs on binos accurate? Is this an industry-wide "problem"?  .......


Not that I'm suggesting the government intervene, but any of you around at the time remember the wild stereo amplifier or receiver power claims? I remember looking for the power (in watts) associated with the following condition. "20Hz - 20kHz, +/-3dB, both channels driven, with no more than 0.01% distortion (for example)" Oh yeah, and driving 8 ohms, not 4.

This was an example of the FCC specs that could be independently _measured_ using a common set of parameters.

When it comes to the binocular manufacturers specs or claims, it might be a guideline to keep in mind that some of the more respected (or "expensive") brands may be more accurate than others. Or at least one would expect it to be the case.

Clear views,
John

#41 MartinPond

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Posted 25 October 2014 - 02:05 PM

That's a great consumer example.  So many makers gave the 'peak power', not averaged RMS.

Specifying a specific method is often (Like ISO-nnn or ANSI-xyz or osme Euro Std.) used for integrity, for things like cars, furnaces, defibrillators,

TVs, stereos, toasters, eye-surgery electrical isolation, engine power, etc..

 

That's the biggest issue, isn't it?   The 'how-to' and the 'this-is' aren't clearly connected.



#42 samovu

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Posted 25 October 2014 - 03:04 PM

That's a great consumer example.  ..........

 

That's the biggest issue, isn't it?   The 'how-to' and the 'this-is' aren't clearly connected.

 

Reminds me of the shell game. Some companies give themselves more latitude when it comes to marketing.  :grin:

 

Sometimes a respected company has little choice but to blur the lines a bit, however. And it's because the consumer sometimes has unrealistic expectations set by so much marketing and specsmanship. One example I can think of is with headphones. Take a look at some of the sound level versus frequency graphs and reconcile it with the frequency range specification of a a pair of headphones and you'll notice one key parameter missing. When the advertised frequency range of a pair of headphones reads something like 30Hz to 25kHz, I think that there may be actually be some output at that range but at a very diminished level to say the least (as in not audible and detectable only with instrumentation). If they stated the usual +/- 3dB as the maximum deviation, the range would be more like from 300hz to 16kHz or something similar. But what consumer looking for bass response and buying on specs would buy that model?

 

Clear views,

John




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