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Exit pupil spec'ed as smaller than objective/magnification

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#1 MikeTelescope

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Posted 12 December 2018 - 03:16 PM

I found a Bushnell Pacifica Xtreme 8x42 roof binocular on clearance, couldn't pass it up.  Good eye relief that I need for glasses with astigmatism correction, waterproof, and so inexpensive I won't be too upset when I break them doing something xtreme in Pacifica.  

 

The spec'ed exit pupil on the box is 4.72mm.  42mm/8=5.25mm.  What can I infer about the fact that the exit pupil is 10% smaller than objective/magnification?  Does this binocular function as a 8x38, or a 8.9x42?    



#2 tmichaelbanks

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Posted 12 December 2018 - 03:37 PM

You could always measure the exit pupil directly with calipers/ruler while pointing them at a light-colored, brightly-lit wall.

 

If they are measured at 4.72 mm, I wonder if that could be caused by masking off the prisms?  I honestly don't know, but others on the forum may chime in.


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#3 Rich V.

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Posted 12 December 2018 - 04:02 PM

It could go either way, or somewhere in between.  The only way to know for sure what you have is to first measure the effective aperture of the bino with the "flashlight test". 

 

https://www.cloudyni...way-to-measure/

 

Once you have the true aperture, divide it by the measured exit pupil diameter and you'll have the actual magnification and aperture.

 

You can best find the exit pupil diameter with a caliper that has a piece of frosted tape stuck onto the back of one jaw; position the tape away from the eyepiece at the distance where it shows the smallest, most focused circle of light (the exit pupil distance) and then measure.

 

Rich


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#4 TOMDEY

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Posted 12 December 2018 - 04:56 PM

I bought this superb Pupil Measuring Instrument via Astromart. It measures both pupil diameter and eye relief, conveniently and accurately!  You just put that round bottom end against the eyepiece (attached to the bino or telescope, of course) and there is the exit pupil with a mm reticule conjugate to it. Didn't cost much, and it works Great!  Tom

 

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  • 043 pupil measuring instrument.jpg

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#5 Cali

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Posted 13 December 2018 - 03:11 AM

TOMDEY

 

Do you have a link for this product?

 

- Cal



#6 SMark

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Posted 13 December 2018 - 03:31 AM

TOMDEY

 

Do you have a link for this product?

 

- Cal

It's a Dynameter and is generally considered optical test equipment and quoted by the job. I've not found any model with a published online price. Would be cool to have one of these laying around...


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#7 Mad Matt

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Posted 13 December 2018 - 05:14 AM

It's a Dynameter and is generally considered optical test equipment and quoted by the job. I've not found any model with a published online price. Would be cool to have one of these laying around...

I don't think it would be too difficult to make one yourself. All you need is an old eyepiece with diopter adjustment (focus) from a binocular and a millimetre grid acrylic ruler.

 

Simply cut a circle with the gradient crosshair in the middle and glue it to the field stop. Then use what ever you have lying around to fashion a short telescoping tube element that allows the gradient crosshair to be positioned at the exit pupil. You also get bonus points for frosting one face of the gradient crosshair you cut from the ruler grin.gif

 

I have not completely made one myself as I don't often feel compelled to precisely measure the exit pupil, but when I am... I simply use a 32mm plossel with the barrel removed, some plastic washers and a precision ruler grin.gif

 

Here are a few pictures to give a general idea of what I mean:

 

Here is the setup:

IMG 6671
 
 
And what I see through the eyepiece... of course it was a bit difficult to get everything lined up and hold the camera at the same time. 
IMG 6672

 

 

These are military 8x40's (Huet Marine) and as you can see, the 5mm EP is correct. 


Edited by Mad Matt, 13 December 2018 - 05:27 AM.

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#8 TOMDEY

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Posted 13 December 2018 - 06:31 AM

TOMDEY

Do you have a link for this product?

- Cal

It's a Dynameter and is generally considered optical test equipment and quoted by the job. I've not found any model with a published online price. Would be cool to have one of these laying around...

I don't think it would be too difficult to make one yourself. All you need is an old eyepiece with diopter adjustment (focus) from a binocular and a millimetre grid acrylic ruler.

Simply cut a circle with the gradient crosshair in the middle and glue it to the field stop. Then use what ever you have lying around to fashion a short telescoping tube element that allows the gradient crosshair to be positioned at the exit pupil. You also get bonus points for frosting one face of the gradient crosshair you cut from the ruler grin.gif

I have not completely made one myself as I don't often feel compelled to precisely measure the exit pupil, but when I am... I simply use a 32mm plossel with the barrel removed, some plastic washers and a precision ruler grin.gif

Here are a few pictures to give a general idea of what I mean:

Here is the setup:

And what I see through the eyepiece... of course it was a bit difficult to get everything lined up and hold the camera at the same time. 

These are military 8x40's (Huet Marine) and as you can see, the 5mm EP is correct. 

Yeah, could make one... I've done a lot of that. But such niceties often left on the to-do list for that time that never quite seems to come. It didn't cost much. Don't recall where I bought it, other than Astromart. It was possibly second hand?

 

The one I have is 6.5-9.5 inches long (depending on where you set the back-focus conjugate to the mechanical interface) and further comprises a relay lens. Its range is from approx 20mm outside the eyepiece mechanical top to 50mm inside (which feature is Great, something you can't measure with tissue paper). More out-range is available by swapping a different base or adding an extension. Very convenient!

 

Come to think of it, That relay lens inside is no doubt a repurposed 1x (?) microscope objective... which, optically is... a 1x relay lens!  So, the manufacturer just availed surplus microscope objectives and (military?) eyepieces for the optics. The eyepiece focus to the reticule is also nice.  Tom


Edited by TOMDEY, 13 December 2018 - 06:33 AM.

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#9 Pinac

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Posted 13 December 2018 - 08:56 AM

TOMDEY

 

Do you have a link for this product?

 

- Cal

 

https://www.trioptic...ment/dynameter/



#10 TOMDEY

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Posted 13 December 2018 - 09:36 AM

Germany. Yeah, that would be it. I don't see that they sell it directly or point to dealers... The one I have was made in Russia.  Tom



#11 mogur

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Posted 13 December 2018 - 06:45 PM

I found a Bushnell Pacifica Xtreme 8x42 roof binocular on clearance, couldn't pass it up.  Good eye relief that I need for glasses with astigmatism correction, waterproof, and so inexpensive I won't be too upset when I break them doing something xtreme in Pacifica.  

 

The spec'ed exit pupil on the box is 4.72mm.  42mm/8=5.25mm.  What can I infer about the fact that the exit pupil is 10% smaller than objective/magnification?  Does this binocular function as a 8x38, or a 8.9x42?    

I think it much more likely that the aperture has been stopped down for some reason rather than the magnification is increased.


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