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Old Tuesday 31st October 2017, 00:12   #1
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Pupil Size and IPD

I mentioned that I would report back after my ophthalmologist visit, but I lost track of which thread it was.

My pupils dilate to 6 mm at the office, and in response to the specific question "Is that the same as in a dark room?" he replied "It is larger." He did not say how much larger. I am guessing that the complete paralysis of the muscles, from the medication which dilates the pupils, allows them to open wider than they do in the dark.

I trotted off to the associated Optometrist Shop, and with the aid of a marvelous little device which one looks into, he informed me that my IPD is 57 mm.

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Old Tuesday 31st October 2017, 15:51   #2
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You seem to have gained 0.1 year since last time. Interesting.
I reckon my pupil size is about 10% larger in total darkness for 20 or 30 minutes than the size I measure in a dim room.
I also use pinholes and Waterhouse stops to judge my out of focus star images at night.
If really fussy I photograph them with a Konica/Minolta Z5 or Z6. The only cameras I know that seems safe, but the instruction book says don't do this.

I think with drops eyes can dilate to 9mm or so. Maybe young eyes.
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Old Friday 3rd November 2017, 23:59   #3
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My understanding is that pupil dilation is fairly rapid (a few minutes max) while the rest of dark adaptation is adaptation of the retina. My experience is that dark adaptation continues to improve over many tens of minutes for at least an hour or more.

There is a fairly simple technique for measuring your pupil size under dark skies described here.

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Old Sunday 5th November 2017, 18:49   #4
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Measuring some Allen keys on a bunch, I found that they are not very accurate, in fact not accurate at all.

I don't know if they are metric but using vernier calipers I think the largest one measures 6.0 to 6.7mm and the next one 5.0 to 5.6mm. Not sure if this is the full extent of the variation depending how they are presented to the eye.
Also not sure if these two are meant to be 6.0mm and 5.0mm.

Perhaps the top of drill bits are more accurate.

Nice fireworks tonight, free display courtesy of neighbours. Probably smog later on.

I am tired and lazy. So didn't try to calculate.
According to the internet the distance between corners of a hexagon is 1.154701 times the distance between flats.
If this is true a 6.0mm Allen key could be as large as 6.9mm and a 5mm Allen key could be as large as

Circle radius r then distance from centre circle to flat side of inscribed hexagon x = square root of 3/4 or 0.75, which is 0.866 plus.
1/0.866 plus is 1.1547.

So I think that Allen keys are not really suitable or accurate enough to determine pupil size using stars.

Last edited by Binastro : Sunday 5th November 2017 at 19:38.
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Old Sunday 5th November 2017, 21:12   #5
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I agree with your math, and respect your wish for accuracy. However, I'd view this method as a reasonable way of getting am estimate. I used it years ago to get a sense of how dilated my pupils were at my regular observing site compared to a much darker site that I was visiting in Arizona.

Using millimeter Allen wrenches, a .15 millimeter error does not seem terribly large. After all, if you had perfectly cylindrical rods and found that your pupil was larger than the 5mm rod but smaller than the 6mm rod, you would still have an uncertainty considerably larger than .15 mm. Having tried it with Allen wrenches, it was not very difficult to hold the wrench "on the flat" against my cheek.
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Old Sunday 5th November 2017, 21:57   #6
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Hi Alan,
The difference with a 6mm wrench is 0.9mm not 0.15mm.
0.9mm is just too large an error.
Basically one would not know if one had a 6mm or 7mm pupil.
In addition, there is the actual uncertainty of say 0.5mm by the method itself, so it could be 1.4mm out.

At night it is difficult to be sure which way the wrench is positioned.

A round rod would be much better.

I use Waterhouse stops and pinholes which I measure to about 0.1mm.
Or the Konica Minolta Z6.
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Old Monday 6th November 2017, 00:45   #7
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You are quite right, it is 15% not .15mm, and .9mm is a big error.
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Old Tuesday 7th November 2017, 17:52   #8
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I measured mine by holding a transparent ruler in front my eyes in the dark and firing the flash on my camera.

IPD by using dial calipers in front of the mirror. Setting the binoculars width correctly seemed to make a big difference in the brightness of the image.
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Old Friday 10th November 2017, 23:18   #9
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One possible way to determine the pupil diameter could be to measure the HVID (Horizontal Visible Iris Diameter) which is constant.
Take a photo under suitable conditions, measure the pupil diameter and the iris diameter on the photo, divide the former with the latter to get the quota.
Finally multiply the quota with the true HVID to derive the true pupil diameter.

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Old Saturday 11th November 2017, 14:58   #10
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Here's a method for measuring pupil size by using a binocular of known magnification with an exit pupil larger than your eye's pupil, a star (artificial or real) and two pieces of opaque sticky tape. I've used this method for years and have posted instructions a few times before.

Place the two pieces of tape on either side of one objective lens ring so that they form a slot of clear aperture between them. Cap the other objective lens. If you are using a real star at night set the binocular to close focus and observe the star. The out of focus star will look like the photo below if the gap between the tape strips forms an exit pupil slot narrower than you eye's pupil. Adjust the spacing of the tape strips until they barely graze the edge of the defocused star image. Measure the clear aperture between the tape strips and divide that by the binocular's magnification and you have your pupil size for that light level.

The beauty of this method is that it can be used to determine pupil size at any light level when a binocular is being held in front of the eye. In daylight or twilight conditions just use an artificial star, like a glitter point of the sun or a pinhole covering a flashlight bulb. Place the artificial star about 4 meters away and set the binocular's focus to infinity.
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Last edited by henry link : Saturday 11th November 2017 at 16:35.
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