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Old Thursday 10th September 2009, 13:09   #1
spacepilot
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How to measure dilated pupil diameter?

Last night I played with a pair of 8x32 bins and a pair of 8x42 bins on my front porch. I was trying to read the license plate of a neighbor's car illuminated by another neighbor's garage lights. To my surprise, both pairs provided about the the same level of details for me to guess the plate number. Now I'm wondering if my dilated pupils are large enough to take advantage of the larger exit pupil size of the 8x42.

To measure my dilated pupil diameter, I'm thinking about staying outside after dark. After a while (I don't know how long it takes for human eyes to fully dilate), I'll hold up a ruler with mm markings up to my eyes, and ask my wife to take a picture with flash. The focusing of my point-n-shoot camera uses a red led to illuminated the object, I hope that doesn't cause my pupils to shrink. So, is that a reasonable way to measure the pupil diameter in the dark?

Ning
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Old Thursday 10th September 2009, 14:27   #2
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It probably takes a little while in the dark for the pupils to reach their maximum size, but for birdwatching is that a realistic thing to expect to happen? We use binoculars in low light (with the setting sun on your face at times), but not in the dark, so the results of your experiment may show that 4mm is all you can expect to achieve in real conditions.

It might also just be that the 8x32s are better binoculars.

I once read somewhere (can't find it now) of a technique where you look at a distant object while holding various size drill bits in front of your eye, increasing the size till you can just see a double image. I can't remember the details, maybe someone else knows.
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Old Thursday 10th September 2009, 14:36   #3
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Ning,

I went through a similar process a little while back for a project. I did what you propose and it worked very well except that I used my measured IPD to scale against. Keep in mind when using a ruler and a camera with a finite focus point that the ruler will be closer to the camera than the pupils so the pupils will be slightly larger than indicated.

Best
Ron
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Old Thursday 10th September 2009, 14:48   #4
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Quote:
Originally Posted by pshute View Post
I once read somewhere (can't find it now) of a technique where you look at a distant object while holding various size drill bits in front of your eye, increasing the size till you can just see a double image. I can't remember the details, maybe someone else knows.
My recollection is fuzzy; I recall that you hold a drill bit up close to your eye while looking at a bright star (at night - duh). With a size of bit that just barely allows you to see the star as double (its image on either side of the bit), that bit represents the size of your pupil. Be sure to keep any stray lights out of your vision, otherwise you will not become fully dark adapted and your pupil may not fully expand (though probably good enough for gov't work). For the most precise measurement, give it about 10 minutes in darkness before trying this.

For some reason I've yet to do it myself.
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Old Thursday 10th September 2009, 15:04   #5
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Google for "pupil gauge". My firewall here at work will not let me get to these sites but I have seen (and made) some of the gauges based of the drill technique, one attached. They describe the use where you can buy them commercially.

I found a couple of JPG's.
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Old Thursday 10th September 2009, 16:58   #6
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Thank you fro the responses, every one.

Ron, I was thinking about the ruler being closer to the camera. I hope with the zoom and macro settings on my camera, my wife can get a usable picture of my eyes from at least a couple meters away, from where the distance between the ruler and my pupils should be negligible.

The drill-bit method and the pupil gauge methods seem very interesting. I will try the drill bits, and may end up getting the commercial pupil gauge, and compare their results to my ruler-photo result.

pshute, I was actually trying to compare my (new to me) 8x32 SE's with the Leupold Cascades 8x42. The Cascades have a very sharp center view, and I have been hard pressed to tell which pair provide better resolution on axis in usual birding settings. I was thinking looking in the dark may give the Cascades an edge because of it's larger exit pupil. But from what I could tell last night, they don't really. Whichever star that I could barely see with one pair, I could also barely see with the other. So if my pupils dilate to larger than 4mm in the dark, as I'm trying to find out, that would be a real testament for the superior optics of the SE's despite their smaller objective lenses.

Ning
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Old Thursday 10th September 2009, 17:17   #7
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Ning, if you can print on transparency film these may be some use to try out.
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File Type: pdf Pupil Gauge 4.pdf (81.0 KB, 288 views)
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Old Thursday 10th September 2009, 17:30   #8
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Ning, if you can print on transparency film these may be some use to try out.
Ron, thank you. I'll print it on paper and copy it onto transparency in the copy shop. Does your measurement with the pupil gauge match the results from the IPD pictures?
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Old Thursday 10th September 2009, 17:40   #9
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Ning, I have never really compared under conditions suitable for accuracy. I have not used the photo method above and the obstruction method at the same time, but I have compared values at different times for the same light value and they compared favorably.

I have not used the obstruction method in dark conditions because it is too difficult to see the values and line up the edges.

Best
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Old Thursday 10th September 2009, 18:24   #10
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Here's a simple way to measure pupil dilation under various light levels when there is a binocular in front of the eyes. I'm quoting myself from another thread so I won't have to write it all out again.

"On the question of measuring pupil dilation with binoculars in front of the eye I've had good results using an out of focus artificial star. For sunlight measurements I set the binocular for infinity focus and examine a glitter point of the sun reflected in a Christmas tree ornament placed about 10' away. The out of focus disc is a pretty well focused image of the eye's pupil or the binocular exit pupil depending on which is smaller. For lower light situations I use a pinhole over a flashlight beam. Of course, at night actual stars can be used.

Just now I made a crude measuring caliper from a piece of stiff wire and measured my pupils with two 8x30 binoculars in front of my eyes. I picked an old Zeiss 8x30B Porro with a 50 degree field and a Nikon 8x30 E II with a 70 degree field to test Jean-Charles very reasonable idea that a smaller apparent field might cause the pupil to open larger. Measurements were made by placing the wire just in front of the objectives and bending it into a "U" shape until the tips of the wire barely show at opposite edges of the image of the pupil. In sunlight I measured an effective aperture of about 18mm for both binoculars (18mm aperture= 2.25mm eye pupil). I could detect no significant difference. I used a flashlight with a pinhole placed in a deeply shaded area under some shrubs for a lower light measurement. The result was about 28mm for each binocular (3.5mm eye pupil). Once again the difference between the two binoculars was too close too call, but I had a slight impression that my pupil was a tiny bit larger looking through the Nikon, the opposite of what I would expect.

BTW, this test is also useful for revealing how much off-axis vignetting is present at different light levels. Just move the disc toward the edge of the field to see the vignette."
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Old Thursday 10th September 2009, 18:33   #11
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Henry, I tried that method with a dial caliper instead of a paper clip and seemed to work well but I did not have a reference at the time and have not gone back and refined it yet. I think the next time I try it I am going to mount the measuring device on the optical line (I meandered around a lot with the caliper) and use an adjustable iris. I suspect this will give real accuracy. I had done the photo method above before reading of your approach.

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Old Thursday 10th September 2009, 19:47   #12
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Quote:
Originally Posted by spacepilot View Post
I was actually trying to compare my (new to me) 8x32 SE's with the Leupold Cascades 8x42. The Cascades have a very sharp center view, and I have been hard pressed to tell which pair provide better resolution on axis in usual birding settings. I was thinking looking in the dark may give the Cascades an edge because of it's larger exit pupil. But from what I could tell last night, they don't really. Whichever star that I could barely see with one pair, I could also barely see with the other. So if my pupils dilate to larger than 4mm in the dark, as I'm trying to find out, that would be a real testament for the superior optics of the SE's despite their smaller objective lenses.

Ning
Well, not really. It just shows that your cascade can't beat out the SE 8x32.

Limiting magnitude tests of a dozen different binoculars from 8x32 to 8x42, under typical mag 5.2-5.6 skies, ranges from mag 9.1 to mag 9.6. However, 5 out of 12 different 8x40/42s that I've tested could do no better than mag 9.1. Well, the 8x32SE also did a best of mag9.1. So finding that the 8x32SE matches the LM of an 8x42 is no decisive finding of superior limiting magnitude performance in the SE, since almost half of all 8x42s in a sample of 12 could do no better.

FWIW, these all beat the 8x32SE by 0.5 magnitude
Celestron Regal 8x42 Roof
Bushnell Legend 8x42 Roof
Fujinon BFL 8x42
Pentax PCF WP II 8x40
Nikon Action Ex 8x40

Had you chosen one of those five for your comparison, you might also have found the 8x32SE would have measured up to a half magnitude less and appeared to fall right in normal line. The difference in area of aperture would indicate about 0.3 up to a maximum half magnitude gain for the larger aperture. That's exactly what testing shows.. Perhaps what you've found by your comparison is that your Cascade is not up to the best LM of an 8x42.

Of course, it's worth mention, none of this has anything to do with resolution. It doesn't test brightness either. What you tested was a combination of light gathering and magnification on stars, for which the standard measure is limiting magnitude, except in this case magnification was equal. This would easily account for getting the same results on the license plates in dark.

So ,although it's probably worth finding out for yourself, nearly everyone can dilate to greater than 4mm in the dark. But I doubt that really has any bearing on the test you did.

edz

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Old Friday 11th September 2009, 13:58   #13
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Henry, that sounds like an interesting method. I'll need to think about it and do some research to understand it.

Thank you for the response, Ed. I was hoping you would chime in at some point. So you are saying, if we had two pairs 8x42 bins, with identical light transmission, but with different resolution, their LM measurements would be about the same? Does that mean if the two pairs of bins get the same amount of light to the retina (very small amount, like from a faint star), regardless of whether two (good resolution) or twenty (not as good resolution) cells receive the light, the brain's perception (whether or not it 'sees' the light) will be the same?

Ning
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Old Friday 11th September 2009, 15:27   #14
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Does that mean if the two pairs of bins get the same amount of light to the retina (very small amount, like from a faint star), regardless of whether two (good resolution) or twenty (not as good resolution) cells receive the light, the brain's perception (whether or not it 'sees' the light) will be the same?

Ning
I can't answer that definitively, as I've never made any note of that. I suspect, if you take your hypothetical example to the extremes as you have, and you consider resolution so poor as to spread the light out over 10x the area, preception of point image brightness would probably decrease. That may not be likely to ever occur.

However, I can say, in all my data, I see no correlation between resolution and light gathering or total light transmission. But you weren't measuring resolution anyway.

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