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Max pupil dilation vs exit pupil

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#26 hoof

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Posted 23 January 2021 - 01:47 PM

As Jon has pointed out dark-adapted pupil size can vary hugely from person to person to say the least. So rather than making an assumption, that's probably wrong anyway, I'd suggest you measure your dark-adapted pupil size. It isn't difficult and there are several methods that a search on CN will show you. This is what I did.

I got dark-adapted for a few minutes to allow my pupils to dilate (there's no need for longer as full dark adaption is a chemical process). I then got my wife to take a flash picture whilst I held a ruler just above my eye. Make sure though you haven't got red eye reduction or pre-flash on.

Unfortunately my maximum pupil size is only 4.5mm (confirmed by my optician) - less than my wife's and way less than Jon Issacs'.


The issue with flash is that many modern flashes actually flash twice, very quickly. Since the retina appears red due to flash, you get the notorious red pupil look that you see with older pictures, double flash is designed to minimize that.

The first flash triggers the pupil shrink instinct. The second flash (after the pupil is constricted) takes the actual picture. This does a surprisingly good job of eliminating red eye in the picture. But if is unfortunate for measuring eye pupil size.

Modern phone cameras likely take a different approach, they likely use image processing to find the pupils and replace the red pupils with black ones. But the flash speed isn’t instantaneous, so the pupil still has time to constrict.

So using flash photography can mislead as far as measuring the eye’s pupil. And that doesn’t include the problems of prefocusing, as a dark room dark enough to fully dilate the pupil is too dark to autofocus, usually. Oh and let’s not forget perspective that can cause the measurement to be off by a millimeter or more, depending on how close the camera is and how far from the eye the ruler is held.

I bought a set of hex wrenches in metric format to measure my eyes. By holding them up and putting them between my eye and a bright star, if I can make the star disappear, the hex wrench is larger than my pupil. So I work down from 9mm until I can see the star (barely). With that method I determined my 46 year old eyes dilate to at least 7mm.

#27 hoof

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Posted 23 January 2021 - 01:53 PM

I've never understood the theory of large exit pupils revealing less sharpness compared to small exit pupils. I am 50 and in the last year realised the advantage of 7mm exit pupils in dark skies.my 7x50,10x70 produce the same small pinpoint stars that my 10x42 and 10x50 do.


All eyes have spherical aberration, and many have astigmatism. This results in part of the light of the star appearing outside the central dot. Since stopping down your pupil (or effectively doing that with exit pupil) you sharpen the view.

This is demonstrated in an extreme if you have your eyes medically dilated. Last time I had that, the world looked hazy and no focusing could bring the world into sharp focus. The astigmatism and spherical aberration created an inferior view, though it sure was bright :) This is likely why we evolved to limit our pupil size in normal low light circumstances, the trade off in image quality for a wider pupil wasn’t worth the brighter view.

#28 warpsl

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Posted 23 January 2021 - 01:58 PM

Even during daylight observing my 7x50,10x70 Porro,s give a strong 3d effect. My pupil in daylight is only 3mm.the link jon provided made note of vitamin A and the importance of it in our diet.carrots,pumpkin,broccoli, salmon,oranges,sweet potato,kale and spinach are foods which help our eyes.apparently bromelien, found in pineapple is good for removing floaters.i drink pineapple juice,but have not noticed a difference.

#29 Mr. Bill

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Posted 23 January 2021 - 02:02 PM

Floaters are forever......can't be removed with diet.


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#30 Jon Isaacs

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Posted 23 January 2021 - 10:17 PM

I bought a set of hex wrenches in metric format to measure my eyes. By holding them up and putting them between my eye and a bright star, if I can make the star disappear, the hex wrench is larger than my pupil. So I work down from 9mm until I can see the star (barely). With that method I determined my 46 year old eyes dilate to at least 7mm.

 

The difficulty I have had with using wrenches and drills is that you are looking for full blockage and you must center the tool so it is aligned with both edges of the pupil.  

 

One can use a camera in a manual mode so there is no preflash.  Another issue is autofocusing, cameras will illuminate the object in order to focus.  

The last time I measured my dark adapted pupil, I used my 19 year old Nikon 4500.  It is a swivel camera.. the original selfie.  It took a while to get the focus right.

 

There is another method that a member came up with that involves defocusing the image of a star.

 

Jon



#31 garret

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Posted 24 January 2021 - 01:32 PM

 

The difficulty I have had with using wrenches and drills is that you are looking for full blockage and you must center the tool so it is aligned with both edges of the pupil.

I have tried to measure my pupil with metrical drills on the moon, I think this method is not reliable, you cannot place the drill close enough to your eyes because your eyelashes make that impossible.

I came up with less then 5.5mm with drills but I know my pupils closer to 6.5mm...

By using a mask over one tube of an binocular entrance (with 7mm exit pupil)  and see for any difference/ brightness in the view, using one eye at the time.



#32 M&P805

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Posted 03 February 2021 - 11:03 AM

For what it's worth, I have spent the past couple of nights studying M44/Beehive Cluster.  First using an alpha 10x42 and low-mid range 10x50, I noticed little difference in the need to avert gaze in order to "brighten" the image (4.2 vs 5.0 exit pupil).  I initially chalked this up to a difference in "quality" of the optics, as to why there would be similar brightness between the two.  Last night I gave the alpha 8x42 bins a try (5.25 exit pupil) and found that averted gaze was not necessary.  The image of the cluster was clear, sharp and bright, despite direct gaze.  I am now a believer in the importance of the exit pupil when viewing an inherently dim target.  It would be nice to have a good 10x56 such as the SLC, however this instrument is not available in the USA.

 

George


Edited by M&P805, 03 February 2021 - 12:01 PM.


#33 Watch&Learn

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Posted 26 March 2021 - 11:05 AM

For what it's worth, I have spent the past couple of nights studying M44/Beehive Cluster.  First using an alpha 10x42 and low-mid range 10x50, I noticed little difference in the need to avert gaze in order to "brighten" the image (4.2 vs 5.0 exit pupil).  I initially chalked this up to a difference in "quality" of the optics, as to why there would be similar brightness between the two.  Last night I gave the alpha 8x42 bins a try (5.25 exit pupil) and found that averted gaze was not necessary.  The image of the cluster was clear, sharp and bright, despite direct gaze.  I am now a believer in the importance of the exit pupil when viewing an inherently dim target.  It would be nice to have a good 10x56 such as the SLC, however this instrument is not available in the USA.

 

George

Just to make sure I am understanding your point correctly, was the view through the 8x42 bino' better than the previous two, due to the larger exit pupil (although the aperture was smaller)? 


Edited by Watch&Learn, 26 March 2021 - 12:55 PM.


#34 Jon Isaacs

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Posted 26 March 2021 - 11:14 AM

I have tried to measure my pupil with metrical drills on the moon, I think this method is not reliable, you cannot place the drill close enough to your eyes because your eyelashes make that impossible.

I came up with less then 5.5mm with drills but I know my pupils closer to 6.5mm...

By using a mask over one tube of an binocular entrance (with 7mm exit pupil)  and see for any difference/ brightness in the view, using one eye at the time.

 

I think masking the objective is a bit tricky because it assumes that your pupil is perfectly aligned with the exit pupil.

 

There's a technique where you defocus a star and then see if you can see the entire image. If the exit pupil is larger than your pupil, you will not see the entire defocused image.

 

When they are nearly the same size, aligning my eye to the image is very difficult. I suspect most of the time, we're not fully aligned.

 

Jon


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#35 M&P805

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Posted 26 March 2021 - 09:32 PM

Just to make sure I am understanding your point correctly, was the view through the 8x42 bino' better than the previous two, due to the larger exit pupil (although the aperture was smaller)? 

Yes, I found the 8x42 brighter and sharper than the other two, although clearly not as magnified.  Viewing in Bortle 5 skies, granted not on the same night.  Cannot exclude difference in cornea from one night to the next.

 

George


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#36 Alan French

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Posted 27 March 2021 - 04:54 PM

I have a pair of 7x42 binoculars. When I use them during the day I'm sure I'm actually using something around a 7x14 to 7x21 because my pupil is only open to 2 or 3mm. I took them with me once when I had my pupils dilated for an eye exam. With the large eye pupil, perhaps closely matching or larger than the 6mm exit pupil of the binoculars, it was impossible to get a sharply focused view. Indeed there was a fairly substantial range of movement for the focus knob that made not detectable improvement to the view.

 

Clear skies, Alan


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#37 M&P805

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Posted 27 March 2021 - 05:57 PM

Eye dilation with a mydriatic is a complicated physiologic effect.  Not only is pupillary constriction “paralysed” but so is the focusing mechanism.  Pharmacological pupillary dilatation is therefore not the same as dark adaptation, and would explain why it would not be helpful for astronomic viewing.

 

George



#38 Alan French

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Posted 28 March 2021 - 01:46 PM

Eye dilation with a mydriatic is a complicated physiologic effect.  Not only is pupillary constriction “paralysed” but so is the focusing mechanism.  Pharmacological pupillary dilatation is therefore not the same as dark adaptation, and would explain why it would not be helpful for astronomic viewing.

 

George

George,

 

My understanding is that visual acuity of the dark adapted eye is about an order of magnitude worse than our daytime vision. I knew that visual acuity suffers greatly when my pupil is dilated by my ophthalmologist and was curious whether the poorer acuity would show up in binoculars, used with their large exit pupil. While my eye's focusing ability might be out of service, I could certainly try improving it with the binoculars on daytime targets. It was quite clear that my eyes were not strutting their usual stuff with a useful, large exit pupil.

 

I was not experimenting with drug induced eye dilation and night sky observing.

 

Clear skies, Alan


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#39 astroneil

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Posted 28 March 2021 - 05:58 PM

nota bene:

 

http://adsabs.harvar...JBAA..114...73L


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#40 hoof

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Posted 29 March 2021 - 01:29 AM

The cornea and eye lens have spherical aberration. This means the outer parts of your cornea/lens focus at a slightly different distance as the middle. In daylight, our pupil constricts which minimizes the effect, much like stopping down a camera lens, improving image quality.

The effect of this is that the fully dilated pupil cannot focus a point light source (say a star) to a point. The best you can do is focus the outer parts to a point (where most of the light is gathered), with the inner parts slightly out of focus. Ironically, the central obstruction of some telescopes help mask this part out, meaning that large exit pupils can have stars looking more like points on an obstructed telescope vs non obstructed telescopes. Stars at low powers (large exit pupils) for me look better on my 6” 43% obstructed astrograph than my 5” apo.

Another side effect of this spherical aberration is at dusk, you can have slightly blurry vision with bright daylight being sharp. It’s because your eyes dilate in dimmer light. “Astronomy” glasses that yield nice tight stars need to be 0.5 to 0.75 diopters stronger than regular daylight glasses for the same reason. My current astro glasses are about -3.75 diopters even though I can read a Snellen in room lighting at 20ft down to the 20/15 line sharp with -3.0 diopter glasses. Once I discovered that (and got the right glasses), I saw stars in the sky at 1x better than I ever had before in my life. If you can get a pair, I strongly recommend it, it will transform looking up at the sky with just your eyes and the glasses.

The eye is fascinating :)
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