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Eye pupil size


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How would you measure your maximum dark adapted eye pupil size?

I found a method somewhere on the web which went something like this...

On a really dark moonless night, take out several drill bits of sizes 5 to 7mm.

"Stick with it, I'm not completely balmy :D "

Get thoroughly dark adapted - say 30 minutes.

Close one eye and hold a drill bit several inches from the open eye whilst looking at a bright star.

The largest size of drill bit that just lets you see two points of light (from the same star), one immediately either side of the bit, will be about the same size as your dark adapted eye pupil.

I have also thought of taking a flash photograph of the dark adapted eye while holding a ruler close to it, but would imagine this could be rather painful if not dangerous to the eyesight, thus defeating the object of the exercise.

Can anyone point out flaws in the above method or put forward any other methods.

I have tables of dark adapted pupil sizes at various ages, but of course these are very general.

The maximum eye pupil size for a particular individual is important in the design of a richest field telescope tailored for use by that person.

I would be very interested if anyone has any ideas on the above.


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I think this should work, though of course you've got to be able to hold the bits exactly square on & not shake about too much.

Thought about this a bit more: it won't work. Light from a star is a point source, even say a 5mm drill bit held at arms length is not reduced to such a small point, it would have to be quite a bit further away. Also if you do get the drill bit in the right location, the light is still a point source & is not spread over your entire pupil. I think!

Edited by DaveJW
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Hi David,

Thanks for the input.

A star is effectively a point source at infinity. The light from the star is being collected by the whole surface area of the eye pupil, which is why a larger aperture telescope see's them brighter than the naked eye. The ray of light that hits the right edge of your pupil is effectively parallel with the ray that hits the left side of your pupil. The parallel rays should be visible either side of the drill bit if it is very slightly (fractions of a millimetre) smaller than your pupil. This is like a parallax effect. Only if your pupil was also a point source, a drill bit of even the smallest size would completely occult the star. Also if the drill bit was larger than the pupil size it would occult the star.

May be it won't work. I can see exactly where you are coming from as well. It can seem counter intuitive, but then it is not an everyday thing try and do.

As you say it needs to be held very still to do this. I am also concerned about the distance required to achieve proper focus on both the bit and the star at the same time. Or maybe for the purpose of this experiment it does not matter if either are in focus, just that light is visible each side.

My brain is starting to hurt now, so I think I will just have to try it and let you know.

Anyone else got any ideas ???



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Preliminary experimental result! :)

This method DOES work for measuring pupil size, but in a different way to what I expected.

Just been outside, no dark adaptation after watching TV all night with dim lighting. Used Arcturus as my test star.

New method...

Take out several drill bits, I took 3, 4 and 5mm.

Focussed on Arcturus with one eye closed and passed the 3mm bit slowly across the star about 50mm from my eye.

Of course the drill bit was out of focus.

The star dimmed substantially as I moved the bit in front of the star but it did not disappear.

Did same with the 4mm and the star dimmed even more than with the 3mm but did not disappear.

With the 5mm it dimmed and then disappeared as the bit got central over my eye.

My un-dark adapted eye was therefore between 4 and 5mm in diameter. Which is about the size I would expect under the circumstances.

The average fully dark adapted pupil at my age should be 6.1mm diameter. It will probably reduce further when I reach 22 :P

When the drill bit, even though out of focus, is passing over the eye, more and more of the eye is being prevented from seeing the light from the star. The area capturing the light is being reduced. If the bit is less than the full diameter of the pupil, then some light will still get through and the star will dim but not disappear. If the bit is exactly the same size or larger than the pupil then the star will dim and then disappear. The smallest size that makes the star disappear is the size of the pupil.

I now need to do this more accurately when fully dark adapted, using some intermediate drill bit sizes or other accurately sized rods. Ideally I would like to get to an accuracy of one tenth of a millimetre.

I'm really please this worked. It had me scratching my head for a while, but now I understand what is happening.

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