Measuring your eye-pupil at home, Berthon's dynameter

[John Baars]

Norton's Staratlas from 1910 shows a little device that will help you measuring your own eye pupil. Later versions show it too.

Originally designed in 1891 for measuring small exit pupils from telescopes in order to calculate the unknown focal length of a telescope, it can be used ( in adapted form) however to measure your own dark-adapted eye pupil too, from your backyard. Might come in handy when considering buying binoculars for night observations.
http://www.southastrodel.com/Page209.htm

A simple home-made version can be made with a piece of cardboard, a ruler, a pencil and a needle.
See my own version below.

Two intersecting lines and pinched with two pinholes for each distance.

How does it work?

Look at a bright star through the 2 mm holes. You will see one hole. Slide the dynameter in front of your eye until you see two touching holes.That will be the size of you dark adapted eye on that particular spot. When you slide further the two holes no longer touch each other.

So how does it look like for an observer with a 4 mm entrance pupil?

Under Bortle 1 skies it might become quite difficult to read the Dynameter, but in all other circumstances it is a nice approximation of the size of your pupil.

So this old-school solution from the 19th century might come in handy when you don't like holding drills of varying sizes or a sharp caliper very close to your eyes. No flash lights needed either.

I have mine since the eighties....

Real antique dynameters can be bought in the unavailablium shop.

[smeyer8015]

You can also use Allen key wrenches. Hold increasing size wrenches in between you eye and the star as close to the eye as possible. Slowly move it side to side. Keep moving up in size until you can no longer see the star. That size will be the appropriate pupil size.

[John Baars]

You can also use Allen key wrenches. Hold increasing size wrenches in between you eye and the star as close to the eye as possible. Slowly move it side to side. Keep moving up in size until you can no longer see the star. That size will be the appropriate pupil size.

Thanks smeyer8015, for this safer solution than the caliper or the drills.

[Bigzmey]

Thanks John and smeyer8015, good to know. Can one really have 8mm exit pupil?

[notFritzArgelander]

Thanks John and smeyer8015, good to know. Can one really have 8mm exit pupil?

Probably a very few have such?

[smeyer8015]

Thanks John and smeyer8015, good to know. Can one really have 8mm exit pupil?

I wonder if the eyedrops the ophthalmologist uses would work?
Ever looked in a mirror shortly after?

[Makuser]

Hi all. As we get older, our eyes don't dilate as wide as when we were younger. That is why I chose my binoculars to have less than a 6mm exit pupil (7x35 and 15x70). Here is something interesting for you. In Renaissance Italy, the Roman women used belladonna to dilate their eyes, as they thought it made them more attractive. In fact, belladonna means beautiful woman or beautiful lady. However, the atropa belladonna plant is a poisonous member of the solanaceae (night shade) family (which includes tomatoes, potatoes, and eggplant). The women would use an extract from this plant on their eyes. But, don't think that the continued use of the modern dilation drops are good thing either. You could end up looking like James Coburn from an episode of the old TV show Outer Limits.

big eyes.jpg (34.92 KiB) Viewed 3014 times

Thanks for the interesting and informative thread John, and the best of regards.

[John Baars]

Hi all. As we get older, our eyes don't dilate as wide as when we were younger. That is why I chose my binoculars to have less than a 6mm exit pupil (7x35 and 15x70). (....)

Thanks for your reaction.
Some experimenting showed me that my eyes still dilate to 6 mm. Quite different from the more than 7 mm when I was 35 or so. I choose my binoculars in the same way as Makuser does. For the same reason I use a 10X50 and a 15X70.
My wife always wonders what I am doing in the binocular-department of several shops. I usually say that I am exploring the centenarian market.

[notFritzArgelander]

.... In Renaissance Italy, the Roman women used belladonna to dilate their eyes, as they thought it made them more attractive. In fact, belladonna means beautiful woman or beautiful lady. However, the atropa belladonna plant is a poisonous member of the solanaceae (night shade) family (which includes tomatoes, potatoes, and eggplant). The women would use an extract from this plant on their eyes. But, don't think that the continued use of the modern dilation drops are good thing either. ......

Belladonna has also been used as a (singularly unpleasant?) recreational drug and is rumored to have been used by Livia to poison the Emperor Augustus.

[Nakedgun]

Hi all. As we get older, our eyes don't dilate as wide as when we were younger. That is why I chose my binoculars to have less than a 6mm exit pupil (7x35 and 15x70). Here is something interesting for you. In Renaissance Italy, the Roman women used belladonna to dilate their eyes, as they thought it made them more attractive. In fact, belladonna means beautiful woman or beautiful lady. However, the atropa belladonna plant is a poisonous member of the solanaceae (night shade) family (which includes tomatoes, potatoes, and eggplant). The women would use an extract from this plant on their eyes. But, don't think that the continued use of the modern dilation drops are good thing either. You could end up looking like James Coburn from an episode of the old TV show Outer Limits.

Thanks for the interesting and informative thread John, and the best of regards.

I loved the Outer Limits during childhood and never miss a chance to watch episodes when they are occasionally broadcast today.

[AntennaGuy]

There's a range of dark adapted pupil sizes vs. age.
See https://pubmed.ncbi.nlm.nih.gov/20506961/
"Dark-adapted pupil diameter as a function of age measured with the NeurOptics pupillometer "
Results: Two-hundred sixty-three individuals participated. For participants aged 18 to 19 years (n=6), the mean dark-adapted pupil diameter was 6.85 mm (range: 5.6 to 7.5 mm); 20 to 29 years (n=66), 7.33 mm (range: 5.7 to 8.8 mm); 30 to 39 years (n=50), 6.64 mm (range: 5.3 to 8.7 mm); 40 to 49 years (n=51), 6.15 mm (range: 4.5 to 8.2 mm); 50 to 59 years (n=50), 5.77 mm (range: 4.4 to 7.2 mm); 60 to 69 years (n=30), 5.58 mm (range: 3.5 to 7.5 mm); 70 to 79 years (n=6), 5.17 mm (range: 4.6 to 6.0 mm); and 80 years (n=4), 4.85 mm (range: 4.1 to 5.3 mm). These values were consistent with studies using infrared photography. The standard deviation was >0.1 mm in 10 (3.8%) participants, all of whom were younger than 55 years."

[Michael131313]

Thanks all for the info. I will try to check my pupil size but I am not going to use belladonna