Two Weird Tricks for Presbyopia

I used to have perfect vision: In my 20s I passed the rigorous Air Force optometry screening for pilot candidates. Now in my late 40s I have developed the inevitable presbyopia that comes with age: The lenses of my eyes have lost so much flexibility that I can’t focus on things as close as I used to. I can no longer read fine print that used to be easily legible. In dimly lit restaurants I struggle to read menus. Both of these can be solved with reading glasses, but I don’t routinely carry those. So I often rely on two tricks to compensate.

The first trick is to increase light. I can read almost anything in direct sunlight.. In a restaurant I pull out my phone and shine its illuminator on the menu. Why does adding light sharpen text that is otherwise blurry or out of focus? There are some interesting optics at work.

The following image contrasts two photos of the same human eye. The photo on the left was taken in a dimly lit room, which has encouraged the pupil to open to let in more light. The photo on the right was taken with daylight coming through a window: The extra light causes the pupil to close.

Photos of the same eye in dim (left) and bright (right) light.

The wider the aperture (which in an eye is the pupil), the greater the focal range of a lens. In sunlight our pupils are as small as they can get, which gives us the maximum focal range. With presbyopia, my lenses can’t accommodate (i.e., change shape to shift focus) as close as they used to, but they can still go far enough that the added focal range from a small pupil brings close objects into focus. The same physics applies to camera lenses, as I demonstrate in the following image:

Illustration: First row: A camera pointed at a pill bottle inside its lens’s minimum focal distance. Second row: The lens aperture shown open to f/2.8, and the photo produced with the aperture wide open (f/1.7). Third row: The lens aperture stopped closed to f/22 and the photo of the same scene

I put a pill bottle closer than the minimum focal distance of this camera lens. A photo taken with the lens aperture wide open (f/1.7) shows that the lens can’t bring the text into focus. Keeping the lens at the same distance and focus setting, but stopping the aperture down almost as small as it gets (f/22) increases the focal range and brings the text into focus. (The photo of the lens in the second row actually shows the diaphragm at f/2.8 because at f/1.7 it can’t be seen.)

Bright light causes our pupil to contract naturally. The second trick is to create an artificially small aperture. You can do this by putting a pinhole lens close to your eye. I do this by closing my fingers and looking through the tiny opening left between two of them at the joint:

David Bookstaber creating pinhole lens for one eye to bring closer objects into focus.

The aperture trick can only compensate so much: With too little light the subject may be in focus but the contrast might be too low to read it. In the camera example: The exposure with the wide aperture was made in 1/640 of a second, but it took 1/4 second to get an adequate exposure at the minimum aperture (holding all other settings constant).

I have a related post on my substack: Why I Dislike Dark Mode.

Autumn Equinox

I happened to be in Salt Lake City during the equinox, where I learned that Utah has outlawed a lot of things. One random example: Happy Hour. Probably Utah also prohibits human sacrifices during equinox celebrations – I didn’t even bother to ask.

So aside from weird rules what else is there in Salt Lake City? Mountains. As my many readers know, I love to photograph interesting celestial events. So I went for a hike to get this equinox sunset:

Salt Lake City autumn equinox, 7500′ ASL

These foothills can be deceptive. I parked at one between the Big and Little Cottonwood Canyons and began to climb straight towards a peak. The first few hundred feet was fine gravel, so I had to kind of scamper from weed to weed to avoid sinking and sliding back. Then I reached sturdier ground which supports thickets of shrub-like oak trees. Those obstruct the view of what’s above, so I couldn’t be certain whether I had a good path ahead. I did eventually run into a trail bearing human footprints (though I saw no one else the entire hike). The trail wound back and forth between rocks and trees, and it was still very strenuous: When I got back I checked a topographic map and found that the straight-line route I followed from 5500 to 7500 feet averaged a 45 degree slope!

Perseid Meteors

Perseid Meteors, composite image by David Bookstaber
Perseid meteors. That’s Jupiter near the middle of the frame.

I set up my old Sony A77II with an intervalometer for an hour and managed to capture these Perseid meteors. I was using my widest lens, 16mm, shooting ISO 1600 with 10 second exposures. It took some real work to post-process and compose this image: I had to find the frames with meteors and stack and align them in Photoshop. The result is nothing great for several reasons, including that (a) meteor shower photos should use a wider lens, and (b) should include a horizon or some ground feature for perspective.

It’s easier to get better results using a smartphone, not only because a typical smartphone has a very wide lens, but also because software can take care of identifying frames that contain meteors and aligning them. After the fact I checked the app store and found at least one cheap app that does this with a built-in “Meteor Mode.”

Father Birds: Killdeer

Killdeer are common birds of the plover family (like the mourning doves I showed in a previous post). Their name supposedly comes from their loud and distinctive call, which bird guides say sounds like “kill-deer,” though I don’t hear that at all. There’s a video of one at the bottom of this post so you can hear and judge for yourself. As with mourning doves, both parents stay to incubate and protect the eggs, which take 3-4 weeks to hatch. Killdeer chicks are precocial, meaning that they can walk and feed themselves soon after hatching. The parents still tend to them and protect them from danger. They learn to fly when they are about 3 weeks old.

Following is a gallery of photos I took of killdeer I found nesting in my yard. I was alerted to their presence by the antics of the mating pair protecting their four eggs. First was a persistent and attention-grabbing tweet. Once my eye found one of the parents, the bird began its famous “broken-wing” act, which is used to lure predators away from their nests: the killdeer will pretend to have an injured wing, dragging it and beating it against the ground while making pitiful cries. The idea is that a predator will follow the seemingly easy prey as the killdeer leads it further and further away from the nest. When the predator is far enough, the killdeer fly away. This actually makes it easy to find the nest: Just go in the opposite direction of where the killdeer is drawing you. The closer you get the to its nest the closer the killdeer will come to you in an attempt to distract you.

So I did find and photograph their eggs, but I didn’t want to overly traumatize the birds so I returned with my 300mm lens to get these photos from a reasonable distance.

The eggs are sort of camouflaged, but leaving them in a clutch like that on open ground still doesn’t seem to me like a great strategy.

Killdeer doing its “broken wing” distraction dance and cry.

Father Birds: Doves

Mourning doves are one of the most common and widespread birds in North America. They get their name from the distinctive (and incessant) cooing sound they make. Male mourning doves are also devoted and protective fathers. In honor of Father’s Day, here are some photos of both parents caring for a fledgling.

Once a pair of mourning doves mates, they usually stay together for life. They work together to build a nest, usually on a tree branch or a ledge. The nest is made of twigs, grasses, and leaves, and is often flimsy and loosely constructed. The female lays two eggs at a time, which are incubated by both parents for about two weeks. The father usually takes the day shift sitting on the eggs.

After the eggs hatch, both parents feed the chicks “crop milk,” which is a thick mix of protein and fat they secrete in their crops (a part of the esophagus where they store seeds) and regurgitate for their chicks to eat out of their mouth.

Mother mourning dove feeding fledgling

The chicks grow to near full size and fledge in just two weeks! In these photos the fledglings are almost as big as their parents and will leave the nest in a few days.


The word for hummingbird in most other European languages is colibri. I can’t find an explanation for why English didn’t adopt that. I put out a feeder with a 1:4 solution of sugar : water, and once they find it hummingbirds come for a drink every 15 minutes for the entire day. Here are photos of one frequent visitor:

Sometimes they perch, and sometimes they maintain a hover while drinking. They get a little territorial about the feeder, even though it has four perches and flowers to drink from: when one hummingbird is at the feeder, often another will approach and provoke it and they’ll flit away rapidly spiraling around each other.

Hummingbird species encompass not only the smallest birds but also the smallest warm-blooded animals. During the day they maintain the highest metabolism of any vertebrate. This requires a nearly constant supply of sugar. (At night, when they can’t feed, they perch and enter a hibernation-like state of vastly reduced metabolism called torpor.)

In terms of flying skills, they can keep their head perfectly still in a hover, even as their wings are fluttering back and forth dozens of times a second. I have seen them hovering at the feeder in the heavy winds preceding a thunderstorm. In the morning and evening I can see hummingbirds hovering and swooping to eat tiny flying insects.

No other vertebrates can maintain a true hover in flight, and the size range for hummingbirds runs from 2g to 20g. This suggests that any smaller and an endoskeleton and/or homeothermic metabolism becomes too expensive for a hovering animal. Any heavier and it becomes too difficult for a biological system to process and supply the energy needed to hover.


My house in Berwyn, Pennsylvania, sat on a hill overlooking dense woods and a small stream. Deer in that part of the state roam suburbia in large herds and, because hunting is severely restricted, their only predators are cars. Every few months a deer would end up dead down by the stream. (Why there? A wildlife officer explained that injured deer tend to go downhill and towards water, and so that’s where they die.) I could tell because vultures would start to appear in the tall trees near my house – black vultures and even larger turkey vultures, which seem to get along quite well. Hard to miss because they are massive birds, and before long they would be perched in large numbers and for days would swoop down to feed on the deer carcass until nothing was left but bones and hair. Here are photos I captured of these impressive birds:

A group of vultures congregated on the ground around a carcass is called a “wake.” Which is a very good description: Vultures are silent, so they seem like respectful mourners … aside from the ones that are actively picking at the carcass. A group of vultures perched in trees is called a “committee” – also a fine description as they will stand together solemnly on a branch for long periods.

It’s hard to convey how large these birds are. Their wings span 5-6 feet, and when they land it looks like their legs are flexing under a substantial weight.