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.

The Fabulous World of Shareware

Today I wanted to convert 26 height measurements I made into a topographic map. Complicating the project was the fact that my data were not spaced at regular cartesian grid points, but rather compass-style at 30-degree intervals from a series of reference points. Within a few minutes of searching online I found this shareware program from 2004 that could take a plaintext file of the xyz points as an input and interpolate a surface, producing this:

Another great piece of freeware that recently simplified some of my other work is this VBA “Decompiler”: I have been pushing the envelope with Excel, and since Microsoft Office is sloppy with its Visual Basic interpreters one must occasionally purge all VBA code from a workbook and reload it – a kludgy process that was automated by the author of that clever tool.

Check your dryer vent

When your clothes dryer says “check vent” you really should. In this case the outside vent cap was unobstructed. But the dryer insisted. I pulled the dryer away from the wall to disconnect the flex duct and discovered it packed with straw – apparently by a bird that thought it would make a cozy nest.

I pulled out a giant wad of grass, cleaned the flex duct, reconnected it, and thought that was the end of the matter. “Check vent” came on again. This time I reached deep into the vent on the dryer side and pulled out another foot-long plug of woven grass!

Cookie Currency

Cookies and Tea
Keep Calm – David Will Fix It

These chocolate chip cookies are a so good that I honor them as payment for home repairs.

Yum … delicious, grey-market barter. Not dependent on fiat currencies, and beyond the reach of all but the most tyrannical governments. You can even have the recipe for free:

Chocolate Chip Coconut Cookies (High Altitude Recipe)

  • 1 C. + 2 T.  unbleached all-purpose flour
  • 1/4 tsp. salt
  • 1/4 tsp. baking soda
  • 6 T. butter, melted
  • 1/2 C. brown sugar
  • 1/4 C. minus 1 T. white sugar
  • 1 extra-large egg
  • 1 egg yolk
  • 2 tsp. vanilla
  • 2 C. chocolate chips
  • 1/3 C. coconut
  • 1/3 C. chopped, toasted pecans (optional)
  1. Heat oven to 350 degrees.  Line cookie sheets with parchment paper.
  2. Cool the melted butter slightly.  Beat melted butter and sugars together.
  3. Add egg, yolk, and vanilla.  Beat until blended.
  4. Add salt and baking soda, beat or mix well.
  5. Stir in flour by hand until just incorporated.
  6. Stir in chocolate chips, coconut, and pecans until just incorporated.  Do not overmix.
  7. Form into balls 2 tablespoons each.
  8. Bake 12-15 minutes or until cookies are just beginning to turn golden.
  9. Let cool on baking sheets for about 10 minutes before removing.