I'm working on an answer for you, but your question is too difficult
for a short answer. You can either check this HN submission in a few
hours, or contact me by email (in the 'about' of my HN profile).

EDIT

The spectrum (pun intended) of vision issues is an exceedingly vast and
complicated field. Your eyes are the biggest part of the equation, but
it's actually more than just your eyes; how your brain tends to evaluate
the things you see also comes into play. Though there are generally
defined groupings and descriptions of issues, it's very important to
remember that your eyes (and brain) are specific to you. The odds of two
people "seeing" things in exactly the same way are minuscule. This
still stands true for even physiologically "identical" people such as
"identical" twins.

Figuring out what works for you personally will involve a good deal of
trial and error experimentation.

Line length is just one example out of thousands.

Have you ever wondered why HN and the essays at paulgraham.com both have
long pages with narrow columns of text? --In one of Paul's essays/pages
he explains how narrow columns of text are easier for him to read, and
since both HN and his own site are his to control, he "wastes" a lot of
horizontal space to get what he wants. For many people, scanning/reading
a long horizontal line is difficult, or better said, more difficult,
than scanning/reading short horizontal lines. This is one of the many
reasons why printed newspapers use narrow columns of text, and why one
of the well accepted styles for academic papers is to use two narrow
columns of text per page. Of course, for some people, long horizontal
lines are not a hindrance at all, so they might wonder why so much space
is being "wasted" to achieve narrow columns.

At least some of the endless wars of opinion on the "correct" way to
format code are simply manifestations of the differences in the way
people see. For example, if your eyes/brain have no difficulty parsing
long horizontal lines, then you'll be more inclined to use them, but of
course, you'll undoubtedly be at least annoying all the people who have
trouble with reading long lines. For some people, the long lines may
actually result in pain due to eye strain/fatigue.

Type Faces (Fonts) and Type Sizes

Just as PG has difficulty with long horizontal lines of text, my brain
has difficulty with most type faces (fonts), and even more difficulty
with switching between varying type sizes. I love books, but the
typography of most books makes reading them far more difficult, and
painful, than they need to be. The same is true for websites, PDF files,
and other documents. On a computer I can force my preferences of type
face and size on whatever I'm trying to read. It does take a lot of
effort to learn how to configure browsers and other programs to
over-ride the style of all websites and documents, but for me, the
results are worthwhile.

Foreground and Background Colors/Brightness

This is one of the most wide spread and well known problems causing eye
strain on computers, but due to fads and fashions, it may never be
fixed. The typical dark foreground text on white backgrounds on
computers is just a skeuomorphism carried over from printed pages of
black ink on white paper. The trouble is, paper pages reflect light, but
computer displays generate light.

The best way to understand this problem is to pick the brightest
lightbulb in the room where you're sitting, and go over and read the
wattage off the top of the bulb. If you're a smart person, you turned
off that light bulb before trying to read the wattage numbers on the
top. If you're a not so smart person, you stared into the sun while
trying to divine the nearly indecipherable written text. If your eyes
are normally sensitive to light, then you only caused yourself some mild
discomfort when reading the lit lightbulb. On the other hand, if you're
one of the people who are extremely sensitive to light (sometimes called
"light allergic" in less accurate terms), then by reading the lit
lightbulb, you just caused yourself a memorable amount of pain.

You can solve this problem by configuring your text editor, browser, PDF
reader, operating system, and other programs to use a "dark" theme or
"Reverse Video" mode. This problem is common enough for even the iPad to
have a "Reverse Video" mode available in its options, but to be honest,
I forgot what Apple called the feature.

Be warned, what works for you might be ugly and it may even unforgivably
offend your more fashionable peers. Fuck em. Seriously. Just fuck em.

It's your eyes/brain/pain. If what works for you is green text on a
black background with a single sized fixed pitch font, then use it. It
might be hideous, but avoiding the pain of eye strain is far more
important than the nuisance of having to put up with the occasional
Matrix jokes.

One important issue to be aware of is color deficiency. Many people are
color deficient, or even color blind, and this is particularly true for
older males. Any eye exam can help you know a bit more about what your
eyes can see easily, and help you pick what looks good to you.

Lastly, both color saturation and brightness levels on your display are
worth testing to see what works for you.

Screen Reflectance (Glossy Versus Matte Finish)

I always love to joke about the reason why Steve Jobs always wore black
turtleneck shirts because without them, he would be able to see anything
on the glossy screens Apple always uses. Oddly enough, there may have
actually be some truth in my little joke, but I'll never get the chance
to ask him.

Anyhow, some people have no trouble focusing through a reflection to the
content on the other side, but for other people, reflections mean
constantly switching focus from the content to the reflection and back.
The result of the excessive switching is often eye strain and pain.

If you have a screen with a glossy finish, then you have two choices;
(1) replace the screen with something that has an anti-reflective matte
finish, or (2) get a anti-reflective filter for your existing screen.
NOTE: This is NOT a "privacy filter" but instead is a thin plastic
anti-reflective sheet that you stick to your screen. The sticky sheets
are removable, so you don't need to worry about making a permanent
change to your monitors.

PRO_TIP: Use an electric blanket to warm up the sticky sheet before
applying it.

Surprisingly, the opposite problem also exists. A glossy screen is, by
definition, more transparent and causes less distortion. Any
anti-reflective filter you use, whether a built-in coating or and add-on
sheet, will reduce the focus of the underlying content. Some people have
a great deal of difficulty with reading/seeing unfocused content, and
like everything else mentioned so far, the result is increased eye
strain.

Refresh Rates and Dot Sizes

The biggest loss in the move between CRT (tube) based displays and newer
LCD (flat) displays is the latter typically has a fixed or "native"
refresh rate. On older high-end CRT's, you can achieve very high refresh
rates if you configure the display and graphics hardware properly. If
your eyes/brain have difficulty with bright things flashing at you, then
the refresh rate of your display becomes very important. Higher refresh
rates are always better for reducing eye strain, but they can be
extremely expensive.

Most consumer equipment has a refresh rate around 50-60 Hz. The more
advanced (and more expensive) equipment can exceed refresh rates of 120
HZ. The trouble with LCD's is, exceeding the "native" rate of the LCD
panel component doesn't actually give you an increased refresh rate,
regardless of what your settings are. In other words, the LCD might be
perfectly happy to lie to you.

Though non-metric, "Dots Per Inch" (DPI) is the common measure of how
many points of light are within a given horizontal or vertical line. The
stated number is usually symmetric across both horizontal and vertical
axises, but not always, so you need to pay attention when reading the
specs of a display. The more dots you have, the better. Well, at least
more dots is usually better unless you are exceeding the native
resolution of an LCD. Again, most all LCD's have a "native" resolution.
You might be able to drive the LCD at a different resolution, but the
results are suboptimal.

Environment Lighting

Though it's somewhat covered above in "Screen Reflectance," as more
light falls on you and things behind/around you, the worse the
reflection problem will be. On the other hand, some people have
difficulty with switching between light/dark environments causing
strain, so a well lighted room can also be a good thing. Some people
need a well lighted room to watch TV, and others prefer a dark room like
a movie theater.

There are some people who simply cannot endure florescent lights without
ending up in a great deal of pain. It's a well known problem but there
are a handful of different causes. In some cases it has to do with the
frequency of the generated light. In other cases it has to do with the
flickering caused by the ballast, poor circuit isolation, poor quality
mains power, or even interference from other electrical fields.

Sometimes the easy answer is to just shut off the over-head florescent
lights and use a normal lamp, with a natural spectrum incandescent bulb,
plugged into a small Uninteruptible Power Supply (UPS). Of course, since
people are different, some can't handle natural spectrum incandescent
bulbs (e.g. overly sensitive to near infrared light), and others can't
handle the spectrum of light provided by typical (cheap) light bulbs.
Using a small UPS is optional, but if your building has dirty mains
power, then the result will be the brightness of the bulb constantly
changing. Even if the change is imperceivable, it can still cause eye
strain, just like the flickering of florescent lights.

If you're unable to shut off the over-head florescent lights, you have
two options; (1) wear tinted glasses (only helps with spectrum issues
but doesn't help with flicker issues) or (2) quit. It doesn't matter how
prestigious your employer is or how much they pay you; nothing they can
offer you is worth harming your eyesight through continued eye strain.

Eye Testing

It sucks to admit that you might be getting old, but more often than not
vision deteriorates with age. Going to a good eye doctor to get your
eyes tested is a REALLY good idea. You probably want to avoid the
cheap mall-based eye stores that sell glasses. You'd be better off
finding someone who actually knows what they're doing, like getting a
reputable eye surgeon in your area to do the testing. It will be more
expensive, but well worth it.

If you are told that you need glasses, get them, but also look into eye
exercises. It is not uncommon to fix mild vision deficiencies through
various forms of exercise to strengthen the muscles you use to focus.
Also be aware that compensating for weak muscles by using glasses can
lead to weaker muscles and the problem getting worse. Eye exercises are
seldom an answer, but it's worth looking into, if you pardon the pun.

Lastly, if you need help with configuring software, my email address is
in my HN profile 'about' section.