Hi Eliot,

On 3 April 2018 at 15:20, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> Hi Dark Themers,
>
>     in an earlier life I worked in optics, designing holographic cameras for
> bubble chamber physics, so I know a little about focussing systems and depth
> of field.  The iris in the eye expands or contracts to allow or restrict the
> amount of light entering the eye, attempting to maintain a constant
> luminosity on the retina so that either not too little, or not too much
> light falls on the cones and rods in the retina.  When the iris is dilated
> (open) the eye allows as much light as possible into it.  You can visualize
> the light from a point (say a pixel) that reaches the iris as a cone.  The
> lens in the eye focuses this expanding cone back into a contracting one that
> comes to a point on the retina corresponding to the position of the pixel
> "out there on the screen".
>
> Focusing is achieved by a muscle around the eye's flexible lens, the cillary
> muscle, which squeezes the lens into a more spherical shape to cause rays
> entering the eye to bend more, brings no objects closer to the eye into
> focus, or by the cillary muscle relaxing, allowing the lens to stretch back
> to a flatter shape, to bend rays less, bringing objects further away into
> focus.  In old age shortsightedness is caused by the lens loosing its
> elasticity and remaining squeezed, and longsightedness by it losing
> flexibility and the cillary muscle losing strength so that the lens cannot
> be squeezed as much.  By the time we hit our 40's many of us will suffer one
> of these two extremes and have to wear glasses either for reading or for
> driving or, in my case, for both.
>
>
> Getting back to the cone of rays from a pixel that the iris and lens
> conspire to bring to a point on the retina, if there is a lot of light
> incident on the eye and the iris is undilated then this cone is much
> slimmer.  The result of these differing cones on focus is called depth of
> field.  When the cone is "fat" depth of field is reduced; only pixels in the
> same plane (actually a spherical surface, not a plane, because the retina is
> spherical; film cameras have planar light receptors; the eye and camera
> obscures etc have spherical light receptors) will be in focus; others out of
> the plane will produce a diffuse circle on the retina.  When the cone is
> "slim" (because more light is incident on the eye) depth of field is
> increased because the size of the out of focus diffuse circle is smaller.
> Consequently, when there is less light falling on the eye, depth of field is
> reduced; the cones are fatter and as the eye roams the cillary muscle must
> work to alter the curvature of the lens to keep things in focus.
>
> The implication for the dark theme is that, while it appears to have better
> contrast (it does not; but more on that below), the real effect is that it
> causes the eye to do more work than a light theme because the amount of
> light entering the eye is less.  So both in the short term and especially
> over the long term the dark theme, relative to the light theme, will tire
> your collate muscle and cause your lens to stiffen or squish sooner.
>
> Why then, if what I'm saying is true, did all those World War Two military
> aircraft use white letters on a black background?  My lotus europa is the
> same.  The disc of the instrument is illuminated by a lamp so one can see it
> at night, and were the panel painted white then, for the same contrast, much
> more light would hit the eye that for white letters on a black background
> and the pilot's (or driver's) night vision would be impaired as the iris
> would contract.
>
> I don't want to fear monger, but I do want to suggest that it is healthier
> and less wearing on the eyes to use light themes.

Great description, thanks!  I went through this 2 or 3 years ago - I
used to like dark themes because they looked "nicer".  Now I'm
exhausted after looking at them for a few minutes.  I also get to
enjoy dealing with multi-focal glasses.

Although I do miss my green-on-black terminal...

> Eliot
> _,,,^..^,,,_ (phone)

How can you type all this on a phone?  I'd be cross-eyed, as well as
feeling like my phone had a dark theme (which it doesn't). :-)

Cheers.
Alistair