more ? on the eye

[Charles V. Britton]

Another eye thing that I have been wondering about. Does anybody have a
good way to estimate the actual size of the retina? I've heard an area of
only a few square MILLImeters and would like verification or otherwise.

[Donald E. Simanek]

Charles,

You *did* mean *retina*?

Here's some facts from the Military Standardization Handbook on Optical
Design, MIL-KDBK-141.

The iris' pupil size ranges from 8 to 3 millimeters in diameter, and can
close from max to min size in 4 to 5 seconds. Both irises respond to
stimulation of either eye alone.

*Critical* seeing takes place only when the image is located on the fovea
in the center of the macula on the retina. This has a diameter of about
2.5 mm. Could this be the source of your info?

The relatively spherical back portion of the eyeball has a diameter of 22
mm. The retina covers about 60% of the surface of this sphere. Shouldn't
be hard to work out an estimate of the retinal size from this info.

While I've got the book open, here's some other info which may be of
interest.

Most of the refraction (and light convergence) occurs not at the lens, but
at the front surface of the cornea. The lens is merely a correcting
element. [Many physics textbook illustrations have incorrectly drawn ray
diagrams of this, showing most of the refraction at the lens.] The diopter
rating of the cornea-air interface is 43, while the entire eye has a
diopter rating of 58, the lens accounting for about 15 diopters. The
muscles which control the lens have pulsations at about 4 to 8 Hz, with
superimposed smaller pulsations at 2 and 0.3 Hz. The resulting
fluctuations of lens power have amplitudes of 0.1 diopter.

The lens diameter is about 8 mm.

Indices of refraction:

Aqueous: n = 1.336
Cornea: 1.376
Lens: 1.42
Vitreous 1.337

Pretty close to water. No surprise.

"When the eye sees only an empty field lacking detail the lens tends to
focus, not at the 20 foot "infinity" of vision specialists, but at about
1 meter. This near-sightedness is called `empty field myopia' for a
bright field and `night myopia' when the empty field is due to darkness."

I'm still looking for help on these questions:

(1) Simple way to show non-sphericity of the eyeball.

(2) Simple way to show the eye's chromatic aberration.

(3) Simple way to show the eye's spherical aberration.

(4) Simple way to show the normal eye's residual astigmatism and coma.

-- Donald