Dear Sci.med.vision members,

One poster raised the issue of optical dead-band,
and then told some one to F. O. when he
asked questions about it -- if you are a professional.

That is NOT the way you deal with scientific
questions.  And it is NOT the way you talk
to the public about this scientific issue.

So here is the correct response:

===============

Subject: Depth-of-focus, and range of values.

Re: Range of "travel" of accommodation.

I think you know this concept as "depth of field",
or optical dead-band.

In engineering it is called dead-band, or DSO, or
Dead Space Operator.

What is means is that a signal is blocked,
and must exceed the dead-band -- for effective
refractive control to exist. Or for that matter
for an image to be blurred on the retina.

Here is the accurate reference:

"Accommodative Oscillation Can Enhance
Average Accommodative Response:
A Simulation Study"

George K. Hung, John L. Semmlow ang
Kenneth J. Ciuffreda

IEEE Transactions on Systems, Man and Cybernetics,
Vol. SMC-12, No. 4, July/August 1982

Statement:

The deadspace operator reflects the optical depth of field, and is
normally a function of pupil diameter, ranging from +/- 0.15 D
for a 8-mm diameter pupil to +/- 0.85 D for a 1-mm pupil [15].

15. F. W. Campbel, "Depth of Field of the Human Eye", Optica Acta,
Vol. 4 pp 157-164, 1957

For a sun-light pupil of 2.5 mm, the depth-of-field would be about 1.2
diopters, as I suggested. (+/- 0.6 diopters).

That is the source I am using.

Thus the eye could have a theoretical positive STATE,
of +1/2 diopter and still have a clear image on the retina,
because of the eye's depth-of-focus.

This explains why you can have no "accommodation",
and good far an near vision.

Otis