THE ACCOMMODATION SYSTEM
The accommodation system operates on incoming rays of light. It detects micro-blur at the surface of the retina, and changes this power of the (internal) lens of the eye, to reduce this micro-blur below a detectable level.
RAYS OF LIGHT – FROM INFINITY.
Figure 1 shows positive status, or stop position.
Figure 1
LIGHT RAYS FROM A 13 INCH OBJECT
Notes on the following two accommodation graphs, Figures 2 and 3.
The range of accommodation (stop-to-stop) is greater than seven diopters. To be consistent, I show a range of 7 diopters. I show this for a farsighted eye, and a nearsighted eye. But what I mean is a normal eye with a positive and negative refractive state (self-measured).
The convention I use in these graphs, is that a value of infinity, is a refractive state of 0.0 diopters. A near object will always have negative status, based on the 1/distant = Diopters equation.
Thus an object at 0.33 meters, will have a value of -3 diopters. Thus the horizontal is labeled in negative diopters (distance from the eye).
To the right, is a postive refractive environment. You can only get this by placing a +1 diopter in front of the eye.
The normal eye show linear movement between these two extremes. (Some children actually read at 4 inches, or -10 diopters.)
The accommodation system must change the eye’s total power, by an amount equal to that change to a near reading position. Thus the vertical line is labeled in positive diopters.
If you read at 13 inches (-3 diopters) your accommodation control-system must change the eye’s power by exactly +3 diopters.
The maximum and minimum stop positions are shown – as labled.
The near stop position, is seldom calculated, since few people read at 4 inches and -10 diopters. If this is and established fact, then the chart could be changed to reflect this truth.
THE EYE WITH A POSITIVE REFRACTIVE STATUS
Figure 1
Shows a positive status, or “stop position”
Figure 2
Figure 3
THE EYE WITH NEGATIVE REFRACTIVE STATUS
The lens of the eye moves between two “stop” positions, the distant-stop position, and the near-stop position. In the range shown, the lens maintains sharp focus on the retina of the eye.
IN THE FOLLOWING TWO FIGURES, THE EYE HAS A
REFRACTIVE STATUS OF +1 DIOPTERS.
The eye’s response to looking from the distance (infinity) to a book at 13 inches. (A change of -3 diopters. )
These drawing are a composite of infra-red measurements, to establish the true behavior of the internal lens of the eye, and its response to both a negative and positive change in its visual environment.
Figure 4
The accommodation response to a step-input of -3 diopters.
The internal lens response to a +1 diopter lens.
Figure 5
The eye’s response to a step-input of +1 diopters.
Please note – this eye has a refractive status of +1 diopter.
READING A BOOK 13 INCHES FROM THE EYE THOUGH A +3 DIOPTER LENS.
Figure 6
This paper contains the reference analysis and drawings taken from an infrared Optometer for your interest.
Reference 1:
Reference 2:
Reference 3: Theory
https://myopiafree.wordpress.com/theory/
Reference Paper on Accommodation, with sketch of accommodation response to step-inputs. L. Stark.
Myopiafree 
Subject: The origin and need for offset, in the eye’s design.
This is how I thought about designing my “first” auto-focused eye.
The accommodation system is an exact replica of what you look at – for 16 hours a day. I think I must “postulate” that as a fact, but I think you will agree ( or at least Antonio will agree).
This is BEFORE I had seen Dr. Young’s data.
So, we now have a two stage control system. The first system, is the “average” of accommodation, on a 16 hour per day basis.
We design are “relativistic”, to control its refractive STATE to the accommodation signal – directly.
But we find this primitive control system, will have a problem with it.
If the average of accommodation is –1/2 diopters (with a 100 day time-constant), our first design will SLOWLY become nearsighted, i.e, negative state of –3/4 diopters.
OK, that will not work. So now we need an “offset”, to make certain this does not happen.
So now we must “offset” the accommodation signal by about +1.5 diopers. With this “offset” applied, our “better design” will have a refractive STATE of +1.0 diopters.
Obviously this limit must be understand as a operational-limitation.
We can not place this relativistic eye, in a –3 diopter environment, or it WILL “servo” its refractive STATE to a “negative value”, and with this negative value, we will not be able to read the 20/40 to 20/50 line.
Using a trial lens, we fill find this eye has a refractive STATE of –1.0 diopters.
That is why I judge that evolution designed a natural eye – with an explicit measurable value of the offset.
OPEN-LOOP ACCOMMODATION PAPER, ABSTRACT:
Open-loop accommodation in emmetropia and myopia.
(PMID:10694894)
Strang NC , Gilmartin BS , Gray LS , Winfield NR , Winn B
Department of Optometry, University of Bradford, England.
Current eye Research [2000, 20(3):190-194]
Abstract
PURPOSE: To investigate the influence of method of measurement and refractive error on the open-loop accommodation response.
METHODS: Open-loop accommodation was measured in darkness (dark accommodation, DA) and using a pinhole pupil (pinhole accommodation, PA) in emmetropic subjects (EMMs, n = 63), subjects with late-onset myopia (LOMs, n = 50) and subjects with early onset myopia (EOMs, n = 51). Further a control experiment examined the differences between DA and bright-field accommodation (BA) conditions in a subset of subjects. All measurements of open-loop accommodation were carried out monocularly using a Canon R1 infra-red optometer in static recording mode. All myopic subjects were fully corrected using soft contact lenses.
RESULTS: A significant variation (p < 0.001) in open-loop accommodation was found between DA and PA, but no variation in open-loop level was observed between the three refractive groups. There was no interaction between these two factors. No significant difference was found between the BA level and DA level in any of the refractive groups.
CONCLUSIONS: Open-loop accommodation response positions vary according to the experimental conditions employed during measurement. No refractive group differences in the open-loop response were apparent