Accommodating of the eye
On this page I talk about normal vision: meaning: a far
point of infinity (possibly using proper spectacles), photopic
vision, an accommodation amplitude of 4D (around 35 years, see below graph), an average acuity of 20/15
(around 35 years), no eye/brain deceases and for this page:
using monocular vision.
When auto focussing (accommodating) the eye/brain combination needs
to do at least two thinks, IMHO:
- a person must be able to experience that the picture is not in
focus: so seeing the picture as sharp as possible in brain.
- the eye's lens must be able to provide focusing capabilities
that matches the needed range.
The accommodation amplitude of the eye depends on the age of the
person. Checking several articles (Chattopadhya
gave the below graph. The reference
accommodation amplitude is defined as 4 D(iopters).
There is a large difference between subj(ective) and obj(ective)
results, certainly for persons below 30 years of age.
The above amplitude does not say where the Far and Near points
are. Using lenses/spectacles one can move these Far and Near
points to the places which are normal for healthy eyes.
And what is the Far point of 100% focussing (so without
including effects due to pupil size: Depth of Field [DoF]) of
the human lens?
- Some people say the Far point is 20 ft/6 m. This looks very
oriented to this 20/20 measurement, but the 20 feet there is
just a clinically expedient reference distance as Snellen
just wanted the repeatable measurement of acuity and thus
used the most common room length in his time. It is not
related to optical infinity or the Far point, but it is called
Effective Far point as it is close (0.164D) to infinity's
experience. This is a typical approximation by a optometrist.
- Some others relate it to the number of photo receptors (~60
lp/mm) as if it is a static device: 17mm focal length and F6.0
and 0.0083mm CoC, gives a hyperfocal
distance of ~5.8 m (close to this 20 ft?), which they
see as the Far point. But the eye is not a static imagining
- Evaluating the resolution of the photo receptors as if the
eye is a static device is not valid; its is a change detection
Fulton, page 72 and 78, 2012) that needs a constant
tremor (amplitude some 30 seconds of arc) to view the object.
This results in an acuity.
So "The high (hyper-) acuity of the eye is determined by the
steepness of the edge of this signal and not by the diameter
of the individual photo receptor." (pers. comm. J. Fulton
- In a lot of literature the Far point is at infinity for an
unaccommodated healthy eye (for instance: Ophthalmology
This is indeed the situation under normal vision conditions.
<Remark: a relaxed eye (so not an unaccommodated
eye) will accommodate on the Resting
point (some 80 cm, and is depending on age)>
I would like to thank the following people for their help and
constructive feedback: Jim Fulton, Bill Glickman, John Krantz and
all the people who provided feedback. Any remaining errors in
methodology or results are my responsibility of course!!! If you
want to provide constructive feedback, let me
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Last major content related changes: Jan. 20, 2013