Abstract
D.Phil.
Under certain conditions the human visual system accommodates and postures at a
position classically known as the dark focus. The dark focus is usually presented as a
spherical phenomenon. The characteristics of the dark focus that can be conveyed by its
spherical presentation are limiting: little representation can be given of the characteristics
of the variation that occurs when measurements are taken of the dark focus (for example
under dark conditions); no indication can be given of any antistigmatic (non-spherical)
change or variation; differences in variation between light and dark conditions cannot be
shown or detected using classical methods of analysis; meridional characteristics of the
dark focus cannot be represented, and so on. In this thesis I have preferred to coin and use
the term dark refraction shift (defined here as: 8F = F dark- Flight) for what has classically
been known as the dark focus. I have done so because I believe that the term is a better
description of what happens to the human visual system under dark conditions.
Multivariate methods of analysis allow for a much more detailed, and complete,
presentation ofthe dark refraction shift and its variation. The limitations of methods used
previously and mentioned above are overcome when multivariate methods are used to
analyze and present dark refraction shift data. This thesis presents the dark refraction shift
and its various characteristics, making use of multivariate methods that are used for this
purpose for the first time.
A Hoya AR550 autorefractor, set to measure refractive state to the nearest 0.01 D,
was used to take 50 measurements at a time of the refractive state of twenty subjects
under varying conditions ofluminance. The autorefractor was re-focused after each
measurement. In the light condition, the subject was positioned in front of the
autorefractor, the room lights were left switched on and the fixation target inside the
instrument was visible to the subject. 50 measurements were then taken under these
conditions. A second set of measurements was then taken under the dark condition. In the
dark condition measurements the subject was left in complete darkness for five minutes
to allow the accommodative system to settle at the dark focus. No fixation target was
visible to the subject who remained in complete darkness for the duration of the dark
condition measurements. 50 autorefractor measurements were taken under these
conditions. Two measurement sessions were conducted where the order of the light and
dark conditions were reversed. In the case of each subject the order of light versus dark
condition measurements was determined randomly for the initial session and reversed in
the second session.
Subjects had to fulfill certain selection criteria; the refractive state had to have a
cylinder equivalent dioptric strength of less than or equal to 6 D, the visual acuity had to
be 6/6 or better in the right eye, subjects with strabismus were eliminated and subjects
with any observable ocular pathology were not accepted. Ten of the subjects were aged
between 21 and 35 years of age. They constituted the prepresbyopic group. The other ten
subjects were aged between 40 and 65 years of age and constituted the presbyopic group.
Each subject had a total of 200 measurements taken, 100 measurements taken in the light
condition and 1 00 taken in the dark condition.