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The IMAGING QUALITY of the well-focussed eye lens and
the turbidity and spread caused by the retina is
expressed in a spread function, being the intensity
Physiological profile of the retinal image of a very small point or
Aspects thin line. Recent values (Gubisch, 1967) are best
approximated by a gaussian function, to enable quick
applicationse Its two-sigma value, the width of the
ve intensity profile at 60.6%, is a measure for the imaging
quality of the human eye. Best values are 300 prad for the point
spread function, 400 mrad for the line spread function, and 500 urad for
the edge spread function (or, for & viewing distance of 250 mm: PSF 15 pam,
LSF 100 um, and ESF 125 um width),
These data have practical consequences for an objective definition of
what is a sharp image, and when to call & blurred image really unsharp.
They also permit to define the Optimum Magnification for a particular
image quality, and the Empty Magnificatione
[Imaging Quality depends on pupil size, being best at about 3 mm;
another reason to design instruments with a 3 to 4 mm exit pupil.
The performance of the eye lens is indeed a subjective property,
and it is a question what should be considered as normal, where
such a high percentage of the Dutch and Japanese population wear
spectacles. But instruments have to be designed for the best eyes,
as such persons should work with ease with a stereoscope. In this
case, an average Imaging Quality would do injustice to the best
workers. Moreover, the eyes' Imaging Quality depends on the Image
Quality of the prints; the lens cannot be aocomodated at optimum
€ © if the details are unsharp.
» A completely different visual property is the judgement of
differences in sharpness between two images. It seems to be possible
to notice differences of 10 to 20%]
fi x
X SZ SS S
iE A
ANNE 20- width
Win
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A point (smatt) a Ant (Hin). andl an edge (chap)
are b£urred Ly Lhe Lye s oplics into un sharb mages
on the retina ; the width 2g Âeing a measure for the LG.
