When an object is viewed through
a very small aperture (a pinhole) a clear image
will always be formed because only coherent rays
of light are able to pass through, so that the
'blur circle' normally formed by an out of focus
eye is reduced almost to the clear point that
would be seen if it were in focus. This means
that, provided there is no opacity of the eye or
impairment of the retina, the object will appear
clear regardless of any refractive error. The
image through a single pinhole is very small and
dim, but by using a regular array of similar
sized holes it is possible to enlarge the field
of vision and improve the overall brightness of
the image while still retaining most of the
clarity of at least the central area.
In practice, the holes are of course rather
larger than an ideal pinhole (which would be
infinitesimally small!) and the size of hole is a
compromise between clarity of resolution and
brightness of illumination. Similarly, in theory
the lens material should be infinitesimally thin
(and at the same time perfectly opaque); in
practice most versions are rather thick so that
the light travels through a 'tunnel' with rather
unpredictable optical results.
Multiple Pinhole Glasses page 2
