Michael Rigby-Jones wrote:

> It's almost certainly astigmatism, it is a very common eyesight
> fault.  AFAIK the rods and cones in the eye are not arranged in a
> manner that give increased vertical resolution.  I wonder what the
> typical eyeball resolution is?  I would think the brain probably
> performs some kind of resolution enhancement.  Dr Webster?

  I agree on all points.  Astigmatism means horizontal and vertical
elements focus in different planes, eyes probably do prefer to pull in
focus in one plane or the other, or one focus may be nearer to the
resting focus and that would certainly explain the observation.

  Cones register detail and colour in the macula (central area), but are
less sensitive and slower, rods register movement and lower light levels
outside the macula.  I know of no evidence suggesting different scale
factors, but I would take the fact that we are good at determining
roundness of objects to suggest that horizontal and vertical scaling are
equal.  (I was tempted to speak of a liking for nicely rounded objects!)

  I'd have to go measure my eye chart at work to tell you the standard
resolution, but it's a four metre chart and the 6/6 type is about 4mm
resolution (IIRC) which is therefore about a milliarcradian.

  The brain performs *superb* resolution enhancement, even to finding
detail that isn't there at all.  Some are better than others at this,
and find employment as politicians etc...

  Actually, the main thing appears to be that the physiological (i.e.,
normal functioning) jitter of the eye muscles "sweeps" the image across
the cones, so interpolating the resolution inherent to the receptor
spacing (remember enhancing ADC resolution by adding noise and
averaging?) and also stimulating the "phasic" (high-pass) response of
receptors.
--
  Cheers,
        Paul B.