T3D Re: Visual acuity

[john bercovitz <bercov@xxxxxxxxxxx>]

P.J.Homer writes:

> As the test pattern is a simple grid of black and white lines it seems to
> me that the rods should be taking as much or more of a part in resolving
> the image as the cones as they have evolved for black and white vision.

I have heard that the fovea is bereft of rods.  Probably Jim C could comment.

> Cones are for colour and I believe there is evidence that there are at
> least three types for the primary colours. This could mean it would take
> three cones to resolve a single white line as white.

I have heard that the eye works a little like a TV backwards: it takes light
levels with all the cones to find outlines and then also uses cones of the
various sorts to broadly paint on the colors.  The consequence would seem to
be that it would be harder to find where one color stops and another starts
than it is to find where the edge of an outline is, if that makes any sense.
I guess you could test this idea by finding two colors with the same bright-
ness and putting them adjacent to each other and then seeing if it is harder
to find the separation between them than between two colors of different 
brightness.  And again: Probably Jim C could comment.  Hope, hope.  8-)

John B


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