Re: Only shades of gray...

----- Original Message -----
From: "Bill William" <iodideshi@yahoo.co.jp>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, June 08, 2004 8:07 AM
Subject: Only shades of gray...

> I hope this is not a REALLY stupid question...
> I am good at asking those,,, but
>
> Does any one know how many shades of grey humans can
> discern, between black and white?
>
> I know the answer will be a variable range, but what is
> the range?
>
> What is the maximum number possible (probable)?
>
> 30? 130? 1300?
>
> If anyone knows of any such studies, I would love to hear
> about them!
>
> Ray
      I think the answer to this lies in books on human
vision. Two factors will affect the range: the level of
illumination and the physical separation of the areas to be
compared. The visual system is very good at comparing
adjacent brightness and color and rather poor when the
objects are separated. There is somewhere a number for the
minimum difference in brightness the eye can detect for
adjacent areas. I suppose this could be considered the
number of steps. I don't know the _total_ range of
brightness but the eye compensates continuously for
brightness both by changing the size of the iris for short
term adjustment, and by varying the amount of visual purple
in the retina for long term variation. The difference in
sensitivity of the eye can vary over a tremendous range. It
seem to me (by memory) that the ability of the eye to
discriminate small variations in brightness varies with
illumination level, there being some optimum level where it
is greatest just as there is for resolution of detail.
   At least some of this stuff is in the _Handbook of
Optics_.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com