In message , Steve Terry
writes

"james" wrote in message
...
In message , Simon Dean
writes
Jaf wrote:
On Thu, 3 Sep 2009 03:57:02 +0100, "Bill Wright"
wrote:

"james" wrote in message
...
In message , Simon Dean
writes
snip
Why do you think there there is no such thing as eye strain?

For sale: Wall mirror. Only looked in once. JF


AFAIK it's the muscles around the eyes that get strained
which lead to so called eyestrain headaches, etc

The muscles which control the eyes' accommodation won't have to do
anything unless the objective's distance varies, and the iris muscles
that control aperture don't do much unless the ambient light levels
change. And even if those muscles do have to work, they hardly use
enough energy to challenge their supply of oxygen.

The brain may get tired by an increased level of concentration but there
is no such thing as 'eye strain'. A camera's lens cap is not provided to
protect it from allowing too many images to pass through it.
--
James Follett.
Http://www.pbase.com/jamesfollett
updated to include 'Wings' air and vehicle show pictures at
Dunsfold.
http://www.jamesfollett.dswilliams.co.uk