Re: Advice, vdu glasses from reading prescription

On Mon, 29 Oct 2007 10:59:47 -0500, Mike Tyner wrote:

More seriously, it's just two ways of describing a football-shaped surface.

Imagine a football. Imagine measuring the curvature along the seams (long
axis) as say, 40 units of curvature. Along the short axis, across the seams,
say it measures 60 units, so the difference between steepest and flattest is
20 units.

You can describe the surface as 40 + 20 or 60 - 20 and you get the same
shape. The "a+b" form always starts with the flatter surface and the "c-b"
form always starts with the steepest. To convert from one to the other, you
must also show that the starting point is no longer the 160 meridian, but
instead it's 070, 90 degrees away. So,

+050+050x160 is equivalent to
+100-050x070.

-MT

A very clear explanation, thank you. I now understand why my first
prescription expressed with -ve values, was actually the same as the next
one even though the numbers are very differnt. The optician told me they
were close enough, but without any explanations, now I get it!
.