Dear NR,

Subject:  To methods of writing a "prescription".

Some write a "plus" and some "minus".

These can be converted to spherical equivalent,
which it to take 1/2 the cylendar power and
add it to the spherical component:

(This ignores the axis.)

Here's the new prescription:

SPH   CYL   AXIS

-6.25   1.75   x15

1.75 =~ 0.85

-6.25 + 0.85 = -5.4 spherical

-6.75   2.50   x10

Here's the year old prescription (prior prescriptions were similar to
this one):

-4.50   -1.75   105

-4.5 - 0.85 = 5.35 spherical equvalent.

-4.00   -2.25   100

Two questions: (1) why would the CYL values change signs, and (2) how
do the axis values relate?

Many thanks for any information.

I am not an optometrist.

Converted to spherical equivalent, the "prescriptions"
show little change.

The two "conventions" do cause concern -- obviously.

Best,

Otis

"The two prescriptions are very similar. There are two different
conventions
for writing eyeglass prescriptions"

correction, but causes confusion for many. The proper and accepted way
is to write a minus cylindar value since astigmastim is grounded on the
back of glasses.