Tony,

CR39 (traditional plastic) will most likely be a spherical design, so look for a
base curve of about +2.50 ± .50 D for best form (minimal off-axis marginal
astigmatism and power error). If aspheric, the BC will be somewhat flatter,
depending on the degree of asphericity. Higher index of refraction lenses will
have slightly steeper BCs for best form optics, although many of these will be
aspheric, hence flatter BCs.

Have an experienced optician dot the OC and draw horizontal and vertical lines
through that point. The vertical line should be ±1mm center pupil for each eye.

Raise your chin so that the lens is perpendicular to the floor. Look straight
ahead- the horizontal line should be ±1mm center pupil (both eyes). If not,
adjust the panto tilt until it is, although the final tilt should be in the six
to twelve degree range. The above assumes no precribed prism.

The dihedral tilt (face form or wrap) should be about +5 degrees.

Vertex distance should be as little as possible without the lenses hitting the
lashes or face. 12mm to 14mm is typical. Longer distances are not desirable but
sometimes unavoidable due to individual facial structures. Regardless, it should
be ±2mm at this power compared to the doctor's vertex distance. However, it's
unlikely that the doctor measured the Vtx so assume about 13mm.

Inspect lenses for waves and warpage (no more than 1 D for the latter).

A visit to the doctor is required if all of the above checks out.

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman