question for Darryl

**harry a saake** · 04-05-2003, 09:49 PM

Darryl, with todays modern design lenses, especially aspherics, does gerstmans law still hold up.

**Darryl Meister** · 04-08-2003, 12:31 AM

Hi Harry,

Sorry about the delay... You'll have to refresh my memory a bit, which rule is Gerstman's? The one related to near centration?

Best regards,
Darryl

**harry a saake** · 04-08-2003, 07:58 PM

Hello Darryl, yes it states for every diopter of power, lens should be decentered in .75 mm

**Darryl Meister** · 04-12-2003, 01:51 PM

In general, you want to keep the center of the aspheric surface relatively close to the pupil (or, more specifically, the line of sight in primary gaze). Unlike a spherical surface, which has the same power across the entire surface, an aspheric surface actually has a center or pole. This is why you can grind prescribed prism in an aspheric lens, but not prism for decentration, which would move the optical center away from the actual pole of the surface.

Many manufacturers recommend keeping this center within 5 mm or so of the pupil. Obviously, for horizontal decentration, this is less of an issue since the centers/poles are generally decentered in to align with the pupils. Rules that apply to near centration are only applicable to multifocal lenses, but are generally not applicable to aspheric multifocals. Aspheric bifocals have a fixed amount of inset, since the pole of the surface is fixed, and laboratories generally grind the optical center to coincide with the pole of the surface. While this precludes the precise calculation of segment inset, it results only in a small reduction in the field of binocular vision through the segments. Since this binocular field is already quite large for flat-tops, the difference will likely be insignificant to most wearers.

Furthermore, aspheric lenses are more sensitive to aberrations produced by improper pantoscopic tilt, so the relationship between pantoscopic tilt and decentration should be respected -- particularly for high powers. (That is, 2 degrees of tilt for every 1 mm that the optical center is positioned below the pupil.)

Best regards,
Darryl