OC Heights

[OPTIDONN]

How many of you take optical center heights? Should they always be taken on aspherics? I know to minimize power error the lenses axis should pass through the eyes center of rotation but if an optical center height is not taken how could the axis pass through the center of rotation?

[QDO1]

How many of you take optical center heights? Should they always be taken on aspherics? I know to minimize power error the lenses axis should pass through the eyes center of rotation but if an optical center height is not taken how could the axis pass through the center of rotation?

Always, on everything

[Darryl Meister]

Oddly enough, I actually overlooked this thread.

While it's certainly not a bad idea to take a pupil height measurement, it's usually not a great ideal to vertically decenter the optical center to the pupil height. Yes, you disrupt the relationship between the optical axis of the lens and the center of rotation of the eye when this is done, but you also increase the maximum thickness of the lens blank. This is a particularly important consideration for high-index and aspheric lenses, since the wearer is actually paying for a thinner, lighter lens.

You may consider raising the optical centers some when fitting the wearer with a low-Abbe material in a high power, in order to minimize chromatic aberration, but even then I wouldn't suggest raising the optical centers all the way to the pupil height. Consequently, if you do take pupil height measurements, you don't necessarily want to specify those heights without some consideration.

[chip anderson]

What Darryl said is true from an asthetics point of view. But If you don't take the vertical height into concidersation, especially on low abbe materials. You can end up with a lot of patient's telling you about those yellow and red outlines around everything. Unfortunately as styles are appearently about to dicatate larger lenses again, you can look for more trouble in this reguard. Unfortunately higher cylinders and higher powers will also accentuate the problem.

Of course this is why poly is a junk material for high powers.

Chip (I hate to disagree with Darryl twice in a week).

[Darryl Meister]

I hate to disagree with Darryl twice in a week

I don't know:

Chip: "If you don't take the vertical height into concidersation, especially on low abbe materials. You can end up with a lot of patient's telling you about those yellow and red outlines around everything,"

Sounds an awful like:

Darryl: "You may consider raising the optical centers some when fitting the wearer with a low-Abbe material in a high power, in order to minimize chromatic aberration."

;)

[fjpod]

Oddly enough, I actually overlooked this thread.

While it's certainly not a bad idea to take a pupil height measurement, it's usually not a great ideal to vertically decenter the optical center to the pupil height. Yes, you disrupt the relationship between the optical axis of the lens and the center of rotation of the eye when this is done, but you also increase the maximum thickness of the lens blank. This is a particularly important consideration for high-index and aspheric lenses, since the wearer is actually paying for a thinner, lighter lens.

You may consider raising the optical centers some when fitting the wearer with a low-Abbe material in a high power, in order to minimize chromatic aberration, but even then I wouldn't suggest raising the optical centers all the way to the pupil height. Consequently, if you do take pupil height measurements, you don't necessarily want to specify those heights without some consideration.

I think this is a balanced approach which deserves a lot of merit. I sometimes wear poly (approx -3.50) and I don't have to look up and down to see chromatic aberration. In fact, I see it mostly when looking side to side. The point is my lenses are properly decentered for my PD, but not for my pupil height. So, as Daryl says, don't necessarily decenter a lens upwards "all the way" to the pupil center if it will significantly thicken the lens.

We use this approach on PDs sometimes too. If a patient wears > +/- 2.00D, and their PD is 60, and their habitual Rx has an MPD of 71, it would probably be a mistake to make new glasses with a 60 PD.

Heck, you can throw a patient off who was wearing a large B frame, where their pupil was a mile above the OC, just by putting them into a more narrow (sensible) frame. Had one the other day. A gentleman had reverted back to his old, thick lenses (+7) in a large B frame, because "he could see better" The new ones were the same Rx, not that narrow, same material, very close base, hands down a better fit. Go figure.

[QDO1]

I measure heights (and a whole load of other measurements) on everything because

• when it absoloutley does matter, I will be uterly fluent with taking the measurements
• it eliminates a lot of potential problems caused by prism, power and ABBE etc

You would be daft not to

[HarryChiling]

I would only move the OC's in anisometropic patients, otherwise you end up with prism imbalance and in some cases (not very many) the patient will complain of double vision, however; even when I move the OC I do take into consideration the aesthetics of the lens and sometimes may split the difference in vertical decentration.