vertical placement of oc's

[ed_fowler]

hi,

not my decision, but i have been instructed to specify the optical center height on all spectacle orders. my experience in the lab business tells me "watch out for thickness issues!"

any thoughts??

thanks,

ed

[the hitman]

I thought this was only necessary for asp lenses or very high powered lenses.

[Jacqui]

I thought this was only necessary for asp lenses or very high powered lenses.

Some are starting to do it on all lenses. I like the idea.

[For-Life]

You can talk about thickness, but we need every edge we can get to promote better vision. If thickness was an issue, then we could play around with PDs also. Maybe not as extreme as lining them up in the centre, but probably a few mms there and there.

I prefer taking the OC.

[HarryChiling]

If you think it's unnecessary just split the B size and you'll get what you always get from the lab. I prefer to tilt the frames slightly when the OC is just above or below datum rather than specify a height and have thickness issues. If the OC is excessively out of the norm it's better to inform the patient that the frame isn't a good fit and just move on.

[Laurie]

Hello 'Boarders,

I used to take vertical OC's only for anisometropes and extremely high +/-. I can see the trend to want to take them now, with SV FF lenses, ect...

A word of caution: when you take vertical OC's, you need to do an additional adjustment/head position first, as there is a displacement of the effective OC height with tilt...(Martin's Rule of Tilt, for the Geeks).

For every 2 mm of tilt, in effect, the OC will be as if it were raised by 1mm. (This is because when light travels straight through the OC, it travels perpendicular to the surface). If we wore lenses orthoscopically (no tilt), then this would be OK. Since we add pantoscopic tilt, the light ray still passes through perpendicularly, which will raise the height as it enters the eyes, as if it was raised 1mm for every 2mm of tilt.

So...to measure:

1. Preadjust the frame front with the correct amt of panto, tilt, wrap.

2. Ask the patient/client to lift their head up, until you see that the lenses are orthoscopic, straight up and down, perpendicular to the floor and ceiling.

3. Spot the center pupils, as if you were doing a PAL fitting cross...

In this above procedure, because you took the measurement preadjusted, with the lenses plane to the face, it automatically takes care of the 1mm/2degree tilt problem.

Or......

Measure the 'as worn' panto, with a protractor, or a measuring device,

and then drop the OC from the Datum Line 1mm for every 2 degrees of tilt) (could explain this much better in person, enabling me to use my 'optical sign-language!)...

Hope everyone has a fun weekend,

: )

Laurie

[QDO1]

hi,

not my decision, but i have been instructed to specify the optical center height on all spectacle orders. my experience in the lab business tells me "watch out for thickness issues!"

any thoughts??

thanks,

ed

Good idea - as it will improve vision, especially off axis.

If thickness was an issue, the power of lens you are using would for sure be strong enough to require vertical centration anyway, especially with the tiny frame sizes in use nowadays

[Darryl Meister]

Your lab is correct regarding lens thickness; specifying optical center heights will often result in thicker lenses. Further, for patients paying for "thinner, lighter" high-index lenses, which typically benefit the most from specifying optical center heights, the visual benefits obtained by specifying optical center heights may be outweighed by the cosmetic consequences involved. Also, the reduction in lateral chromatic aberration afforded by specifying optical center heights will typically be greater than the improvements due to lens tilt for most prescriptions in modern frame styles, until you get above the +/-4.00 diopter range or into frames with exotic sizes or lens tilt.