PD's

[chip anderson]

I really hate to admit this but I remember reading 30 years ago (back when I did only contact lens fitting and it didn't matter) that hyperopes )(or was it myopes?) need to converge 2 mm more than myopes (or was it hyperopes?) Question is: 1) Which has to converge more?
2) Should I order near PD's inset 4 mm (per eye (instead of the usual 2 per eye) for these people?

Yeah, you can laugh at me for being in the business this long an not knowing this.

Chip

[shanbaum]

Originally posted by chip anderson:
I really hate to admit this ...

Hmm... may be time to haul out the Darryl.

I don't know why convergence would differ per se, but, I do know that superfluous base in prism is physiologically more tolerable than base out. I know some O.D.'s believe that a small amount of base in prism is desirable. To under-decenter myopes, and over-decenter hyperopes, would ensure that any prism induced by mis-centration would at least be in the preferred direction.

[Darryl Meister]

Hi Chip/Robert,

Yes, myopes wearing spectacles need to converge less than hyperopes. Since minus lenses produce base in prism when the eyes converge in from the optical centers to read, the base in prism (which causes the image to be displaced towards the apex) reduces the amount of convergence necessary. Conversely, plus lenses produce base out prism while reading, which increases the amount of convergence necessary.

Consequently, for myopes you would reduce the bifocal inset required and for hyperopes you would want to increase the inset. (Some modern progressive lens designs do this for you automatically.)

Robert is right about tolerating more base in prism at near (while reading), since this would reduce the convergence necessary. (It might have some small effect upon accommodation though, since the two are very closely linked.) However, base out prism is more tolerable for far vision, since the eyes have more positive fusional vergence (i.e., ability to converge to compensate) than negative fusional vergence.

Best regards,
Darryl

[shanbaum]

Originally posted by Darryl Meister:
Yes, myopes wearing spectacles need to converge less than hyperopes. Since minus lenses produce base in prism when the eyes converge in from the optical centers to read, the base in prism (which causes the image to be displaced towards the apex) reduces the amount of convergence necessary. Conversely, plus lenses produce base out prism while reading, which increases the amount of convergence necessary.


Doh!

[pedseye]

Don't worry Chip, I have been doing this a long time too and just when I think I have taking PD's down to a science, someone new, usually and O.D. or M.D. tells me of another way and they all make sense then I get confused and just do it the way that has worked for me. I have a question though and I'm not to embarrased to ask, when using a P.D. stick for monocular P.D.'s, where do you have the person focus. Does it vary according to the type of lens? I have currently been having them look at my nose for reading glasses and at my right eye for distance right and my left eye for distance left. The distance PD's are consistantly on the money every time I do this. I read it somewhere a long time ago. I have been told to have the patient look in the distance and dot their pupils for distance PD's, but they never stay focused when I get in front of them to mark the lens. Pupilometers are not as acurate for me either. What method works best for you guys? Also, what is the best way to measure PD's for kids? And do you measure differently if they are prescibed a high hyperopic rx than a high myopic rx? Always looking for what works BEST !!!
Thanks !

[chip anderson]

As to the accuracy of pupilometers, many years ago when I was primarily a contact lens technician I won a pupilometer as a door prize. After a while I was filling in on a busy day in the optical department working with a journeyman optician (40+yrs experience), he measured all his patients with a ruler, I did all of mine with the Pupilometer. When the patients came back to pick up their glasses, we swaped patients (not intruments). In over 40 patients we didn't find over a millimeter variation. Show's to go you, is one more accurate (normal myopes/hyperopes, etc.) no, just depends who's hands are using what.

Chip

[pedseye]

So Chip,

I guess you prefer the pupilometer? How do you measure for kids with a narrow PD? I would love to hear your method.

[chip anderson]

Dear Pedeye: Just because I use a pupilometer that doesn't mean that I can't use a ruler. As to children and adults with narrow P.D.'s. if the eyes are straight, I measure inner iris of one eye to outer iris of the fellow eye. If not I try, marking the demo lens, trying to measure the center of the straight eye (and the other if it will hold straight with the fellow blocked) to the center of nose.

Sometimes I just have to guess (been at this a long time and a pretty good idea what P.D.'s are just looking).


Chip

[Di822]

Some kids won't sit still long enough to measure with a pd rule so I get them to fixate on the wall behind me and I quickly dot their pupils with a marking pen. Then I just measure from dot to dot for a distance pd. So far it has always worked. I use the pupilometer to measure adults, but I still dot the pupil. Then I have something to compare. I am usually within .5 mm each eye.

[Sara]

Here is very simple method of taking PD in babies.Shine light in the baby's eyes and measure distance between corneal reflections.This is distance between visual axes of eyes which is actual PD.
I hope this student has been helpful.
Sara

[This message has been edited by Sara (edited 03-09-2001).]

[chip anderson]

Note To Sara: Don't forget that the distance the light is from the eye matters. If the light is within 1 meter of eye, you must allow for 1 mm convergance (add 2mm to P.D.) per eye. Also test will not work if patient has an eye that turns in or out (even with fellow eye blocked), then you must use the P.D. of the straight eye.

[Sara]

uncle chip,
reply me on this:
babies do not physiological prefer to converge because they are hypermetropic when born, hence have blurred near vision.
meauring real PD(visual axes) by method of corneal reflections,it has no relation to convergence,eye deviation or strabismus.You can take babies to window,observe corneal reflection and measure distance between reflections(do we call it hirscberg spots?)even deviating eye gives off corneal reflection.
in babies incorrect method of measuring PD is by measuring inner canthus to outer canthus, because of epicanthul folds it gives wrong results.

Sara(one more year to qualify)the student

[chip anderson]

Sara: Unless I am mistaken hypermetropic does not mean that they cannot fixate on a light. I was referring to using a penlight or other near source light. Sunlight and outside reflected light would not cause convergence. Of course these light sources are not available in all environments.

I think (although I am no longer sure) that a baby with very little near vision at all will converge on a light from a less than 1 meter source.

[Sara]

Hello Uncle Chip,
Being a student with more years to go, lots of questions flashes across my mind.I hope I am not bothering anyone of you & you all find me interesting.
OK now for measurement of PD by corneal reflections,convergence that takes place as you said,I think we use principal of corneal reflection for measuring PDs by pupilometers.
Though I always use PD rule,I've seen we measure distance PD using pupilometers at very close range(pupilometer lenght of about 10 cms).What happens to convergence?
Uncle Chip,I know you will not fail to explain me this;I wait for your response.
Sara

[This message has been edited by Sara (edited 03-21-2001).]

[chip anderson]

Pupilometers have built-in compensation for convergence. You will note the dial that allows you to measure the distance for which the glasses will be used. Usually infinity for distance, infinity less 2mm per eye for near. And 40 mm for near alone. For special purpose such as computers, pianists etc, you set it at whatever distance the patient will be looking at, infinity or less.


And no dear chile you do not bother me, when someone asks me for advise, I find it a little flattering.

[Sara]

Thanks for acknowledging dear uncle.
Sara