Base Curve Selection

[Aleyz2020]

Really, all you have to do is call any of the major lens manufacturers and they will gladly send you a base curve chart. It's not a difficult task at all!

[John R]

Originally posted by Aleyz2020
the flatter you can keep the base curve, the less thickness you'll have. Al

I here this said a lot these days but cant seem to get it some how :hammer: .. A +2.00 sph on any base is going to have the same c/subs.
I can see that it may look thinner due to the lower curves but......

[Darryl Meister]

Believe it or not, the +2.00 will be slightly thinner on flatter base curves (which provides one of the advantages of aspheric designs). The same holds true for minus lenses. The larger the diameter, the more noticeable the difference will become. If you imagine a plus lens with zero edge thickness, the center thickness basically becomes the difference in sag heights between the front and back curves. (The sag, or sagitta, of a surface is its depth at a given diameter.) Steeper curvatures produce greater differences between the sag heights of the front and back surfaces, resulting in a greater center thickness. If you download my OpticsLite spreadsheet, you can play around with this effect.

Best regards,
Darryl

[John R]

Thanks Darryl, I get what you are saying.

[Texas Ranger]

This is always an interesting discussion, and I cinsider Daryl and Mikes comments right on, when I started in this mess, everything was glass lenses with the cyinders ground on the front side, and the Base Curve was the inside ocular surface, until AO came out with minus cyl. lens and new equipment and tools were designed for ocular surface generating, fining and polishing. Some of our biggest headaches were in the early 70's when we had so many folks going from +cyls to -cyls.
I always specify base curves when I order lenses surfaced uncut. First of all, about 95% of them are current clients, I know what they're wearing and don't want an arbitray BC change by the lab when all the rx change is +or - a .25. Sometime folks order more than 1 or 2 pairs of glasses, and the matching base curves needs to apply to each of them, as close as possible. I think this is important because some rx powers are 'borderline' as to which BC's that the lab might choose to put the job on. There are valid reasons to not match bc's, like in progressive myopia; i'll always take the ocular curve as the guideline as to what front 'base' curve to order. this process has saved many a remake. sometimes it's a most important consideration when folks have a monocular developing cataract or post IOL. just flip-floping base curves can make a remarkable difference in wearing comfort.

[jimbobby]

I appreciate all of the responses to my question about base curves. I have had a couple of opportunities to hear Michael Disantos in person ; and agree that his seminars are informative as well as interesting. I have not had the same luck with lens sales reps.: they can generally get me a chart but have no idea why their company selects certain bases. Maybe I can answer my own question: Please give me feedback on this thought:::::::::

BASE CURVES ARE SELECTED BECAUSE OF MACHINING REASONS RATHER THAN OPTICAL REASONS.


Consider: a +2.50 poly aspheric lens, with eyesize of under 50mm to avoid edge aberrations. Vision Ease suggests using a 5.50 B.C.; but they also make : 8.50, 7.50, 6.00, 5.50, 4.25, and 2.50 base curves. All of these base could be used to create a +2.50 lens power.

B.C. NOM. OCULAR FOCAL LENGTH CORRECTED MAG.
CURVE CURVE OCULAR

8.50 8.53 6.37 .438618 6.15 1.048
7.50 7.59 5.37 .4318448 5.18 1.047
6.00 6.01 3.75 .4307442 3.57 1.044
5.50 5.59 3.37 .439897 3.14 1.044
4.25 4.32 2.12 .447832 1.85 1.042
2.50 2.54 0.37 .4583442 0.05 1.038


Given that most of the labs are still using glass laps 1.53 index in fining and polishing: the manufacturers selected the best "possible" base curve given this constraint. Also notice that if we arbitrarily choose a base curve; the focal length gets longer and the imagine magnifacation gets weaker. My guess is that the lensometer would read +2.50 in each case. If we wanted the best optics for the pateint with a focal length of .4000 we should use the corrected ocular laps and these would only be possible with Gerber foam laps. Still notice that the magnifacation still decreases as the curves are flatten. I`m done (a guy with to much time on his hands).

[eyesguy]

Base Curves

HI:

One of the best works I have seen on base curve selection is "The Design Of a General Purpose Single Vision Lens Series" by John Davis, Henry Fernald, and Arline Rayner. I had the pleasure of working with John and Henry back in AO's glory days and with John more recently. John's still sharp as a tack and really knows his stuff. But, as Darryl says, getting copies of John's material is tough.

For what it's worth, I still have the above mentioned paper from my old AO days. As long as it did not get out of hand, I would be willing to make a few copies.

Regards,

eyesguy

[Homer]

Jimbobby,

you stated:

BASE CURVES ARE SELECTED BECAUSE OF MACHINING REASONS RATHER THAN OPTICAL REASONS.


What is your rational behind this statement?

[Darryl Meister]

Re: Base Curves

eyesguy said:
One of the best works I have seen on base curve selection is "The Design Of a General Purpose Single Vision Lens Series" by John Davis, Henry Fernald, and Arline Rayner.

Yes, that was a great piece -- although it was really more of a white paper for their Masterpiece lenses. Anything put out by Tillyer, Davis, Fernald, Rayner, Whitney, Lamar, Winthrop, and the rest of those AO guys was top notch stuff.

Periodically, I publish some of the late Don Whitney's papers over in the ophthalmic optics forum (generously provided by his son, Dick, a second generation AO lens guy).

Best regards,
Darryl

[John R]

Re: Jimbobby,

Homer said:
you stated:
BASE CURVES ARE SELECTED BECAUSE OF MACHINING REASONS RATHER THAN OPTICAL REASONS.
What is your rational behind this statement?

Well i know that we do this sometimes, why should we make our lives harder.
I guess it boils down to money, the harder a lenses is to make the more likly its going to break. Even the best of us taking a job round arn't fast enough when the machine decides it dont want the job on it any more.
:hammer:
There also is the problem now with the latest computer controled 3 axis gens in that you cannot fool them into doing something that they think is outside their range which you could do on a old hand gen.

[nscaleduck]

Base Curve Selection

I've been an Optician for 30 years and I have recently taken a job in a gov't optical lab that employs many younger people trained by the military. There is an debate going on about base curve selection. Here is an example:
OD +1.00 -0.50 x 90
OS +3.50 -0.50 x 180
Most military OD's don't specify base curves, so separately, I would choose a 6D base curve for the OD and an 8D base curve for the OS (split base curves). But, since the patient will be wearing these together, I would choose the 8D base curve so that light would enter the lenses at the same angle and present a better appearance.
I would like to know your experience with split base curves and the effect this may have on patients. Also, if you know of any written reference I can go to on this subject. Thank you

[Darryl Meister]

Putting the +1.00 D on the 8.00 Base won't produce a significant reduction in performance, but will indeed balance the image magnification and cosmesis between the two lenses.