Base Curve Selection

[jimbobby]

Iim looking for a book, chart,etc that will give some general insight into the slection of Base Curves and thus ocular (back) curves which gives more comfortable vision for the patient depending on his/her prescription. I understand that in the past few years we gone away from the traditional curve charts in favor of the flatter European curves. The lenses look better but where is the limit/or is there a limit? Recently the Retailers are asking that Rxs be put on what ever curve is available-------but does this result in good optics for the Patient?

[Homer]

Welcome Jimbobby....

What makes you think that anyone on this fourm knows anything about base curves? We just let our labs make all of the decisions so we have more time to type our non-political, completely neutral points of view for everyone else to read.

The important thing is to contribute, it doesn't really have to mean anything or even make much sense.

Invite in a couple of lens reps. to take you to lunch and let them tell you all about "the best" base curves .... at least you will have some good lunchs.

[optigoddess]

In my opinion, the best information I have come across re: the base curves is by Michael DiSanto of Bell Labs (I think in Ohio????) He wrote a wonderful resource book on just about everything optical - sorry I don't have the name of it --

[Joann Raytar]

I remember reading a nice article online that talked about "true base curves" versus "cosmetic base curves." Now I just have to pour through my bookmarks and find it. At least that will give you a short article and a bibliography to reference by. Now, where did I put the link?

[JRS]

Mike doesn't work at the Bell Labs anymore - although in a way he does, since he works for Essilor as an Instructor. You can reach his voice mail @ 1.800.377.4567 x5824. He might be able to tell you where to get his book - forgot the name of it myself.
Darryl has written some very nice pieces on BC selection. Go to his web site and I believe they are listed there. Also review the download section of Optiboard for other Darryl insightful articles.



The old method for determing the BC - for spherical lenses, looked something like this:

Ideal Base = 6.25 + Sphere + (Cylinder * 0.25) [ - (Prism * 0.25)]

On aspherics you tried to hold the ocular curve in the -3.00 realm.

Now, with all the specialized lens designs/materials, you are better off using the suggested BC that the manufacturers supply to the labs and to the software companies. At least for the majority. The optician and/or doctor may need to specify a BC, as they deem nessessary to benefit the patient.

[Darryl Meister]

I think the single most complete source on base curve selection is probably in Brooks' Understanding Lens Surfacing (Butterworth-Heinemann). There are also pretty good sections in Clinical Optics, 2nd Ed. and System for Ophthalmic Dispensing, 2nd Ed. (both also published by B-H).

I've read many other articles and books on base curve selection and have found many to be inaccurate and/or incomplete. I have included a few general pointers for you below. These will probably answer most of your questions about base curve selection:

1. You're not concerned with the ocular curve but rather the front "base" curve. This is the curve that the manufacturer supplies finished, and the curve with which the laboratory will perform its calculations. Lenses must be ordered from the lens supplier by this finished front curve, not the back ocular curve. Also, the back curve alone really has no inherent effect upon vision. For instance, the front curve affects magnification and the relationship between the front and back curves affects the spectacle lens aberrations bothersome to the wearer. While you should understand the principles of base curve selection, try not to concern yourself too much with base curve selection since the laboratory generally does it for you.

2. Flatter base curves will generally provide better cosmetics, including reduced thickness, bulge, and weight. They will also be retained more easily in certain frames. However, these improved cosmetics come at the expense of optical performance. Flatter lenses with spherical base curves produce more of the aberrations that blur vision away from the center, and will narrow the wearer's field of clear vision. To avoid this situation, use aspheric base curves, which provide the flattest, thinnest, and lightest lens designs available without sacrificing peripheral optical performance. The wearer's eyes will also look more natural. It should be noted that laboratories using older equipment may have trouble processing the flat back curves necessary for aspheric lenses -- though this shouldn't be an issue too much today. It is particularly important with aspheric lens designs to use the recommended base curve, since their peripheral optical performance will be even more sensitive to base curve changes.

3. Do not match base curves unless absolutely necessary. Matching base curves is sometimes done to reduce adaption difficulties after prescription changes. However, prescriptions work best optically on their recommended base curves. Keeping the patient in the same base curves as his/her prescription changes may reduce the optical performance provided by the lenses. The changes in image size and distortion produced by the prescription change will generally be more significant than the same changes caused by using slightly different base curves. And, except in rare instances with really sensitive wearers, the patient should adapt to small base curve changes within a week or two. There are probably only three occasions wherein you should consider matching base curves: 1) When the patient has -- or will likely have -- an unusual sensitivity to the subtle changes in vision caused by spectacle lenses, 2) When the patient is getting a second pair of similar eyewear, or 3) When the patient is replacing only one lens.

4. Using steeper front curves to provide for eyelash clearance is generally not the best approach, and will cause both optical and cosmetic issues. When possible adjust the frame to avoid this situation.

5. In general, your best bet is to use the manufacturer's recommended base curves. Manufacturers generally supply lenses conforming to "corrected curve" or "best form" principles, which essentially means that the base curve has been chosen in an attempt to minimize the bothersome aberrations that occur in the periphery of the lenses. These aberrations narrow the field of clear vision. For finished lenses, manufacturers have already taken the guess work out of it for you, and supply them with the correct base curve. For semi-finished lenses, manufacturers supply the laboratory with charts illustrating the available base curves and the recommended prescription range for each. The laboratory will generally use such a chart (which may reside electronically in their lab software) to make your lenses. Since these recommendations will generally differ from product to product, and from manufacturer to manufacturer, you should consult the appropriate base curve selection chart for a product before considering a substitution. Also keep in mind that lens suppliers advertising "flatter" base curves in spherical (non-aspheric) designs may be deliberately sacrificing optical performance for marketing purposes.

6. If you do find it absolutely necessary to specify a base curve, remember that the ANSI Standard allows a tolerance of 0.75 D. However, in practice base curves generally only come in 2.00 diopter steps from a given manufacturer, greatly eliminating the amount flexibility that you really have for a given prescription. So, if you order that new High-index Photochromic Mega Progressive from Brand X, don't expect the lab to adhere to that 0.75 D tolerance. You should be particularly cautious when specifying or substituting aspheric base curves. Also remember that lens clocks (or measures) do not measure aspheric surfaces correctly. (See other OptiBoard postings for the correct use of lens clocks.) Moreover, the base curves of certain progressive lenses are optimized for the intended prescription range. Substituting base curves with these progressive lenses may negatively impact their performance.

Anyway, I hope you have enjoyed this presentation of "All About Base Curves," by Darryl Meister. ;)

Best regards,
Darryl

[optigoddess]

does anyone have some asprin.....

JRS:

Thanks for the update on Michael - I have had several classes from him and think he is an AWESOME instructor. I absolutely love the book he wrote - would recommend it for anyone.


Darryl:

You bring up a good point albeit a thorn in the side of many opticians.....that is the famous "phrase that pays" as it were:

Match Base Curve

I worked alongside a doctor who practically "rubber-stamped" the phrase on all rx's. It was the book that Michael wrote that illustrated so well the Corrected Curve Theory and just WHY it wasn't so good to "match base curves". (referencing 3 & 5 of Darryl's post).

I always thought it was a "control" issue from the Doctors, however. That is, until I had a discussion from someone currently in optometry school. We have a part-time ABOC (I think he may be in 3rd year in optometry school) who came to work one day not too long ago and said something to the effect that what they covered in class recently, was that CHANGING the base curve of lenses affects the magnification of the lens by (it seems to me) a relatively small %%% (can't remember the exact amounts) and THAT's why Dr's want the base curve kept the same.

So, I have a question, do Doctors want opticians to MATCH BASE CURVE meaning, "follow the corrected curve theory" and follow what the manufacturers create.......OR do they want us to make un-godly rx combinations.....that work AGAINST corrected curve theory..... ?????????

Another thing that is frustrating....is receiving the note to MATCH BASE CURVE but you can't because you are trying to update the patient into an aspheric or hi-index lens and you CAN'T Match because the new lenses don't come in the base curve that is within the .75 D tolerance range.

It seems as though Doctors are taught ONE THING....and opticians/lab techs are taught another.....

[Darryl Meister]

Sadly, optometrists are taught less and less about ophthalmic optics as their curriculum becomes more and more focussed upon primary eye care. The good news is that this provides an opportunity for opticians to step up to the proverbial plate and assume the role of the optical expert, providing guidance to their optometric colleagues. (Note that I use the term "colleague" and not "nemesis," as some would have you believe.) It will be interesting to see how many opticians choose to do this.

To answer your question... Yes, when an OD writes "match base curves," he/she is generally wanting you to use the wearer's previous base curves for the new eyewear, regardless of the manufacturer's recommendations. As we have previously described, this is to reduce the adaptation issues associated with the changes in magnification and distortion produced by base curve differences.

As I mentioned in my post, I generally recommend against this practice and would have a conversation with the prescribing OD if he/she did this on a regular basis. If you are switching a patient into an aspheric and/or high-index design, matching base curves may be a particularly bad idea. Aspheric lens designs work best when used with their recommended prescription ranges, and their optical performance is particularly sensitive to this. Moreover, substituting base curves with aspheric lens designs defeats the whole purpose of using them in the first place.

Best regards,
Darryl

[David Wilson]

Another great posting, Darryl. Your comments in your second posting also apply down here. I believe that optometrists are concerned about adaptation and will often blame non adaptation on base curve problems (it is a convenient scape goat). Their main concerns are primary care and contact lenses, both of which they do extremely well.
I would also like to add some comments on fitting. Single vision lenses, whether aspheric or best form (from Tscherning's ellipses) should be fitted according to the principal axis/center of rotation rule. That is, the OC should be dropped 1 mm for every 2 degrees of pantoscopic tilt. This ensures that when the person looks away from the principal axis at a given distance in any direction they will be looking through the lens at the same angle.

Regards
David

[Aleyz2020]

As a lab man of 25 years, until recently becoming licensed, the rule of thumb on determing base curve has always been to try and keep the backside curve as close to 6D as possible. Although for cosmetic reasons the flatter you can keep the base curve, the less thickness you'll have. The reasoning on keeping the back curve close to 6D, it creates less distortion to the patient. That's normally where you run into problems with patients with high Rxs or astigmatism. Keeping the base curve within one step of their old one is usually the safest.
Al

[Bev Heishman]

Darryl

I agree with your listing and also site your articles with Jim Sheedy when teaching staff and others. Another excellent source is by John Davis in Clinical Ophthalmology on lenses.

Bev

[desktopgrass]

I choose base curves that will fit in with my limited tool supply.... i try to have my fining polishing tools between 4 and 7 diopters.
I use almost exclusivly 4,6,and 8 base blanks

[Darryl Meister]

Re: Darryl

Originally posted by Bev Heishman
Another excellent source is by John Davis in Clinical Ophthalmology on lenses.

Sadly, few people have access to John's materials. (And they are not for the faint of heart! ;) ) He has been one of the foremost authorities on corrected curve lens design (and ophthalmic optics in general) since he worked in the lens design/engineering group at American Optical. He is a brilliant man, who was also nice enough to send me many of his articles while I was working on my Master's thesis years ago.

Best regards,
Darryl

[Bev Heishman]

Darryl,

How true. I also sited his works in my master's paper. Those that have access should read. Those working in an MD office will usually find Clinical Ophthamology either in a group library area or in a MD private office it is a wonderful tool to pick up, read and learn.

Best Regards,
Bev

[Spex]

Originally posted by optigoddess
In my opinion, the best information I have come across re: the base curves is by Michael DiSanto of Bell Labs (I think in Ohio????) He wrote a wonderful resource book on just about everything optical - sorry I don't have the name of it --

I have an article written by Michael entitled "The Basic Truths About Base Curves". I have several copies of it that I can send to docs that make an issue out of a note printed on all of their Rx forms that states "Match Base Curve". It is a very good article on base curves. If I can get it scanned, I will attach it to another post.

Mike definitely knows his stuff!!!