Question about choosing Base Curves

**GPK** · 05-16-2013, 07:06 PM

I've recently started in the industry and have been shown a couple of contradicting ways to get a base curve. I want to give two rx examples and if you wouldn't mind please tell me what base curve you would choose for them each.

1. Groove Rimless Frame:
Right Eye -1.50 sph 2.00 cyl 180 axis 2.50 add
Left Eye -2.25 sph 3.25 cyl 175 axis 2.50 add

2. Plastic Bevel Frame:
Right Eye 0.50 sph -0.50 cyl 74 axis 1.50 add
Left Eye 0.50 sph -0.50 cyl 53 axis 1.50 add

I've personally been using Vogel's formula and for the first example I would have pulled about a 4 bc. For the second I would have pulled around a 5.50 to 6. I noticed on Vogel's formula that it says to use the spherical equivalent if you have an rx with a cylinder. Would I have pulled the right curves? I would be pulling by adding 5 instead of 6 though.

**Robert Martellaro** · 05-16-2013, 10:29 PM

Originally Posted by GPK

I've recently started in the industry and have been shown a couple of contradicting ways to get a base curve. I want to give two rx examples and if you wouldn't mind please tell me what base curve you would choose for them each.

1. Groove Rimless Frame:
Right Eye -1.50 sph 2.00 cyl 180 axis 2.50 add
Left Eye -2.25 sph 3.25 cyl 175 axis 2.50 add

2. Plastic Bevel Frame:
Right Eye 0.50 sph -0.50 cyl 74 axis 1.50 add
Left Eye 0.50 sph -0.50 cyl 53 axis 1.50 add

I've personally been using Vogel's formula and for the first example I would have pulled about a 4 bc. For the second I would have pulled around a 5.50 to 6. I noticed on Vogel's formula that it says to use the spherical equivalent if you have an rx with a cylinder. Would I have pulled the right curves? I would be pulling by adding 5 instead of 6 though.

Looks good, assuming minus cylinder in example #1, low index of refraction materials, and spherical (not flattened and compensated with asphericity). Your best bet though is to follow the manufacturer's recommended base curves.

**Gera** · 05-17-2013, 03:38 AM

Sound good for me .
Every manufacture have the base curve chart

**Plausible** · 05-17-2013, 05:50 AM

Choosing a base curve solely on power is incomplete given the frames we have available today. Consideration of base curve should include information about the frame that is being used. The frames curvature and the frames bevel design should be considered along with the patients rx power and specific lens needs. False front frames have been written about here on this board before and also come to mind a base curve specific. A complete evaluation to determine the lens base needed will lead to a lens to frame fit that is second to none where a lens base selection solely on power may produce a product that does not perform as good as it could nor look as good as it could. I don't know how many times we have received orders to do with lens base request too flat or too curved for the frame sent in with the job. For some of these jobs we have to use a wrap specialty lab but have occasionally tried to edge a few odd combinations. The results are not very good when a 2 base lens is requested to be put into an 8 base wrap frame or vice versa.

**John@OWDC** · 05-17-2013, 06:47 AM

I should know better but...

The large lens companies employ doctoral level optical engineers who in turn employ high-end computer programs to determine the precise individual base curve series for a set of lenses.

Why on earth are we trying to mess around with that?

Are not we taught to match the frame to the Rx not the Rx to the frame?

And, is not base curve actually a dated concept at this point since free-form is creating lenses with aspheric front and back surfaces?
Aspheric meaning not a consistent lens surface, not aspheric as what we call an "aspheric" lens.

You say that you are "new to the industry" so I really would like to know if there is a program or school that is teaching you to second guess and reinterpret each Rx for base curve applications?
Did you really spend classroom hours on this?
Do you feel this was a productive use of your time, your employers time and time for patient interaction?

**MakeOptics** · 05-17-2013, 07:20 AM

Originally Posted by John@OWDC

I should know better but...

The large lens companies employ doctoral level optical engineers who in turn employ high-end computer programs to determine the precise individual base curve series for a set of lenses.

Why on earth are we trying to mess around with that?

Are not we taught to match the frame to the Rx not the Rx to the frame?

And, is not base curve actually a dated concept at this point since free-form is creating lenses with aspheric front and back surfaces?
Aspheric meaning not a consistent lens surface, not aspheric as what we call an "aspheric" lens.

You say that you are "new to the industry" so I really would like to know if there is a program or school that is teaching you to second guess and reinterpret each Rx for base curve applications?
Did you really spend classroom hours on this?
Do you feel this was a productive use of your time, your employers time and time for patient interaction?

I couldn't agree more. I personally feel these 100 year old + methods need to be reevaluated and certain techniques should be put to pasture. As you've mentioned vogels formula was a neat trick back in the day when you wanted to avoid pulling the volumes of text to find the correct base curve. But we have excel spread sheets and programs that can do that along with base curve charts from manufacturers. I think optical programs need to update their courses to be more inline with current tech.