Optical Formulas Part 2

Optical Formulas Part 2

When we first started writing blogs and sending out our monthly IcareFocus Newsletter, we did not know what to expect. We are often humbled by the amount of interest in our blogs. We are also sometimes surprised by the success of some and the lack of interest in others.

I have to admit that when I put together last month's blog going over ABO Optical Formulas, I was pretty much expecting it to flop. I figured there would be some newer dispensers out there that could use the info and maybe a couple of salty ones like myself who would appreciate the recap but did not anticipate much noise.

Well, it seems that more were interested than I thought. So here we go with part two of digging a bit deeper into some of these exciting formulas. This time around I am going to dig into the basics. This will be geared toward those who are newer to the industry.

To be a good dispenser, one must be able to know what a prescription lens will look like in a frame. Of course, this is dependent upon several factors which include Rx, base curve, and the amount of decantation (see last month's blog on how to figure this).

The main type of lens that we use for 99% of our patients is called a Meniscus lens. This is simply a lens with aA meniscus lens has a front curve and back curve plus front side (convex) and a minus backside (concave). The difference between these two surfaces is what gives us the power or Rx in the lens. We will dig a bit deeper into this soon. 

Lenses can be broken down into three major categories: Plano (no power), Plus, and Minus. 

There are some simple ways to easily tell if a lens has a plus or minus power by just looking at the lens.

Plus lenses will have a steep front curve and a flatter backside curve. They will also have a thicker center and thinner edges. Plus powers will create magnification when looking through and demonstrates  "against" motion when moved side to side. This is done by holding the lens out about 12 inches from the eye and when moved side to side, images will move in the opposite direction of the lens motion.

Minus lenses will have a flat front curve and a steep back curve. They will have a thin center and thick edges. Images will appear smaller while looking through and will demonstrate "with" motion. Images will move in the same direction as you move the lenses. 

Nominal Lens Formulas

We read the power of lenses in diopters. The power of a lens once again is derived from the difference in the front side curve (base curve) and the back side curve (ocular curve). By either adding or subtracting the given curves of a lens we can determine the power of the lens. We can also use the Rx and figure out what the backside curve should be based on a specific front curve. This is known as the nominal lens formula. 

The Formula for the Power of a Lens

F (front curve) +  B (back curve) = P (power)

Example 1 - You have a lens with a front curve of +6.50 and a back curve of -3.50.

F+B=P or +6.50 + (-3.50) = +3.00

Example 2 - You have a lens with a base curve of +2.50 and an ocular curve of -8.50.

F+B=P or. +2.50 + (-8.50) = -6.00

Plus lenses will have more plus (front curve) than minus (back curve) and minus lenses will have more minus (back curve) than plus (front curve). 

Knowing this formula and understanding it will give you a base to begin understanding optics. You will seldom, if ever, use this formula in the office as you have lensometers that will read the lens more accurately. 

The Formula for the Backside or Ocular Curve

One of the key elements to being a good dispenser of glasses is choosing a frame that not only fits the patient correctly but also the lenses. Knowing and understanding what a backside curve will be based on the front curve is a very useful tool.

Having a mid-to-high minus patient wanting to purchase a wrap frame can often turn bad. Using the nominal lens formula to figure out the backside curve can be useful.

Knowing the nominal lens formula will help you understand how to fit a patient with the right frame An example would be that you have a patient with a -4.00 OU and they choose an 8-base wrap frame.

Simply use the formula: Power - Front Curve = Backside Curve or P - F = B. Which is -4.00 - (+8.00) = -12.00.

You are now able to get a mental picture of just how bad this job will look and how bad the visual acuity will be for the patient.  It would be best to get the patient into a flatter frame. If you have to use this frame, you will know it would be better to do it in a 6-base lens which will give you a -10.00 backside. This will fit better in the frame, be a bit thinner, and have better optics.  

Next month we will dig into compound lenses and the ever-popular optical cross in our 3rd and final optical formula blog. Make sure to bookmark all 3 of these pages in your browser for easy reference.

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