CONTINUING EDUCATION, 1 CE Credit – $9.99, 1 Hour, General Knowledge, Level 1, Release date: October 2007, Expiration date: October 31, 2012

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“How do I Calculate That?”


Having been a dispensing optician for over 15 years, there are very few things that surprise me anymore. Constant turn over in the optical business, patient complaints, doctor complaints, managed care, lens spoilage, etc., etc., etc. I’ve worked in retail optical, an optical lab and finally as office manager of an optometry practice. I’ve been around the blocker. Pardon the optical humor.

One thing that really amuses me is when the patient has vertical imbalance and we need to recommend bicentric grinding -- otherwise known as slab-off. My amusement stems from the fear in the eyes of the average optician when they need to calculate slab-off. It is certainly a complicated topic, but not one that should be overwhelming. Since it is a topic that we will all encounter, and since we do not want anyone else to do our calculations for us, what follows is a review of the vertical imbalance subject. After all, this is one of those areas of opticianry that seems to fly by a lot of good people in the profession.

Why Slab-Off?

To understand why slab-off is sometimes required, a little background information is needed. It’s important to remember viewing an object is a team effort. Working in pairs, the eyes produce a fairly extensive visual field, fuse an image stereoscopically (in 3-D) and converge when looking at an object closer than twenty feet. This is simply known as binocular vision or stereopsis. When a patient sees normally, the images focused on the retina appear approximately the same size, shape and location. However, when the prescription that is required for each eye has a significant difference problems ensue.

When the patient has the same refractive error in both eyes but with a significant difference in power it is a condition known as anisometropia. In optical terms, anisometropia is typically a difference in refractive value of 1.00D or more. These patients will generally present one of three ways: congenitally, post-trauma or after cataract surgery. A similar condition is known as antimetropia. Antimetropia occurs when there is a substantial difference in which one eye requires a concave, or “minus” prescription, while the other is corrected using a convex, or “plus” prescription.

When the prescription is the same or similar between the two eyes the patient will experience no prismatic effect as they move their eyes behind their lenses. The prismatic effect that one eye gets as the eye moves away from the optical center will be canceled out by the prismatic effect that the other eye receives. For instance, if one gets base out the other will get base in and the net result is no prism and no problem for the patient. However, if there is a large difference in power between the two eyes one eye will develop much more prism than the other eye and now the patient will have displacement of the image. When they are pre-presbyopic the patient simply learns to turn their head to view objects. This keeps their eyes at the optical center of their lenses and they will not induce any prism. Once they become presbyopic though, we now advise them to move their eyes down into the segment and the prism returns!

Now that they have a segment for their presbyopia they can no longer rely on looking through the optical centers in order to avoid the prismatic problem. Now they must drop their eyes into the segment to get the benefit of the reading prescription. This simple act creates the prism and the result for the patient is a splitting of the image as they try to read. The patient will also feel uncomfortable trying to read for any prolonged period of time. They will frequently feel strained, sometimes nauseous and occasionally get headaches from the prism.

Our mission is to correct the vertical imbalance in their prescription so that they will be comfortable in their new multifocals. We have six methods of correcting this imbalance: Two pairs of glasses – one for reading and another for distance, dis-similar segments, r-compensated segments, Fresnel press-on prisms, prism segments and of course, slab-off.


Slab-off is also known as bicentric grinding. Bicentric grinding is a type of lens fabrication process in which base-up prism is ground on the lower portion of the lens to redirect light as a patient gazes downward. This technique allows the patient to fuse the images and prevent prism as the eye travels down the vertical meridian of the lens. The term slab-off came about because the technique makes the finished lens look as if someone removed a small section of the bottom portion. Of all the ways to correct vertical imbalance, slab-off is by far the most common technique used ---but it is not always the best understood.

Choosing Sides

Patients with anisometropia or antimetropia and who have presbyopia, needing bifocals or trifocals to see up close, will require slab-off in order to see comfortably up close. Since they require multifocal lenses, they must drop their gaze from viewing objects at a distance to utilizing the multi-focal area of their lens. Therein lies the problem and the need for slab-off. Slab-off can be ground on the front or back of the lens or molded into the front in the case of reverse slab-off. Regular slab-off is ground base up on the most minus or least plus lens in the 90th meridian by the lab. This is done to correct vertical prismatic imbalance that results in diplopia when looking at something at the near point. Reverse slab-off is molded base down on the least minus or most plus lens in the 90th meridian at the segment line of the multi-focal lens. Since the slab-off correction is already in the lens blank, a lab will only need to cut the back curve for distance which is something that can be done easily and is less prone to error. Reverse slab off is used more extensively than regular slab off for this reason. Typically, slab-off is done on a flat top bifocal because it provides reasonably good cosmetics.

How Much Slab-Off Do I Need?

Never leave this question to the lab! You are the trained professional and you are the one responsible for what is dispensed! NEVER trust anyone to calculate anything for your patient. Calculating the amount of slab-off required is relatively simple. Since we’re most concerned with the vertical balance of the lens, we need to determine how much power resides in the 90? meridian. Consider the following prescription:

OD -4.00 -2.00 x 030
OS -1.00 -2.50 x 045
Add +2.50

We need to find the power at 90?. You’ll need to follow these steps to determine powers in the 90° meridian for each eye:

  1. Subtract the axis for each lens from 90. For the right eye, we have 60. For the left, 45.

  2. Now you need to find the sine of each angle. For the right, we have .8660
    For the left, 0.7071.

  3. Squaring these results with the x² function, this will yield .75 for the right, 0.4999 (or 0.5) for the left.

  4. Taking this result, we multiply it by the amount of cylinder in each lens yielding
    -1.50 D for the right and -1.25 D for the left.

  5. We will now add these amounts to our spherical power and yield -5.50 D for the right and -2.25D for the left. These are the powers of each lens at 90°.

Since most labs use reverse slab-off, we want to use the lens with the least amount of minus power, or most plus. This criteria, designates our left lens as the “carrier” lens -- the lens taking or “carrying” the slab-off.

Next, we need to take into account the reading depth of the lens. This is determined by simply using 8 mm. Why 8 mm? This has been studied many times over the years and the average amount that a human being drops their eyes to read is 8 mm.

Now we have the power and the reading depth for each eye. We will need to apply Prentice’s Rule to determine the amount of prism for each eye. We determined the right eye had a power of -5.50 D at 90° and the left -2.25 D. First we subtract the difference between the two eyes and then apply Prentice’s Rule, we yield the following:

3.25 D X 8 mm/10 = 2.6 ? base down OD = the imbalance between the two eyes. If we can provide slab-off of the same amount in the opposite direction we will eliminate this prismatic imbalance and the patient will be able to read comfortably for any length of time they wish. Or as in the case of reverse slab-off, we will put the same amount of prism and base direction in the other eye and this to will balance the two eyes. For this example we would order 2.5 ? base down in the OS in reverse slab-off. Note: Reverse slab-off is only available in certain quantities (1.5, 2, 2.5, 3, 3.5, 4 & 5) which is why we rounded this answer from 2.6 to 2.5.

Hopefully, the next time your patient needs slab-off, you will feel more comfortable performing the calculation. Not only is that rewarding in and of itself, but you will also have a better understanding as to why it’s needed to help our patients see their best and maintain their comfort. With a little questioning of your patients you will be surprised how many of them have issues with vertical imbalance that have gone uncorrected in the past.

With contributions from: Brian A. Thomas, P.h.D, ABOM

John Dick, ABOC, FNAO

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Posted: 11/13/2012 7:05:43 PM

Thank you for the insight (pardon the pun) on slab-off. I was able to follow everything you wrote and the example given, but what do we do when the axis is above 90?
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