This is the 5th installment of my Fitting Contact Lenses Course. So far we’ve learned:

• Knowing Who Can’t Wear Contact Lenses
• Knowing Who Can Wear Contact Lenses
• How To Convert A Glasses Prescription To Contact Lenses
• How To Convert A Glasses Prescription To Contact Lenses: Astigmatism

The next step is to get your patient a free trial contact lenses so that you can assess their fit and performance.

This lesson is going to be centered around ordering your patient/client’s first pair of free contact lens trials. I wrote it assuming your don’t have trial fitting sets in your office and have to order your trial lenses from the companies. If you do have the fitting sets, you can just get your trials from there (assuming they are well maintained).

Working With What’s Available

At this point, you’ve successfully converted your patient/client’s glasses prescription to contact lenses and you know what contact lens trial pair you want to order. However, the exact numbers that you calculated in your conversion is not always available for you to order. To avoid problems and unnecessary delays, it’s important to order contact lens trials that are actually available.

Limitations in Sphere Power

+6.00 to -6.00

If the sphere number you want for your patient’s contact lens trails is between +6.00 and -6.00, you shouldn’t have any problems finding what you are looking for. Contact lenses generally come in 0.25 steps in that range.

The problem you may come across in this range is when your glasses to contact lens conversion gives you a sphere power that ends in an eighth diopter step, i.e., .12, .37, .62 & .87. You will have no choice but to choose from one of the two nearest quarter diopter step, i.e., .00, .25, .50 & .75.

If this happens, you can use your professional judgement to choose the one you think would work best for you patient. However, if there’s some uncertainty you can always order and try them both.

For example, if your glasses to contact lens conversion is:

Since neither of those sphere powers are available in contact lenses, a safe bet would be to order -5.00 and -5.25 for the right eye, and -5.50 and -5.75 for the left eye.

Over +/-6.00

If you need a contact lens trail with a power that is higher than +6.00 or -6.00, you will be more limited. Outside of the +/-6.00 range, most contact lens brands become available only in steps of 0.50, i.e., -6.00, -6.50, -7.00, -7.50, -8.00 etc.

So if you’re looking for trail contact lenses with a sphere power ending in .25 or .75, what do you do? Well, at this point you should be expecting to have to try 2 or 3 different combinations of powers before you find the one the works best for your patient. The best thing to do would be to order the contact lens trials in both powers that bracket what you calculated in your conversion.

For example, if your glasses to contact lenses conversion is:

Since neither of the those sphere powers are available in typical (non-extended range) contact lens brands, a safe bet would be to order -7.00 and -7.50 for the right eye, and -8.50 and -9.00 for the left eye.

Start by having the patient try on the lower power and see if holding a -0.50 trial lens over the eye makes a significant difference in the vision. If it does, you’ll be happy that you ordered the stronger power.

It’s A Trial & Error Process

Keep in mind that it may still be possible that despite this, your patient will be need something that’s even lower or higher that what you have ordered. There are many factors that influence this such as specific contact lens designs, patient age, patient visual demands, etc.

Additionally, it is important to keep in mind that whatever sphere power ends up working best for any given brand does not necessarily translate to other brands. Every time you try a different brand, it is advisable to re-test the vision and perform a contact lens over-refraction to find the best sphere power for each brand you try.

Limitations in Cylinder Power

What’s Available

Cylinder power is perhaps the most limited parameter in contact lenses. There are 3 cylinder powers that every contact lens for astigmatism have. They are:

• -0.75
• -1.25
• -1.75

Many brands also have:

• -2.25

A few brands have:

• -2.75

As you can see, the cylinder powers in contact lenses are quite limited and only appear in 0.50 diopter steps. This is due to manufacturing limitations as well as the fact that when the cylinder is only slightly off, it doesn’t affect the vision all that much.

To Round Up Or Down?

If you need a cylinder power that isn’t available, should you pick the closest lower one or the closest higher one? In this situation it’s typical to round down to the nearest available cylinder power. People who wear contact lenses for astigmatism do best when they have the lowest possible cylinder power that still allows them to see 20/20.

For example, if your glasses to contact lenses conversion is:

The right eye’s contact lens cylinder power should be -1.50 but due to restricted availability we have to pick between -1.25 and -1.75. It is quite possible that the patient would be able to see perfectly and equally well with either one. If that is the case, it would be best to opt for the lower cylinder of -1.25.

It’s also possible that when you convert a glasses prescription to contact lenses, the cylinder ends in a eighth diopter step. When this happens, simply choose the nearest available cylinder (a cylinder ending in an eighth diopter step never falls squarely between two available cylinders.

For example, if your glasses to contact lens conversion is:

For the right eye: -1.62 falls between -1.25 and -1.75 but it much close to -1.75.

For the left eye: -1.37 falls between -1.25 and -1.75 but it much close to -1.25.

Limitations in Axis

Excluding extended range and specialty contact lens brands, contact lenses for astigmatism are available in axes of steps of 10.

When you convert a glasses prescription to contact lenses, the axis does not change. This means that you could end up with an axis that’s any number between from 1 – 180.

When picking the contact lens free trial for your patient, what you have to do is choose the axis that is the closest to the glasses axis.

Of course, you’re going to encounter many prescriptions where the axis ends in a 5. When this is the case, I tend to order both axes that straddle the glasses axis so that I don’t have to wait on another delivery if that one I ordered wasn’t the right one.

Assessing The Vision/Examining The Fit

Once you’ve ordered all the contact lens trials you think you’ll need for your patient, the next step is to have the patient try them on assess their performance. This is where you’ll make the decision as to whether what you’ve ordered is adequate, or if you have to make adjustments.

This is what I discuss in the next installment of my Fitting Contact Lenses Course.