Each issue, Tech Tips will explore some interesting aspect of optical technology. This month, we look at the other side of an RX lens.
An eyeglass lens prescription has two sides.
You’re a conscientious optician who spends a lot of time and energy making sure that your patients receive the most appropriate lens designs and treatments, the best frame options, and the most accurate prescription. Like many conscientious opticians, you work diligently when performing final inspection and verification to ensure your patients are getting the best eyewear. In fact, you even have a checklist to be absolutely sure you’ve covered every aspect.
Assuming that the prescription the doctor wrote was perfectly accurate, you assume the patient will see perfectly well with their new eyewear, but that shouldn’t be a foregone conclusion. Why? The reason is because there are two sides to an eyeglass lens prescription-the power side and the magnification side.
As an optician, you know that lenses have power because they are essentially a series of progressively powered prisms arranged base to base (for plus lenses) or apex to apex (for minus lenses). This prism arrangement is what enables the optical material to converge or diverge light. Interestingly, it’s also the cause of another optical characteristic-magnification.
You might think of magnification as a side effect of lens power. If a lens has power, it’ll have magnification. This means that while the prescription lenses you dispense help your patients see clearly and sharply, they also alter the size and shape of the things seen through them. For plus lenses, things appear larger, and for minus lenses they seem smaller, while cylinder lenses make circles look like ovals. In addition, plus lenses make objects appear closer to the wearer, more compressed, and in a narrower field of view, while minus lenses make things appear smaller, expanded, and are in a wider field of view. The stronger the power, the greater this effect becomes.
Think of it this way:
Magnification makes objects appear a little weird and alters their position in space relative to other objects in the field of view. When your patients come in for eyewear, they expect to leave your office seeing clearly and comfortably and they pay a good deal of money for that. They didn’t pay a penny for seeing weirdly, but that’s what happens sometimes due to the effects of magnification.
What can you do about this? Actually, not that much. Remember, magnification is a side effect of powered lenses and while there are some ways to compensate for it a little, it’s a natural property of lenses that gets their power by using two lens sides that are curved differently. Your best defense against this problem is a good offense-be aware of what can cause the trouble and let your patient know that it might occur.
Changes in power, base curve, thickness, or index will cause a magnification change, which may affect visual discomfort. Problems often occur when patients change lens powers with a new prescription. That’s because the lens’ curves have changed as well as the lens’ vertex distance. Its index of refraction may also have changed.
Here’s a common scenario. A patient uses single vision lenses and gets a new prescription with new powers and a different base curve, vertex distance, and material index than their last pair. Their old lenses used spherical base curve while their new lenses are aspheric. Under these conditions, it’s not uncommon for the wearer to mention that things seem weird through their new lenses. They see clearly, but things seem a bit odd. Knowing that this can occur, you should prime your patient at the time of lens selection that this might happen so they won’t be upset when they pick them up. By warning them, you’ll be a hero. But not warning them, you’ll be the bad guy.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.