![]() ![]() Let’s try another one using that +12.50 from above. Just keep in mind that the brain is already doing the compensation and we are working from there. I know, I know, confusing to say the least. NOT to correct them back to the original amount written on the Rx. That power, raised or lowered from what was actually written on the Rx is what we want. If their brain is seeing the lens as being weaker we would raise the power. ![]() If their brain is seeing the lens as being stronger we will lower the power. Now we just need to make a lens that matches what their brain is seeing. Their brain is already doing the adjustment on its own. There is a difference between those two ideas. We are trying to correct them, to wearing a lens that they perceive as it is worn, as providing them with +18.00. We are NOT trying to correct them back to +18.00. To correct this the lenses would need to be remade at +16.22. This customer would perceive that their glasses were +1.78 diopters stronger than than they actually are. It is -0.005 not 0.005 because the lens moved away from the eye (follow the rule above) 0.005 is 5 millimeters converted to meters which is what the formula is based on This tells us that our result should be something above +18.00! Our rules above tell us that a + lens moved further away from the eye will increase in perceived power. The customer wears the glasses at 15mm away from the front of his cornea. The Rx was written with the lens in the phoropter placed 10mm away from the front of the cornea. Let’s say a customer brought you in a pair of glasses with the Rx of +18.00 OU. If the lens is moved closer to the eye you make d positive or leave it alone If the lens is moved away from the eye you make d negative or add your minus sign ![]() In this case we will figure out what the patient is perceiving in a pair that has already been made.ĭE is the power being perceived by the wearer.ĭ is the distance between the where you want the lens and where it actually is. How Does the Vertex Distance Calculation Work? They are both plus powers, but +8.00 is “more minus” than +9.00. They are both minus powers, but -7.00 is “more plus” than -9.00. “Less minus is more plus less plus is more minus.” This is a good time to review a tricky concept: The most important thing to remember when understanding the concept of vertex distance is that in ALL circumstances, more vertex distance equates to more perceived plus power. In the past the tool used most often to measure vertex distance is was the distometer: Thankfully, most patients now receive replacement lenses during cataract surgery and there is no longer a need for extremely hyperopic post-surgery prescriptions. Even the slightest difference between refracted and fit distances would make a dramatic difference for the patient. It was very common for prescriptions to reach +20.00D or beyond! These powers made it very important that eye care professionals knew how to compensate for vertex distance. This meant that their eyeglasses had to compensate for all of the ‘plus power’ that was normally supplied by the crystalline lens. These “aphakic” or literally “no lens” patients had no crystalline lens inside their eye. Note: Free form lens design is calculated to the nearest 1/100th diopter not 1/4th.ĭid You Know: That just forty years ago patients who had cataract surgery did not receive an inter-ocular-lens (IOL) implant. Part of free form lens design is to provide the wearer with the perceived amount that is equal to what the Rx was actually written for. If an Rx should call for +12.50 that perfect +12.50 is only perceived by the wearer when the glasses are worn at the same vertex distance as the phoropter or refraction device used to determine that need. This is as good a time as any to remember that “close-is-good” – you always want a lens to sit as close to the wearer as possible.ġ) It explains why the prescription for contact lenses and glasses are often differentĢ) It can explain why a high-power prescription can be perceived as being “off” by a customer even when it is actually filled 100% correctly (*sort of)ģ) Because you must know how to do these calculations to work with contact lensesĤ) Because you must grasp the concept to pass your license exams 5)*Because vertex distance is a part of modern compensated prescriptions. The actual power of the lens does not change. Note: The vision through the lens will be perceived by the brain as having more or less power. Because the eye is a self-focusing system, lenses placed in front of it will be perceived as having more or less strength depending on the distance they are set from the eye. The eye is actually a series of lenses that make up a focusing system. Vertex distance is the distance from the front surface of the cornea to the back side of a lens that is mounted in a frame and being worn by the patient. ![]()
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