A plus lens can be thought of as two prisms arranged base-to-base while a minus lens has its prisms arranged apex-to-apex.
What is a lens? The answer is not as simple as you might imagine because lenses have a lot of properties. Essentially, they interact with light in some way. For example, they transmit, reflect, absorb, refract, converge, or diverge light. Some lenses are clear while others are colored. Some refract light while others do not.
PRISM VIEWS In their simplest sense, ophthalmic lenses can be thought of as being constructed by two prisms arranged base-to-base for plus lenses or apex-to-apex for minus lenses. While this might seem a simple explanation of lenses, it actually tells us a great deal about their behavior and characteristics (see Fig. 1). It makes plus lenses converge and minus lenses diverge. Eyecare professionals (ECPs) know that prism breaks light into the colors of the rainbow—known as dispersion. Lenses do the same thing and we rate that characteristic using a system called “Abbe value.”
Notice how the center of the target rests on the first reticle circle. For most lensometers, this arrangement represents 0.5 prism diopters.
How much prism does a +3.00D stock lens have? Most ECPs would say zero because that’s the way stock lenses are manufactured. But let’s take this one step further. Lenses are actually comprised of a series of progressive prisms arranged base-to-base or apex-to-apex. In fact, if you assessed 12,000 points around a +3.00D lens, you’d find 11,999 points that read a prism value while only one point reads zero prism power, its optical center location. Every other point on the lens reads some prism value.
PROGRESSIVE LOGIC How can you prove that a lens is a series of progressive prisms? Take a +3.00D lens and center it in the lensometer so it reads zero prism. Now push the lens up so the target center falls on the first circle of the reticle (see Fig. 2). If I asked you what you saw in the instrument, you’d say 0.5 prism diopters. That’s funny because this lens has no prism, right? It’s a +3.00D sphere. Try moving it up again until the target rests on the second circle. Now it should read 1.0 prism. But this is a spherical lens with no prism, right? Keep doing the experiment and you’ll see that every single point on a lens has prismatic value except for one—the optical center.
Since lenses are progressive prisms, they affect eye movements. This explains why patients obtain base-in prism when they converge their eyes to read with their minus powered eyeglasses and why they obtain base-out prism with plus lenses when doing the same task (see Fig. 3). This is especially important with progressive lenses where it’s imperative to have the eye track down the center of the intermediate corridor. In fact, many quality progressive lens designs compensate for this prismatic effect by adjusting the corridor.
Understanding that lenses are a series of progressive prisms arranged base-to-base or apex-to-apex helps ECPs understand many aspects about lenses including their focusing behavior, color aberration, magnification, and a host of other important characteristics.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.
As the left eye of this person looks through a minus powered lens, base-in prism is received. If this same person had a plus lens in the other eye, base-out prism would be experienced when converging to read.