In addition to polishing lens surfaces to remove deposits and scratches, a GP lens modification unit can be used to change the power of a lens, too.

Why modify the power of a contact lens in-office? Let’s say at your patient’s annual exam, you find she needs an extra –0.50D of power; otherwise, the lens is in good shape and doesn’t need replacing. Making small adjustments to the lens power can be a significant time-saver for your patient if you can just change it on-the-spot rather than waiting for a new lens to arrive. Changing power is quick and straightforward to do. However, it only works if the lens is spherical on the front surface (so spheres and back-surface torics work).

Some words of caution: modifying the power could void the lens’ warranty, so ask your lab about its policy. It’s also possible that you could break or distort the lens; you should have your patients sign a written notice before you attempt to do this on a habitual lens.

Practice on an old contact lens first to see how long it takes to make changes, and check your work frequently on the lensometer so you don’t overdo it.

Finally, high-Dk materials may warp with excessive heat, so use plenty of polish and don’t touch the lens to the pad for more than a few seconds at a time.

Adding Minus Power

Use the flat sponge pad and put the contact lens on a spinner tool. Lightly touch the front of the contact lens to the midperiphery of the sponge, adding polish as you do so (Figure 1). Keep the spinner tool perpendicular to the sponge. Apply enough pressure to get the lens spinning on the tool, but not enough to push the entire surface of the lens into the sponge. Only the central area of the lens should touch the sponge. You can also just use a suction cup, but the optical results are better with a spinner.

Figure 1. Adding minus power (View a video of this here).

The longer you touch the contact lens to the pad, the more plastic is removed and the more power change you will get. Practically, you can add up to about –0.75D to –1.00D; more than that, and the optics may get distorted.

Adding Plus Power

Use the same flat sponge. You can use the spinner to hold the lens, but hold the tool near the bottom so that the lens doesn’t actually spin. Push the entire front surface of the lens into the very center of the sponge (Figure 2) and hold it there for a few seconds; lift the lens, add a bit more polish, and repeat. By pushing the sponge into the center of the contact lens, more plastic is removed from the periphery of the contact lens than the center (because the rotation speed is faster in the sponge’s periphery). The end result is a shift to more plus power.

Figure 2. Adding plus power (View a video of this here).

Adding plus takes longer than adding minus. You likely can add only +0.25D to +0.75D with this method.


With a little practice, you or a well-trained technician can make minor power changes as needed and provide a valuable service for your patients. CLS