Understanding how lens tilt effects lens power is super important. Just bending a frame a little can make a difference!

All of the following is based on the assumption that the lens OC is directly in front of the wearer’s pupil/visual axis.

You also need to know that in addition to vertex distance, lens tilt also effects the perceived power of a lens. As covered in All About Contact Lenses, you will actually use the vertex compensation formula. However tilt compensation is rarely, if ever, used in the daily life of an optician. But, and it is a BIG BUT, you do need to know about it, so hang in there.

The moral of the story is, Never discount what a little frame bending might accomplish!

VISUALIZE IT! Think of a lens suspended in air with the ability to move in an infinite range of motion. Now bring that down to an exaggerated movement of what could happen to a lens mounted in a frame.

Imagine the lens tilting through all the positions from retro to neutral to panto.

Imagine the lens tilting through all the positions from positive face form, to neutral to negative face form.

See that panto tilt occurs along the 90th meridian.

See that face form tilt occurs along the 0-180 meridian.

The rules are:

Lens tilt in the 90 or lens tilt created by retro, neutral or panto angle changes the perceived sphere power and creates cylinder power in the 90th meridian.

Lens tilt in the 180 or lens tilt created by face form, positive, neutral or negative changes the perceived sphere power and creates cylinder power in the 180th meridian.

This is why you sometimes see people wearing their glasses with the temple bends 2″ above their ear! They are adjusting the lenses so they provide additional power and/or cylinder. Pretty smart!

If you can grab a a nice thick lens you can probably see why these changes happen. When you tilt a lens you are upsetting the entire optical process of refraction. You are disrupting the interaction of the eye’s optical system and the lens system as well.

This unwanted cylinder aberration is known as marginal astigmatism or oblique astigmatism. It can be distracting to the wearer and even create a “swim” effect that makes them nauseous.

However, because of the unique nature of lenses mild face form and mild pantoscopic tilt are desirable in fitting frames and will actually improve vision for the wearer.

This is why a free form lens is called “compensated” and will not read accurately on a basic lensmeter. A free form lens takes into account the wrap and tilt and adjusts the Rx accordingly.

Here is the math behind it:

Assume you have a +12.00 diopter spherical lens in polycarbonate.

You take that lens and tilt it 15 degrees.

You started with a simple +12.00 diopter sphere and by tilting the lens you ended up with a perceived power with a sphere power of +12.25 and almost a full diopter (0.879) of cylinder!

Certainly something to keep in mind when dealing with any strong lens.

The moral of the story is, Never discount what a little frame bending might accomplish!