Merchant Processing Services

THE WORLD OF PROGRESSIVES is complex. There are hard designs, soft designs, combination designs, short corridors, long corridors, and now free-form or digital surfacing. At last count, there were over 200 different types of progressives on the market. Ugh! It’s enough to make eye care professionals pull their hair out. How is an ECP to muddle through the lens maze and decide what is best for the patient?

Thinking Progressively

It is easy to see why progressives are the lens of choice for most presbyopes. Cosmetically, it appears to be a single vision lens without the age revealing lines of flat top bifocal or trifocal. Visually, progressive lenses are the closest to natural vision. With a progressive, there is a prescription for all the field of vision in front of the patient. Therefore, a progressive meets most of the visual needs for most patients.

Since a progressive is the first choice for most patients, it is best to define exactly what a progressive is. Many individuals call it a no-line bifocal, but this is actually a misnomer. The closest thing to a no-line is actually a blended or Ultex bifocal. Instead, a progressive is a lineless multifocal that is designed in such a way that the add power gets progressively stronger the closer to the bottom of the lens it gets. 

Now, a progressive lens is designed to reach maximum add power by utilizing a corridor. A corridor is the small, hour glass shaped area of the lens that the eye follows to get from the distance prescription - through the intermediate - to the reading prescription. It allows the eye to seamlessly and comfortably change focus without “image jump.” It is the corridor that lens designers focus on when creating the optimum progressive.

Is it better to be hard or soft?

The hard or soft debate has been going on since the first soft lens design was introduced in the 1970’s by Varilux. Originally, this was a huge issue in the optical field and with good reason. Hard designs have a high level of precise blending in the unused, or distorted, portion of the lenses. This gives a much clearer reading and distance portion, but can cause more of that upset, dizzy feeling when the eye travels through the corridor. Not a good idea for someone with vertigo, balance problems, or people who are sensitive to motion. Some of the traditional characteristics of a hard lens include:

• Large and stable near and distance zones

• Spherical curves in the distance

• Shorter progression corridors

• High astigmatism on the surface

On the other side, soft designs have the blending more spread out and the blends enter the corridor more, resulting in the "soft" part of the design. An aspheric blend, this helped eliminate dizzy and distorted feeling, but it sacrificed some of the crispness of vision in the corridor. A soft lens design is the opposite of the hard in terms of traditional characteristics.

Today, the debate continues, but it doesn’t really need to. In actuality, most lenses today are neither hard nor soft, but a combination of the two blending designs. Myopes need clearer vision at the distance so the distance part of the lens has harder blends and softer blends at the near to help with the distortion. Hyperopes need clearer vision at near so the opposite is done. So, hard design, soft design, it really doesn't matter since neither one is usually produced in its true original form anymore.

The Long and the Short of It

Traditionally, a long corridor was the only lens option available. Initially, this was not a problem because the styles of the seventies and eighties were larger. In the 1990’s this all changed when frame styles became smaller and smaller. Since the average corridor at the time was 17mm, this forced the ECP to do one of two things: cut off the reading area or “bump up the add” to prevent the strongest reading area from being removed. This resulted in either a loss of near vision from the add removal or a distorted intermediate because of the extended transitional area that the longer corridor lenses have. Fortunately for the ECP and the patient, this changed with the development of the first short corridor progressive in 1999. 

With the introduction of the AO Compact, the progressive began another change. Though many manufacturers initially just dropped their minimum fitting height to compete, this resulted in a 15% loss in add power. Realizing that in some prescriptions the loss was unacceptable, within three years most of the major lens manufacturers had their own short corridor lens designs. Today, many lenses have an average corridor length of 10 to 20mm, a much shorter height than the average corridor length of 17mm just a decade ago. The end result is a wider group of lenses to fulfill the patient’s visual and cosmetic needs.

Be Free From It All With Digital

Free-Form is a way of digitally surfacing a lens, not an actual lens design. In conventional lens surfacing, a semi-finished blank is placed in a generator and the back of the lens is ground away. The resulting rough surface then goes through a process of fining and polishing, utilizing either a soft or hard lap. Because there is limited ways that a lens can be processed, traditional lenses can only have spherical and cylindrical surfaces and can only be produced with accuracy up to +/-.12 diopters. However, Free-Form lens surfacing has changed this.

In Free-Form surfacing, a computer numeric controlled (CNC) generator is controlled by a point file software system. This software can apply over 10 million calculations to create a data file that includes the patient’s prescription, parameters and desired lens material. The data file is then sent to the CNC generator and the lens surface is cut into the lens using a natural diamond point cutting tool. An additional factor in the lens grinding process that is unique to Free-Form surfacing is that the front as well as the back surface of the lens can be ground. 

As a result of the dual surfacing as well as the exact nature of the cuts, almost any type of lens surface can be cut into the lens. After completing the time in the generator, the lens can proceed directly to the polisher, skipping the fining process. A soft pad polishing system is then applied to the lens to safeguard the new curves on the lens surface that usually has only about 1 micron variation in depth. The effect for the patient is:

• Wider fields of vision

• Clearer vision while looking down

• Greater prescription accuracy at distance, intermediate and near

• Smoother transition through the corridor

Seeing Clearly

Progressives are the visual and cosmetic choice for most patients. They provide a wide range of designs that enable presbyopic patients to see and look their best, regardless of lifestyle and frame choice. It is the responsibility of the ECP to work with the patient to decide which progressive will work best for the patient’s needs. By being up to date on the latest technology, knowledgeable of current and past designs, and by actively listening to the patient, the ECP can navigate the progressive muddle and pick the best lens possible.

Carrie Wilson
BS, LDO, ABOAC, NCLEC