Photo by Ned Matura
By Palmer R. Cook, OD
The Fitting Cross is an important landmark for fitting PAL lenses of all kinds. As such, it deserves a careful look and some analysis. It is an industry-agreed-upon marker that can serve you well if you want to deliver the best PAL technology when fitting progressive addition lenses (PALs). Oddly, it doesn’t represent any point in a lens of optical importance; however, it is a landmark that allows you to position the real points of importance for good lens performance.
Understanding the anatomy of the Fitting Cross can help you use it more effectively. The fitting cross has three components. The first is its horizontal bar, which defines its vertical location (i.e., the height above the lowest point of the lens). Its vertical bar defines its lateral location on the lens (i.e., the monocular PD). The monocular PD is the horizontal distance from the Fitting Cross to the centerline of the frame’s DBL (Distance Between the Lenses—also known as the shortest horizontal separation of the two lenses).
The third component of the Fitting Cross is the point at which the two bars cross. This fixes its exact position or location, which makes it useful. Typically, we may say, “The fitting cross is 22-mm high” and leave it at that, but what is left unsaid, yet understood, is the monocular PD must be the other coordinate.
WHAT DOES IT REALLY DO?
The Fitting Cross is a landmark that simultaneously defines the location of the point in the lens that gives the prescribed refracting power for distance (DRP), the point that gives the refracting power for near (NRP) and the point that gives the location of the prism reference point (PRP). The PRP is the point that gives the needed prism or the absence of prism if prism is not needed. In most prescriptions, prism is not required, so the PRP is the optical center of the lens. The location of the fitting cross is also the top of the corridor.
Most opticians and technicians mark a cross at the center of the patient’s pupil. They do this because they were taught that this is the way it should be done. Unfortunately, this encourages the misconception that the Fitting Cross is the point through which the patient views distant objects. In fact, much is made of the fact that the patient should have a comfortable (chin up – chin down) head tilt when the Fitting Cross is placed, but the level of the Fitting Cross is also the point at which the corridor starts (i.e., where plus power begins) so the patient must look somewhat above the Fitting Cross to see clearly at distance. At the Fitting Cross, the upper part of your lensmeter’s ray bundle passes above the corridor, and the lower part of the ray bundle passes through the top of the corridor. That’s why you cannot accurately measure the refractive power of the lens unless your lensmeter is centered higher in the lens to get an accurate reading.
It is important to remember that patients can, will, do and must move their lines-of-sight higher than the Fitting Crosses to see clearly at distance. To do this, they lower their chins.
The lateral position of the vertical bar of the Fitting Cross is determined by the monocular PD, so it need not be marked. When you mark the pupil’s center, with the line-of-sight leveled using a mirror, and with the patient’s head correctly positioned, you are marking the point the patient must seek for his distance power (i.e., the DRP). The DRP will be above the Fitting Cross by a few millimeters as specified by the lens manufacturer’s design. For the PureSite Ultima that distance is 6 mm, and the Fitting Cross would be marked 6 mm lower.
If your patient views his own eye (with the other eye covered) in a flat mirror that is vertical, his line-of sight will be horizontal. If you position your eye at the same height as the patient’s eye, it will be easy to mark the height of the DRP to the left side of the eye, at center of pupil height (Fig. B) if you don’t block the patient’s view in the mirror.
So, is the Fitting Cross a good thing or not? The answer is easy. It is a tool, and used properly, it is very useful indeed. For every lens design that you fit, you should be familiar with the distances from the Fitting Cross to the critical lens points including the DRP, PRP and the NRP. That knowledge, coupled with an understanding of your patient’s tolerance for vertical prism* will allow you to dispense better performing eyewear. ■
*Optometrists measure the vertical fusional zone by gradually increasing vertical prism until the patient experiences blur, then a separation of the primary image into two separate images. The prism is then gradually decreased until the separate images merge into a single image—the recovery point. This test and others are used in a comprehensive examination to give an understanding of the patient’s tolerance for unwanted vertical prism. Once you, the dispenser, know the induced vertical imbalance at the DRP and NRP levels, you can ask the prescribing doctor about the patient’s tolerance for imbalance to achieve a comfortable, wearable eyewear.
Contributing editor Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics in Cincinnati, Ohio.