Read the bolded part of this article by Rnadall Smith. I've, in the past, measured the OC for aspheric lenses 2 mm below where I've dotted the pupil. This article seems to tell me that for every 1 degree of panto on the frame, I need to take the OC down 2 mm. Perhaps I completed missed something or I'm just brain dead today, how does everybody else measure for aspheric lens fitting?
Aspheric Designed Lenses
By Randall L. Smith,
Aspheric lenses have been used since the early 1900s. With the advent of computer-assisted design and computer controlled surfacing equipment, it has become possible to create surfaces of almost any complexity. The main interest in designing aspheric surfaces in recent years has been to improve the cosmetic appearance of the lens and reduce its weight for medium- and low-powered prescriptions. The front surface of the lens is aspheric (i.e., a flatter curve) and the back surface is used for the cylinder component of the lens.
Aspheric designed lenses are a product to recommend to all your patients with prescriptions of plus or minus 4.00 or higher. Aspheric surfaces result in lenses that are thinner, flatter and lighter in weight than lenses with conventional spheric base curves. For a minus lens, the aspheric front surface becomes gradually steeper from the center toward the edge. For a plus lens, the aspheric front surface flattens from the center toward the edge.
Aspheric lenses require a higher degree of skill in the laboratory and the dispensary. When working with aspherics in the laboratory, the major reference point has a fixed location because of the aspheric design. No prism for decentration may be ground. A sagometer that measures for a 50mm diameter will not give accurate results on an aspheric lens. Use the information supplied by the manufacturer or a computer program designed for that brand of aspheric. The true base curve and sag will be helpful in calculating tool curves, but not for thickness. A lens gauge cannot be used to measure the amount of asphericity. Moving the lens gauge across the surface may indicate the front surface is aspheric, but it is not an accurate measurement of asphericity.
It is important for the laboratory and the dispenser to record all the information supplied by the lens manufacturer to reduce possible problems if a lens must be remade. The dispenser must be aware, when using aspheric lenses, off-axis power errors caused by improper fitting are larger than for spherical lenses. The dispenser must place the optical axis of the aspheric lens surface so that the optical axis ray will pass through the center of rotation of the eye. The optical centers of the lenses must be placed monocularly horizontally and vertically. Using a corneal reflection pupillometer will assist you in obtaining the most accurate placement of the optical center horizontally. The optical center of the lens should be dropped 1mm for every 2 degrees of pantoscopic tilt. Most modern frames have between 8mm and 12mm of pantoscopic tilt.
The benefits of aspheric design are:
1. The magnification/ minification of the eye and surrounding area, as seen by an observer, is reduced.
2. The spectacle magnification/minification is reduced for the patient.
Some patients may experience adaptation problems, especially in high-prescription corrections. Problems can originate from improper fitting procedures, changes in magnification and the changes in the amount and location of reflected images from the front and back surfaces of the lenses. Dispensers are using a combination of aspheric design and high-index material to reduce thickness for minus patients. High-index lens materials tend to have lower abbe values and lower transmission levels. Anti-reflective coatings can help improve the performance of the lenses.
Patients with prescriptions in the higher power ranges will appreciate the premium nature of aspheric lenses. With proper patient education and accurate fitting measurements, aspheric lenses will be more cosmetically appealing and optically superior than lenses the patient has been wearing.
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