Take a careful look at your next lens formula. Then really look at your patient. You have all the starting points for picking the best lens material, for guiding the frame selection, for making the correct eye size and bridge size choices, and even for making vertex-dictated power adjustments. By making the right choices you will turn that lens formula into comfortable, attractive and well-performing eyewear. The lens formula gives you the light bending information, but it also contains a lot of other clues that are easy to overlook. Each of the clues deals with a factor that, by itself, may be minor. In optics however, factors tend to be additive. Here are the clues top-notch dispensers look at in every lens formula and how each clue bears on the final eyewear design.
Increased center thickness in plus lenses means greater magnification. In strong plus powers this means a reduced field of view for the patient and an undesirable “magnified eyes” effect. Added thickness means added weight in all lenses. The determination of the strength and location of the strongest meridian of a lens is a quick and easy way to focus your thinking on the lenses you are ordering. To find the power of the strongest meridian in any cylinder lens, simply combine the sphere power and the cylinder power. The power of the strongest meridian will be either the sphere power or the combination of sphere and cylinder power, which ever is greater. For -3.00D -2.00D x 180, combining -3.00D with -2.00D gives -5.00D, which would be the power of strongest meridian in this lens. For +3.00D -2.00D x 180, combining +3.00D with -2.00D gives +1.00D; so in this case the power in the strongest meridian would be the sphere power, +3.00D. The strongest meridian is 90 degrees from the axis if the sphere and cylinder powers combined are greater than the sphere power. The strongest meridian is in the axis meridian if the sphere power is greater than the sphere and axis powers combined. For -3.00D -2.00D x 180, the strongest meridian is at 090, and for +3.00D -2.00D x 180 the strongest meridian is at 180.
The boxing system is the industry’s accepted system for eyewear measurement and is the foundation for understanding the lens/frame relationship for any power. | |

•If the strongest meridian falls close to the 180° meridian the “minimum possible” lens thickness will be more affected by decentration, so it will be more important to select a frame that has a frame PD that equals, as closely as possible, the anatomical PD. •For strongest meridians that fall close to 90° or straight up and down, your lab can achieve a thinner, more attractive lens by not using a deep (i.e. large B measurement) frame. •Beware of the strongest meridians that fall close to the meridian of the effective diameter, or ED (see section below on “Understanding the ED”) This affects the “minimum possible” lens thickness. For strongest meridians in minus powers the thickest edge is at or near, depending on decentration, the meridian of the ED. For strongest meridians in plus powers, the center must always be thick enough to allow sufficient material at the edge of the lens at, or near, the meridian of the ED. | |

Defining Details BD is a notation that indicates a “base down” prism component is needed with the thickest part (base) of the prism directed vertically downward. BD: BI: BI is a notation that indicates a “base in” prism component is needed with the thickest part (base) of the prism directed horizontally inward toward the patient’s nose. BO: BO is a notation that indicates a “base out” prism component is needed with the thickest part (base) of the prism directed horizontally outward away from the patient’s nose. BU: BU is a notation that indicates a “base up” prism component is needed with the thickest part (base) of the prism directed vertically upward. Combining Powers: Combining lens powers is an algebraic combination. For example +1 combined with +1 is +2, and +1 combined with +3 is +4. And +1 combined with -1 is 0, while +2 combined with -3 is -1. With a little practice and perhaps a look your old algebra book, you’ll catch onto it in no time.Decentration: This is the distance in millimeters that the distance MRPs must be displaced from the eyewire centers.Frame PD: This is the distance between the centers of the eyewires. To calculate the frame PD, simply add the eye size and the bridge size. A 4820 frame has a 68 frame PD.Major Meridians: These are the two meridians made up of the meridian of the axis power and the meridian that is 90 degrees from the axis. Locating the Meridian of Greatest Power: The easiest way to visualize this is to draw an optical power cross with one line in the axis meridian and the other at a right angle to it. Note the sphere power on the axis meridian and the combined sphere and cylinder power on the other axis. The meridian with the larger number is the meridian of greatest power. Meridian: Ophthalmic lens power is specified according to a pattern similar to the face of a clock. The horizontal (180) meridian would be a line passing through the three and the nine. The vertical (090) meridian would be a line passing through six and 12. The one to seven line would be the 060 meridian, and the 10 o’clock to four o’clock line is the 150 meridian. It is a good idea to indicate meridians with three digits, that way you can be sure that a mistake such as the 0 being left off an axis 110 is not taken as axis 011. Minimum Lens Thickness: There is a limit to how thin a lens can be surfaced. This minimum thickness is determined by practicality and by established standards such as the ANSI Z80-1999. In plus lenses, lens blanks are sometimes overly thick to allow a greater diameter. For small eyesizes it is often necessary to surface the lens thinner, rather than simply edging down a large blank, to achieve a first-rate result.MRP: The MRP (major reference point) is that point in the lens that gives the prescription. Bifocals have two MRPs. One is the distance MRP and the other is the near MRP. PD: This is the distance from the center of one pupil to the other. It varies depending on whether the patient is looking at distance or converging to look at a near object. Prentice’s Rule: The amount of prism at any point in a lens is equal to the distance in centimeters from the optical center of the lens times the power of the lens in diopters. The base of the prism will be toward the optical center for plus powered lenses and away from the optical center for minus powers. (e.g. 10 millimeters (1cm) from the center of a +2.00 lens the amount of prism is 1 x 2.00 or 2 prism diopters—with the base toward the optical center of the lens.) For cylinder lenses, the power used in the calculation is the power in the meridian that passes through the point being considered. Refracting distance: This is the distance from the strongest test lens in the refracting instrument to the front of the eye. Many refractors have devices to help estimate this distance. Spectacle Lens Vertex Distance: This is the distance from the lens to the eye. Except for steeply curved lenses in large eyewires, simply measuring the distance from the cornea to the plane of the spectacle front is adequate to determine a spectacle lens vertex. | |

l When the prism part of a lens formula varies from what is “usually” done, a call to the prescribing doctor may help you avoid making an error in ordering. l When lateral prism (i.e. BI or BO) is specified, try to avoid decentration as much as possible by careful frame selection, unless the optical center of the lens is shifted toward the geometric center of the eyewire by the addition of the prism component. •When vertical prism—i.e. base up, (BU) or base down (BD)— is specified, avoid, as much as is practical, using a frame with a deep (large B measurement) lens shape.
Always comparing the right and left lens powers is good technique. This comparison is especially important for the powers in the up-and-down (90°) meridians. For every cylinder lens formula, the power in the 90° meridian can be easily calculated by combining the sphere power with the power of the cylinder in the 90° meridian (see sidebar at right). •Patients do not easily tolerate vertical prism differences—often called vertical prism imbalance—between the two eyes. Individual tolerance varies, but a difference between the right and left lens of one prism diopter (1s) or more of vertical prism can cause asthenopia, adaptation problems, reading difficulties and even diplopia. Patients with single-vision lenses and vertical prism imbalance usually learn to drop their heads to read through the lenses at a point closer to the optical centers. Careful placement of the MRPs helps these patients avoid induced prism problems. •For patients with vertical prism imbalance who must look down to read, slab-off design or reading lenses should be considered. It is especially important to select a design with a short distance from the prism neutralization point to the reading level when ordering PAL lenses for these patients. The distance from the prism neutralization point to the distance neutralization point also requires careful consideration because this shows the amount of induced prism that must be tolerated for distance vision tasks when there is a vertical imbalance.
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