The prescription, or lens formula, is the starting point for picking the best lens material, frames, and eye and bridge size. The right choices will turn that lens formula into comfortable, attractive and well-performing eyewear.
AXIS AND EYEWEAR SELECTION
The lens axis describes the major or principal meridians of the prescription, the meridian of greatest and least power; thickest edge of a minus lens and thinnest edge of a plus.
Lens powers are separated into two principal meridians, the axis and 90° from the axis. Illustration A shows cylinder axis location when looking at the spectacle wearer.
To find the power of each meridian in any cylinder lens, simply compare the sphere power and the combined power of the sphere and cylinder together. At the axis, there is only sphere power; 90 degrees from the axis, lens power is the sum of the sphere and cylinder values. For example:
Rx: OD +3.00-2.00 X 045, OS +2.25 Sph.
For a +3.00 -2.00D x 045 lens, the two lens powers are +3.00D and the combination of +3.00D and -2.00D or +1.00D; so the power in the two meridians is +3.00D@045 and +1.00@135. A +2.25 sphere lens has the same power at all axes on the lens.
Rx: -0.50-2.50 X 090 OU.
In this lens, the two powers are -0.50D and the combination of -0.50D and -2.50D or -3.00D. This lens would have a -0.50D@090 and -3.00D@180. While a new dispenser might think that there is little power in these lenses because of the -0.50D sphere power, due to the axis the lenses are really -3.00D@180. This will significantly impact visible edge thickness.
PRISM
Prism is required when the line of sight must change direction. A prescription or lens formula with prism, specifies the amount and the direction of the base (thickest part) of the prism. The amount of prism is usually, but not always, the same in both eyes.
• Usually, but not always, horizontal prism is either base in (BI) in both eyes or base out (BO) in both eyes.
• Usually, but not always, vertical prism is downward in one eye and upward in the other.
• When the prism part of a lens formula varies from what is “usually” done, a call to the prescribing doctor may help avoid making an error in ordering.
• When BI or BO prism is specified, try to avoid decentration. The optical center of the lens is shifted toward the geometric center of the eyewire by the prism component.
• When base up (BU) or base down (BD) prism is specified, avoid, using a deep (large B measurement) lens shape.
Patients do not easily tolerate vertical prism differences, often called vertical imbalance. Individual tolerance varies, but a difference between the right and left lens of one prism diopter (1Δ) or more of vertical prism can cause asthenopia, (eye strain) adaptation problems, reading difficulties and even diplopia (double vision). Patients with single-vision lenses and vertical prism imbalance usually learn to turn their heads to read through the lenses at a point closer to the optical centers. Prism imbalance tolerances of 2/3Δ horizontally and 1/3Δ vertically are generally acceptable. For patients with greater vertical prism imbalance who must look down to read, slab-off design or reading lenses should be considered.
LENS FORMULA TIPS
· If the strongest meridian falls close to the 180° line, lens thickness will be more affected by decentration. Select frames where the patient’s pupils are close to the geometric center of the lenses.
· Thicker lens edges in minus lenses are less attractive.
· Increased center thickness in plus lenses means greater magnification. In strong plus powers this means a reduced field of view for the patient and undesirable “magnified eyes.”
· Added thickness means added weight in all lenses.
· For strongest meridians that fall close to 90° or straight up and down, avoid deep (i.e. large B measurement) frames.
· The effective diameter (ED) of a lens shape is defined as twice the longest radius of the shape. The longer the ED, the thicker the finished lens will be.
· For strongest meridians in plus powers, the center must always be thick enough to allow sufficient material at the edge of the lens. In fact, lens thickness is determined by minimum edge thickness at a plus lens’ thinnest point.
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