Base curves and their effects on optical performance can be a confusing and frustrating issue for opticians. Sunshine Optics always tries to maintain aesthetics while not compromising the optics of a patient’s prescription. Where aspheric base curves are concerned, that mainly applies to single vision and bifocal lenses, since all progressives are aspheric by default. These excerpts from “All About Base Curves” by Darryl Meister can be used as a handy reference guide:
1. You’re not concerned with the ocular curve but rather the front “base” curve. This is the curve that the manufacturer supplies finished, and the curve with which the laboratory will perform its calculations. Lenses must be ordered from the lens supplier by this finished front curve, not the back ocular curve. Also, the back curve alone really has no inherent effect upon vision. For instance, the front curve affects magnification and the relationship between the front and back curves affects the spectacle lens aberrations bothersome to the wearer. While you should understand the principles of base curve selection, try not to concern yourself too much with base curve selection since the laboratory generally does it for you.
2. Flatter base curves will generally provide better cosmetics, including reduced thickness, bulge, and weight. They will also be retained more easily in certain frames. However, these improved cosmetics come at the expense of optical performance. Flatter lenses with spherical base curves produce more of the aberrations that blur vision away from the center, and will narrow the wearer’s field of clear vision. To avoid this situation, use aspheric base curves, which provide the flattest, thinnest, and lightest lens designs available without sacrificing peripheral optical performance. The wearer’s eyes will also look more natural. It should be noted that laboratories using older equipment may have trouble processing the flat back curves necessary for aspheric lenses — though this shouldn’t be an issue too much today. It is particularly important with aspheric lens designs to use the recommended base curve, since their peripheral optical performance will be even more sensitive to base curve changes.
3. Do not match base curves unless absolutely necessary. Matching base curves is sometimes done to reduce adaption difficulties after prescription changes. However, prescriptions work best optically on their recommended base curves. Keeping the patient in the same base curves as his/her prescription changes may reduce the optical performance provided by the lenses. The changes in image size and distortion produced by the prescription change will generally be more significant than the same changes caused by using slightly different base curves. And, except in rare instances with really sensitive wearers, the patient should adapt to small base curve changes within a week or two. There are probably only three occasions wherein you should consider matching base curves: 1) When the patient has — or will likely have — an unusual sensitivity to the subtle changes in vision caused by spectacle lenses, 2) When the patient is getting a second pair of similar eyewear, or 3) When the patient is replacing only one lens.
4. Using steeper front curves to provide for eyelash clearance is generally not the best approach, and will cause both optical and cosmetic issues. When possible adjust the frame to avoid this situation.
5. In general, your best bet is to use the manufacturer’s recommended base curves. Manufacturers generally supply lenses conforming to “corrected curve” or “best form” principles, which essentially means that the base curve has been chosen in an attempt to minimize the bothersome aberrations that occur in the periphery of the lenses. These aberrations narrow the field of clear vision. For finished lenses, manufacturers have already taken the guess work out of it for you, and supply them with the correct base curve. For semi-finished lenses, manufacturers supply the laboratory with charts illustrating the available base curves and the recommended prescription range for each. The laboratory will generally use such a chart (which may reside electronically in their lab software) to make your lenses. Since these recommendations will generally differ from product to product, and from manufacturer to manufacturer, you should consult the appropriate base curve selection chart for a product before considering a substitution. Also keep in mind that lens suppliers advertising “flatter” base curves in spherical (non-aspheric) designs may be deliberately sacrificing optical performance for marketing purposes.
6. If you do find it absolutely necessary to specify a base curve, remember that the ANSI Standard allows a tolerance of 0.75 D. However, in practice base curves generally only come in 2.00 diopter steps from a given manufacturer, greatly eliminating the amount flexibility that you really have for a given prescription. So, if you order that new High-index Photochromic Mega Progressive from Brand X, don’t expect the lab to adhere to that 0.75 D tolerance. You should be particularly cautious when specifying or substituting aspheric base curves. Also remember that lens clocks (or measures) do not measure aspheric surfaces correctly. Moreover, the base curves of certain progressive lenses are optimized for the intended prescription range. Substituting base curves with these progressive lenses may negatively impact their performance.