Lens Power range limited by base curve

Written by Optician Club on . Posted in Freeform Rx Lenses, Optician Tools, Progressive Lenses

Manufacturers typically produce a series of semi-finished lens blanks, each with its own base curve. This "base curve series" is a system of lens blanks that increases incrementally in surface power (e.g., +0.50 D, +2.00 D, +4.00 D, and so on). Each base curve in the series is used for producing a small range of prescriptions, as specified by the manufacturer. Consequently, the more base curves available in the series, the broader the prescription range of the product. Manufacturers make base curve selection charts available that provide the recommended prescription ranges for each base curve in the series, let's check this lens power range limited by base curve.
 
A Typical Base Curve Selection Chart
Power Range
Base Curve
+8.00 D to +4.75 D
10.00 D
+2.25 D to +4.50 D
8.00 D
+2.00 D to -2.00 D
6.00 D
-2.25 D to -4.00 D
4.00 D
-4.25 D to -7.00 D
2.50 D
-7.25 D to -12.00 D
0.50 D
 
The base curve of a lens may affect certain aspects of vision, such as distortion and magnification, and wearers may notice perceptual differences between lenses with different base curves. Consequently, some practitioners may specify "match base curves" on a new prescription. Some feel that these perceptual differences should be minimized by employing the same base curves when the wearer obtains new eyewear. This would conceivably make it easier for particularly sensitive wearers to "adapt" to their new eyewear. It's not proper to request a lab to provide over -6.00 with Base 8 front curve.
 

The “best form” or "corrected curve" shape of a lens is one that provides as close to the the same vision at center and in the periphery as possible, as the eye moves normally behind the lens. For finished lenses, manufacturers choose both front and back curves; for semi-finished lenses, a recommended series of front curves are provided usually in a base curve chart.

Lens power range limited by base curve

 

The choice of base curve influences a number of optical and mechanical factors, which are important in both traditional and free-form lens design.

This is one of most important fact to determine which type of lenses should be used for a specific prescription, check more about   Advantages and disadvantages of FreeForm.


Optical Factors Associated with Base Curves:
1. Off-center vision quality
2. Magnification
3. Geometric distortion

Flatter lenses generally compromise off-center vision quality in conventional lens designs using spherical base curves. Aspheric and optically optimized free-form lens designs are not subject to the same optical limitations, since the design can correct oblique astigmatism using local changes in curvature. However, as the base curve deviates farther and farther away from the "ideal" best form base curve, the asphericity or optical optimization required also increases. And the lens design then becomes more sensitive to factors such as lens tilt.

Flatter lenses generally produce less magnification through the center of the lens but more distortion (pin-cushion in plus powers, barrel in minus powers), which is caused by a change in magnification away from the center of the lens. Fortunately, the visual system will often "adapt" to these magnification effects within a week or so, since the brain essentially "remaps" the new visual input based upon the expected visual input and other sensory input. Aniseikonia can also become an issue in the presence of significant anisometropia.

Check more about Mechanical Factors is Associated with  Lens Thickness and Base Curves

 

 

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