This allows flatter, thinner lens forms to be utilized without compromising peripheral optical performance.
This difference in curvature can offset the difference in focal power at a given angle of view. These lens aberrations can be reduced or eliminated by varying the form, or relationship between the front and back curves, of the lens by selecting the appropriate base (or front) curve.Īlternatively, the use of asphericity can neutralize the oblique astigmatism produced by the lens through the use of surface astigmatism, which introduces a progressively increasing difference in tangential and sagittal curvature away from the center of the lens. The average dioptric distance of the tangential and sagittal from the ideal image plane of the eye, or far-point sphere of the eye, is referred to as the power error or mean oblique error this aberration is analogous to an error in prescribed sphere power. The difference between the tangential and sagittal focal powers, which is essentially equal to the amount of cylinder power between the tangential and sagittal meridians, is referred to as oblique astigmatism this aberration is analogous to an error in prescribed cylinder power. The default is Automatic, which uses a 2.0 mm minimum for low-index lenses and a 1.5 mm minimum for high-index lenses. The minimum center thickness of minus-powered lenses can be selected. Reducing the viewing distance will also result in a near vision effectivity error through the center of the lens, which is the difference in vergence for finite object distances due to lens form. The object distance can be varied to assess the performance of the lens over a discrete range of viewing distances from 25 cm to Infinity. The actual stop or center of rotation distance utilized in the optical analysis calculations is determined by adding 13.5 mm to this vertex distance. The vertex distance can be varied to assess the performance of the lens over a range of vertex distances from 5 to 20 mm. These results should be disregarded.įrom the Options menu, the following assumptions may also be changed: Note that, in higher prescription powers, off-axis performance may become erratic at extreme viewing angles as the mathematics involved break down. Click and drag the red horizontal vernier line to set the 'Off-Axis Optical Analysis' calculations to a specific point of intersection on the back surface (that is, ray height in millimeters) or to a specific angle of view (that is, ocular rotation in degrees).Set the lens material from the Material drop-down list.Set the asphericity of the front lens surface using the Asphericity slider control.Īsphericity eccentricity values from −10.0 to +20.0 can be selected.Set the front curve of the lens, in the refractive index of the lens material, using the Front Curve slider control.įront curve powers from −4.00 to +16.00 D can be selected.Set the diameter of the lens using the Diameter slider control.ĭiameters from 30 to 76 mm can be selected.Powers from −12.00 to +8.00 D can be selected. Set the back vertex power of the lens using the Lens Power slider control.
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