OPTICAL CORRECTION

  UNIFOCAL

Distant Vision /Near Vision

The unifocal lens give an unique optical correction, for the none  PRESBYOPIC eye, and this for all distances of vision. For the  PRESBYOPIC eye, the unifocal lens give an optical correction either for distance or near vision..

Horizontal centration of spectacle lenses

When an eye is directed to a point on an object the image of that point will be produced on the fovea. The line joining the object point and the fovea is the eye's visual axis. It is implicit in this statement that the eye's entrance and exit pupils also lie on the visual axis. If a spectacle lens is placed before the eye so that its optical axis coincides with the visual axis there will be no prismatic effect and the object will not be displaced, otherwise the eyes have to correct this displacement, and eye strain will build up. 

Decentration to provide the correct centration distance.

In many cases, a spectacle frame is dispensed where the distance between centres of the frame is larger than the subject's CD. In order to position the optical centres of the lenses directly in front of the subject's pupils, decentration must be ordered to shift the optical centres so that their horizontal separation corresponds with the subject's CD. For example, if the distance between centres for the frame is 70mm and the subject's CD is 64 then there is a discrepancy of 6mm and the lenses must be decentred inwards to compensate for this. If just a binocular CD of 64 is given then the decentration must be assumed to be divided equally between the eyes and the decentration is given simply by:

Decentration = (distance between centres - CD)/2

In this example the decentration is given by (70-64)/2 = 3mm each eye.

If monocular CDs are given, such as 33/31 (which sums to 64mm as before), then the decentrations for each lens will differ. Again assuming the frame dimension to be 70mm, the decentration for each lens can be calculated from:

Decentration = distance between centres/2 - monocular CD

In the above example the decentration for the right eye would be 35 - 33 = 2mm in.

Whereas the decentration for the left eye would be 35 - 31. = 4mm in. Needless to say, the sum of the monocular decentrations must be the same as the difference between the distance between centres and the binocular CD.

Effect of best form lenses

Look for Hoya Nulux Ep, Nikon See Max, Rodenstock Impression MONO; Seiko bi-asphrical lenses.