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I have a pt with an astigmatism of -4.00 OU and she is concerned with the visual effects of her eyes through the lenses. Just wondering what the best way to minimize this effect (shrinking effect) in the edging process. I know that polishing edges can reduce some of this effect. Any other tips? Do Asheric lenses help? Higher index? Thanks for any tips or light you can shed upon this subject.
Minimizing the vertex distance will have the greatest influence on minification in this case. You can do this through proper frame selection and adjustment. While edging the lenses, using a guided mini-bevel positioned near the front of the lens will also reduce the vertex distance.
Increasing the base (front) curve will reduce minification but, since this generally increases the vertex distance, the net change in magnification isn't necessarily affected much when manipulating the base curve of minus lenses.
Keeping the size of the lens small through proper frame selection will also help a great deal in minimizing how conspicuous the lenses are. In addition to reducing lens thickness and the appearance of the reflections of the lens edge, this also reduces the area of the face seen through the lens aperture by an observer, so you will see less of the face minified through the lens.
Reducing the visibility of the edges by decreasing thickness (using high-index or rolled edges) or by applying edge treatments (such as light tints or edge polishing) will also reduce the "coke bottle" effect and the appearance of those "power ring" reflections, which are created by internal reflections of the lens edge.
Best regards,
Darryl
Thank you Darryl for this helpful response!
By the way, I should have mentioned this in my earlier post, but remember that the maximum edge thickness of a minus cylinder lens will be at 90 degrees away from the axis of the prescription. So you would want to pay particular attention to the size of the frame through that meridian. High cylinder at axis 180, for instance, would benefit from a vertically more shallow frame shape.
There are probably also some eye makeup or cosmetic tricks that will help your patient make the apparent vertical (cylinder at axis 180) or horizontal (cylinder at axis 90) compression of her eyes less noticeable.
Best regards,
Darryl
Darryl, don't you also want to recommend aspheric along with high index?Quote:
Originally Posted by Darryl Meister
Yes, Asphericity would definitely be a good idea, and many high-index lenses utilize some sort of aspheric design. For that matter, while my post was focused only on the topic of cosmetics, an atoric or an optimized free-form design would be essential for good optical performance given this much cylinder power.Quote:
Darryl, don't you also want to recommend aspheric along with high index
Best regards,
Darryl
Bi-aspheric lenses would be my reccommendation. It works for my patients.