High Index question for Darryl and others

[OPTIDONN]

This is something that I am having a hard time explaining to people. I feel that I kind of know why this is but I want to be sure. When a patient puts on a pair of high index lenses for the first time they opten complain that as they turn their head there is a kind of image movement that was not present in their previous cr-39 pair. I notice this when I switch from my small glass lenses to my rectangular 1.66 lenses. As I turn my head it seems that every thing moves as well. Do you get what I'm saying? Like I said I have been having a hard time explaining thins. What causes it? Is it due to prismatic effect of looking obliqueley (spelling may be off...long day with the kids!) through the lens?

[Darryl Meister]

It could just be the normal "with" (minus) or "against" (plus) motion you see when moving an object relative to the optical center of lens, which has perhaps become more apparent in the larger frame. It could also be due to a processing aberration (wave) if it's a semi-finished lens. Or it could also be due to a difference in geometric distortion between your new lens and your old lens, which will vary with base cuve.

[OPTIDONN]

"geometric distortion"? What is that?

[Darryl Meister]

Check out the section on "Secondary Lens Aberrations" in Ophthalmic Lens Design.

[OPTIDONN]

oh...thanks! So if for instance you held a flat peice of glass and as you moved it around the images seen through the glass tend to move then you began to move around a curved peice of glass and as you moved it around in front of you the images seen through it move less. This is what I experience. Is it just the typical "with" movement exagerated due to the rectangular lens shape? Or is it caused by the fact that as the eye rotates further away from the optical axis of the lens the distance from the lens to the eye's center of rotation increases causing all them funky off axis aberrations? This is a complaint many people have when they try high index lenses for the first time (however it takes them only a few minutes to adapt).

[Bezza]

Just a quick point to add here, I noticed this when I first started out in optics and switched from CR39 to 1.74 aspheric as my RX is around -7.50D. At the time i was a complete novice and was quite dissapointed at the poor VA's i was getting due to all the abberations and the constringence also was something i had never spotted before. A few degrees increase in frontal bow can make a huge difference to this i soon discovered, and my VA's were vastly improved.
Had another pair of D&G rimless made up recently and unfortunately I didnt realise that the mount was going to be virtually impossible to adjust, as a result i was unable to achieve a frontal bow anywhere close to that of my other pair, the difference in VA between the two pairs of glasses is quite considerable.
Knowing what i know now I can say that the various abberations can be reduced quite a lot by increasing frontal bow when using high index lenses. You have to consider that given how flat these lenses can be that the back surface of the lens will be further away from the centre of rotation of the eye at the lens periphery than it will be at the centre thus increasing the effect of these abberations.