Ophthalmology & Optometry/lens index for my Rx
QUESTION: Dr. Axelrod, i'm trying to order reading glasses. My Rx OD +1.50 ADD 2.50 (+4.00), OS +1.75 ADD 2.50 (+4.25), PD 31/31
lens options are
1.57 Mid-Index Single Vision
1.53 Trivex Single Vision
1.59 Polycarbonate Single Vision
1.61 High-Index Single Vision
1.67 High-Index Single Vision
Can you recommend the best lens for my Rx? Is the lens thickness really different between 1.57 and 1.67 in my case or is it distortion at the edges and curvature that matter here? TIA
ANSWER: Hi Robert, you'll want to look at what are called 'lensmaker equations' as the answer depends on a few additional factors. since your lenses are thick in the center and thin at the edges, your concern would be center thickness. most companies use stock lenses of a given diameter and 'cut out' the lens shape for your frame. The larger diameter a lens blank is the thicker it needs to be so you may want to ask or know 'what is the smallest lens blank size that will cut out'. That will depend on the size of the frame and must involve the position of the lens within the frame so that your pupils will be properly aligned (must know the frame PD and your own PD). So given the relatively small difference in the refractive indices you list and the power of only +4, I would want to know the Abbe value or some measure of the material's distortions. You'll probably end up with mid index but if you get a good optician they can get specifics from their lab.
You'll have the largest control over lens weight and thickness if you get the smaller eye sizes of frames and use the smallest lens blank that will cut out. At your powers, the index doesn't make as big a difference as these two factors. To control distortions you can ask for aspheric lens design or 'HD lens design'
It's a great question and the marketplace is way to full of hype and uninformed sales staffs.
Mitch Axelrod, OD
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QUESTION: So if the index is not so critical in my case i can go with 1.57 but the lens needs to be aspheric or HD or else i will get distortion? I've had lenses made before that caused straight object like sheet of paper or monitor appear concave. Thinks the lens was 1.57 or 1.59 but most likely not aspheric or HD if it caused distortion. Thank you.
ANSWER: Hi Robert,
The biggest factor for what you've described is the front curve of the lens called the 'base curve'. Center thickness is also a big factor for magnification effect (you want focus, not necessarily magnification). Higher index lenses reduce thickness and reduce magnification by having flatter base curves but changing base curve, either increasing or decreasing, compared to what your brain is used to will cause shape distortion like curving. If you have a pair of glasses you are used to, just have the optician measure and duplicate the base curve and center thickness. Good opticians can sometimes tell if you have a polycarbonate (worse) lens material or a better plastic. Typically, CR39 material has the most glass-like optical properties but is not a high index material. Polycarbonate is impact resistant for safety applications, but historically was never a good choice for ophthalmic lenses except for safety. They are a bit better now, and as they are light and a high index material they are promoted. They used to be called 'the poor man's high index material'. They still suffer from color aberrations (rainbow effect at edges of white lights)away from the optical center, though.
As lens design and manufacture has evolved it is easier/cheaper to make better optics; even today's cheap lenses are better 'curve corrected' than older ones. so don't angst too much, go with something and see what happens. your brain will adapt to any new 'view' given a week or so.
Mitch Axelrod OD
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QUESTION: Dr. Axelrod, i've found a site where the cheapest lens index starts at 1.5. Frame dimensions 51x22, smallest available. Asked for aspheric lens but they can't make one with 1.5 index. Aspheric lenses start at 1.6 and up and are significantly more money. Should i go ahead with the cheapest index 1.5 or get aspheric 1.6+?
If the lens blank for 1.5 leaves too thick of a lens, go for the aspheric, but best to ask the company to tell you the center thicknesses of the lens blanks that would be used for your frame size, then you will know the difference for sure. Again, also ask their opinion of the optics and Abbe value.