lens types and abbe values

[saffron1]

Hi, I am new to the forum here, and am trying to get the absolute
thinnest pair of glasses, with the best optical clarity. I would like
to know if anyone knows the abbe value of a 1.74 high index, 1.8 or
1.9 glass lens. I have had problems in the past with chromatic
aberration in both a polycarbonate and 1.67 high index lens. I am
wearing a Trivex lens now, which is optically great but much thicker
than I would like. I am thinking of ordering glass lenses from visions
of canada but they are quite expensive so I want to be sure these
lenses would be best before I order them. My prescription is -7.25
with -1.75 astigmatism in both eyes.

I am also wondering if the glass lenses will make the eyes appear more
normal in size than the plastic do. I have heard that aspheric lenses
are better for this, but I also heard that is mostly in farsighted
cases.

Thanks

[Paul]

I don't think glass will make your eyes look any different, and I
think it's going to be very hard to find aspheric lenses for myopia.
Glass would be about as thick as CR39, which is pretty thick. I tend
to use smaller lenses and standard index material to reduce thickness
and get better optics than polycarbonate. In the "old days," when
lenses were all glass, small lenses were common because they reduced
weight.

If you're used to Trivex, you might find materials other than glass
and CR39 to be disappointing, though someone here may suggest a
particular material I'm not familiar with. If it's been years since
you've used polycarbonate, they have improved the materials, though I
still notice a bit of color at the edges of some of my glasses.

[Beaugrand®™©]

There are some things you can do to have thinner lenses without paying
outrageous prices for high-index lenses.
With a near-sighted prescription (such as your -7.75) the thickest
part of your lens is around the edges; the part of the lens directly
in front of the pupil is only 1mm thick (2mm for "safety" lenses).

To keep the lens thin, use a smaller lens. For example, my "TV
viewing," -5.50, 1.56 index semi-rimless glasses are 50mm wide and 5mm
thick at the edge, but my older "Socializing" 1.56 index, -5.50, 55mm
wide lenses are 11mm thick at the thickest part of the edge. Same
diopter value, same index, not much smaller but a whole lot thinner.

I suggest 1.56 or 1.64 index lenses no wider than 50mm for a thinner
look.

You can also disguise the thickness of the lens by using a thicker
plastic frame, rather than a rimless or semi-rimless or even a full-
rim metal frame. My plastic "Socializing" frames are 6mm thick around
the rim, so less of the edge of the lens is visible.

WHL

[Paul]

Another thing to do to minimize thickness is to work with bridge width
and lens size to make the optical center of the lens close to its
physical center. Many of my lenses have their optical centers closer
to the inner edge. With a minus lens, this makes the outer edge
thicker.

My best metal pair for thinness has 38 mm lenses with the optics very
well centered. Even though they're made from CR39, the lenses are
pretty thin.

Also keep in mind that to get that last few percent of extra thinness
with a more exotic material will likely add a lot to the cost (similar
situation to buying hi-fi music equipment), as well as having worse
optical quality.