Below are some examples of recent work we were asked to undertake. The metal glasses had very strong +5.00 Diopter Spherical powers. Usually we'd expect these lenses on this type of prescription to give a result of lenses that look thick and chunky with overhangs on the top and bottom of the frames and over magnification of the wearers eyes behind the lenses. Not a desirable look.
The sunglasses below in the plastic frame have a set of lenses in excess of +8.50 Sph Dioptres. Again, we'd expect these lenses to be very chunky with a very thick centre magnifying the eyes, and overhanging lenses from the plastic rims.
So, the question is, how did we thin these plastic lenses to this degree ? The lenses should look considerably thicker due to the strong prescriptions here, yet they don't. Read below for the answer that may be of use to you, or you should consider if on a budget !
When it comes to ultra thin lenses and strong prescription glasses, we have a number of ways of thinning the lenses for a better thinner result, depending on your budget. Obviously it is always preferable to use ultra thin Index lenses, but, as the cost of ultra thin lenses can be considerable, there are a few tricks that can be employed to save you money. A standard thickness lens has an index number of 1.5 (plastic). It is not generally used for the prescriptions above. The usual solution would be to use a high index lens. A 1.6 index plastic lens is available and would be a little thinner, but a 1.67 index lens is a better option being substantially thinner than a 1.5 index lens. The thinnest plastic lens available is a 1.74 index lens, although the difference in this option and a 1.67 index lens is minimal on most prescriptions and can be prone to more peripheral distortion on the edges which can take some time to adjust to and is only recommended on very strong prescriptions. We offer these options, but there is something else in the equation when thinning lenses that doesn't cost anything extra at all.
So, here's the thing. With + lenses (as opposed to - lenses), the size of the lens blank used before being cut into the frames has a major impact on lens thicknesses both in the centre of the lenses and the edge thickness.
We at Ilkeston Factory Specs Laboratory, always calibrate the lens blanks (the plastic your prescription is put on to) so that it is only just big enough to cut into your glasses frame considering the lens centres (pd) and lens heights (for varifocals). This is a bespoke process we always use rather than simply using off the shelf pre-made lenses with bigger blanks than necessary.
The smaller the blank lens used the thinner the lenses turn out
and because this process doesn't usually cost much more to use, we generally don't charge extra for small blanks. In a nutshell, if your prescription is +5.00/+6.00 Diopters territory, you may be able to get away without the high cost of ultra thin lenses. It would be possible to get a reasonably thin lens result with standard cheaper cost 1.5 index standard lenses, as long as you have or choose a frame that is small (not to wide across). 46mm to 48mm eyesore glasses frames are ideal for this prescription and if you're on a budget, you might like to keep your costs down using a 1.5 index lens for a lower overall cost.
So, a 1.5 index lens can still give good results. Needless to say, a higher index lens and a small blank and narrow frames can give even thinner results
Remember, size is the key!