Posted 02/12/2017 12:40:58
How do you choose the right style and size of spectacle frame if you have relatively high powered lenses ?
This blog looks at the effect of size, shape and type of lens on the cosmetic effect of the finished Spectacles
How to choose the right spectacle frame when you have a high prescription.
I will write about prescription considerations first, as these very often over ride style in the sense that putting a very thick lens into a frame can ruin the appearance of a seemingly good choice.
Generally speaking, prescription needs to be taken into account for powers above minus 4.00 (myopia or short sight) and plus 4.00 (hypermetropia or long sight).
Minus powers are thin in the lens centre and thick at edge and the further the edge is from the centre , the thicker the edge becomes; also because of internal reflections there are a series of power rings visible towards the lens edge. In addition, as the power increases the thicker the lens becomes.
Lens thickness can be partially offset by using a higher refractive index material, giving a thinner lens for a given power ,this is again worth considering for powers over 4.00. Standard plastic lenses have a refractive index of 1.49 , this can be increased up to 1.74 in plastic and 1.9 in glass, although the price approximately doubles for each increment of index. Anti reflection coatings which can be a real nuisance to keep clean, reduce the power ring effect and can greatly improve cosmetic appearance.
These considerations are the same for plus powers, where the aim is to reduce centre thickness. A further problem exists for plus powers because of the magnification effect, particularly with high powers. It is important for the optician to choose the smallest blank size possible for a given frame in plus powers only, the aim is to have a knife edge at the rim to give minimum centre thickness.This means that rimless or supra style frames are best avoided in high powers as they require at least 3mm of lens edge thickness, there by increasing centre thickness by 3mm more than otherwise necessary.
The golden rule is that the higher the prescription you have the smaller the frame you should have and also the rounder the frame since with a completely round frame all point at the edge are equidistant from the centre, giving the minimum possible centre or edge thickness.
Varifocals or bifocals
Here you need sufficient depth, particularly with varifocals to fit the reading portion in, there should be at least 18 mm of depth below pupil centre and the deeper you go the more reading are you get.
I will consider style in my next blog.