Why are these lenses thicker than my old pair?September 1, 2015
Sometimes we hear this from accounts wondering the same thing. Let’s look at the possibilities.
-Did the lab make them thicker than they needed to be? Possible, but remember, labs always work with the thinnest part of any lens. In minus, they work with the thickness at the center (OC) of the lens. In Plus, they work with the thickness at the edge farthest away from the OC. They also look at how the lens has to be mounted. There are mainly 5 factors by themselves or in combination with each other that will determine thickness.
- Prescription power.
- Material’s index of refraction.
- Size of frame.
- Shape of lens/lens edge.
- Prescription Power– This is the area that most of us look at first. The thinking is that if the prescription did not change that much, then the lenses should be about the same thickness as the old pair. Probably, as long as factors 2, 3, 4, 5 have not changed. If they have then they could receive thicker lenses. Also, if the Rx went from SV to a progressive, they may be thicker. This is due to the OC placement in the frame. In SV, the OC’s height is ½ “B” measurement of the boxing system. The OC (also the PRP) in a progressive is 0, 2, 4, 6mms (depending on the manufacturer’s progressive style) under the fitting cross which is usually placed higher in the frame. Minus would tend to be thicker at the bottom and Plus lenses would tend to be thicker at the top. Hence, prism thinning.
- Index of refraction– The higher the index – the thinner the lens. This is correct most of the time as long as factors 1, 3, 4, and 5 do not change.
- Size of frame– If the size of the frame increases – the thickness will increase. If the size of the frame decreases – the lens thickness will decrease.
- Shape of lens/lens edge– The more round the shape – the thinner the lens. The more rectangle/aviator shapes- the thicker the lens will be.
Plastic eyewire type frames generally lends itself to a thinner Plus lenses (the lens edge is cushioned/protected by the plastic).
For metal eyewires, Plus lenses are a little thicker to prevent chipping/flaking on the outer edge. The metal does not provide as much of a cushion as plastic.
Grooved edge- Plus lenses will be thicker in order to have at least a 2mm minimum edge thickness at its thinnest edge. This allows enough stock on each side of the groove to make it less likely to chip or flake on the exposed edge. Low powers in Minus lenses may have to be made thicker for the same reason.
Drilled/Notched Rimless lens- Plus lenses have to be made thicker in order to have enough stock to support the mountings. Low Minus lenses may have to be made thicker for the same reason.
- Decentration– In general, the more the decentration – the thicker the lens. Plus lenses will be thicker nasally and Minus lenses will be thicker temporally.
In general, Minus lenses are around 1.5 to 2.0mm center thickness (depends on prescription) and the Plus lens edge (farthest edge from OC) is 1.8 to 2mm. So if the lens is “too thick” check the thinnest parts of the lenses. But keep in mind; it may have to be thicker because of the other factors coming into play.
Food for thought:
0.1mm = 0.0039”. A human hair is around 0.0030”.
So 0.2mm thinner is the same as 2 ⅓ hairs thinner.