Aug 25 - 16 | So you are thinking about getting new glasses... but you are worried that the lenses will end up looking thick and heavy. What are the factors that influence optical lens thickness in glasses? Read on...
[#1] Prescription: The higher your degree of nearsightedness or farsightedness will influence your lens thickness. A lens to correct nearsightedness (myopia) will be thicker at the perimeter of the lens and a lens to correct farsightedness (hyperopia) will be thicker in the middle of the lens. There is not much that you can do about your prescription - it is what it is - so use the other 4 factors to help minimize lens thickness.
[#2] Diameter: The larger a lens is across, the thicker it will be. This consideration is often trumped by fashion or function (e.g. progressive bifocal) but try and minimize diameter to limit lens thickness.
[#3] Material: There are basically six difference plastics that are used to make lenses (glass rarely used these days). These plastics are rated for their ability to bend light and there ability to sharply focus light. They are identified with numbers (index) and/or names such as 1.6 or 1.53 or Polycarbonate. Some retailers try an brand a particular plastic with a fancy name but they are all essentially the same six. If you are offered a "Thinner-Lighter Lens" - find out what the index actually is to properly compare...in order or thickest to thinnest: 1.5 (CR39) > 1.53 (Trivex) > 1.59 (Polycarbonate) > 1.6 > 1.67 > 1.74. The worst optical quality (sharpness) plastic is polycarbonate but it is also one of the most break resistant. Optical quality also is influenced by the design of the lens (e.g. aspheric) but this would not significantly impact the thickness. As a rule-of-thumb, you do not need to consider a mid or hi-index lens for prescriptions from -2.50 to +1.50. Note that a 1.74 lens (thinnest) can reduce lens thickness by approximately 33% over a 1.5 (CR39) lens.
[#4] Mount: The three most common frame mounts are: grooved, semi-rimless and drilled. A grooved mount has the metal or plastic frame completely around then lens. The lens has a raised lip at the edge that sits snugly into a groove in the frame. This mount will allow your lenses to be thinnest. If you are nearsighted, a semi-rimless or drilled mount will have little impact on the thickness when compared to a groove mount. However, If you are farsighted these latter to mounts will cause your lenses to be thicker. This is because the edges of your lenses have to me made thicker to carry a groove set into the lens edge. Or in the case of drill-mount, the lens have to be make strong enough as not to break with stress.
[#5] Surfaced or stock: This factor is probably the most confusing and misunderstood - even for people in the industry. If you are farsighted (i.e. a '+' prescription like +3.00), a surfaced (custom made) lens will deliver the thinnest end product (any material). The selection of which to use gets messy because is a balance between wholesale cost and value to consumer plus all of the above factors. Even when it is the best choice, a surfaced lens is not always selected by many optical retailers because it is far more costly to fabricate which negatively effects profit. If you are nearsighted (i.e. a '-' prescription like -5.50), it does not matter if the lens is surfaced or stock. The decision will be made on availability and cost as the thickness is the same.
Questions? Come in and see us at StudioEyeCare . We will help you navigate! Dr Jason Morris
On Facebook at drjasonmorris
[#1] Prescription: The higher your degree of nearsightedness or farsightedness will influence your lens thickness. A lens to correct nearsightedness (myopia) will be thicker at the perimeter of the lens and a lens to correct farsightedness (hyperopia) will be thicker in the middle of the lens. There is not much that you can do about your prescription - it is what it is - so use the other 4 factors to help minimize lens thickness.
[#2] Diameter: The larger a lens is across, the thicker it will be. This consideration is often trumped by fashion or function (e.g. progressive bifocal) but try and minimize diameter to limit lens thickness.
[#3] Material: There are basically six difference plastics that are used to make lenses (glass rarely used these days). These plastics are rated for their ability to bend light and there ability to sharply focus light. They are identified with numbers (index) and/or names such as 1.6 or 1.53 or Polycarbonate. Some retailers try an brand a particular plastic with a fancy name but they are all essentially the same six. If you are offered a "Thinner-Lighter Lens" - find out what the index actually is to properly compare...in order or thickest to thinnest: 1.5 (CR39) > 1.53 (Trivex) > 1.59 (Polycarbonate) > 1.6 > 1.67 > 1.74. The worst optical quality (sharpness) plastic is polycarbonate but it is also one of the most break resistant. Optical quality also is influenced by the design of the lens (e.g. aspheric) but this would not significantly impact the thickness. As a rule-of-thumb, you do not need to consider a mid or hi-index lens for prescriptions from -2.50 to +1.50. Note that a 1.74 lens (thinnest) can reduce lens thickness by approximately 33% over a 1.5 (CR39) lens.
[#4] Mount: The three most common frame mounts are: grooved, semi-rimless and drilled. A grooved mount has the metal or plastic frame completely around then lens. The lens has a raised lip at the edge that sits snugly into a groove in the frame. This mount will allow your lenses to be thinnest. If you are nearsighted, a semi-rimless or drilled mount will have little impact on the thickness when compared to a groove mount. However, If you are farsighted these latter to mounts will cause your lenses to be thicker. This is because the edges of your lenses have to me made thicker to carry a groove set into the lens edge. Or in the case of drill-mount, the lens have to be make strong enough as not to break with stress.
[#5] Surfaced or stock: This factor is probably the most confusing and misunderstood - even for people in the industry. If you are farsighted (i.e. a '+' prescription like +3.00), a surfaced (custom made) lens will deliver the thinnest end product (any material). The selection of which to use gets messy because is a balance between wholesale cost and value to consumer plus all of the above factors. Even when it is the best choice, a surfaced lens is not always selected by many optical retailers because it is far more costly to fabricate which negatively effects profit. If you are nearsighted (i.e. a '-' prescription like -5.50), it does not matter if the lens is surfaced or stock. The decision will be made on availability and cost as the thickness is the same.
Questions? Come in and see us at StudioEyeCare . We will help you navigate! Dr Jason Morris
On Facebook at drjasonmorris
