Have you ever experienced the following? You had a thorough eye exam by your Optometrist or Ophthalmologist and purchased a new pair of eyeglasses with all the bells and whistles, with expectations of seeing like an eagle. You put on the new eyeglasses and did not focus very well and worse yet, you felt a little dizzy. Your immediate thoughts were, the lab made the glasses wrong, I messed up during the exam, or the doctor is a quack. A fourth possibility is the ABBE VALUE of your lenses.
Different lenses have different properties. The density varies, known as index of refraction, and the ABBE VALUES are different for the various materials. The ABBE VALUE is a number given to describe the amount of chromatic aberration of an ophthalmic lens material. I've read messages from the list stating that their glass lenses seem to provide the best focus. Here is a list of ABBE VALUES. The higher the number, the less amount of aberration.
|MATERIAL||INDEX||ABBE VALUE||Crown Glass||High Index Glass||High Index Glass||Plastic CR-39||Mid Index Plastic||Mid Index Plastic||High Index Plastic||High Index Plastic||Trivex||Polycarbonate|
Chromatic aberration is when white light is broken up into component colors. A color fringe may be noted around a light source or around an object. In reality, my patients never complain of color fringes but rather complain of blurriness and peripheral distortion.
Note that crown glass has the highest ABBE number (least aberration) and polycarbonate has the lowest ABBE number (most aberration). The higher index of refraction means a denser material and therefore a thinner lens. Cosmetically, thin lenses appear more attractive but has more aberration. They also cost more. High index glass has more aberration than standard crown glass.
Different manufacturers may have a different ABBE VALUE for the same exact lens. You might have experienced a previous pair of eyeglasses provided better vision when you thought there were no changes in prescription nor type of lens material with your new pair.
Polycarbonate lenses are used extensively for safety eyewear such as in your shooting glasses and has the most aberration. Keep in mind that most of us may see just as well with a low ABBE VALUE lens as compared to a high ABBE VALUE lens.
Crown glass is most commonly used for eyeglass lenses with an index of refraction of 1.523. There are however, 50 glass formulations used today for various optical applications, all with its unique properties. Perhaps this can explain the different qualities of our scopes as we dicussed previously.
Ernst Abbe was a brilliant German mathematician and physicist. Carl Zeiss was so impressed with Ernst Abbe that he was hired by Zeiss in 1866 and later became the owner of the company.
You might want to print out this list and refer to it when you purchase your next pair of eyeglasses. An informed shooter is a better shooter. If any of you have unresolved issues with your present eyeglasses, as usual, I'll be happy to answer any questions the best I can.
Norman H. Wong, O.D.