Sunday, January 8, 2017

Programmer's new glasses or how to buy new glasses

    Hi again in 2017! It will be the best year so far! I decided to write about some topics which are not strictly related to programming but they live within the same ecosystem. I've been recently looking for a new pair of glasses and I encountered... yeah, some interesting facts! One may ask what can be interesting in ordinary glasses. Well, many things.
I would like to highlight that I am not an optician nor an optometrist so one may notice some simplifications in my analysis. But hey! You do not need to be an expert to buy a pair of glasses! I am just a customer who wanted to buy a new pair of glasses as I noticed that the clarity of my vision had become worse. So I had an oculist appointment. The oculist performed computer vision test and then he used a phoropter to test just one configuration delivered by the computer. I was able to read the row at the very bottom of the card so it seemed to be fine. I received the new glasses prescription and I was ready to go to the optician to buy my glasses with the parameters established by the oculist. I would classify the parameters of the glasses into two categories: 

1. Fixed (rigid) parameters - the ones determined by a qualified person (oculist or optometrist) during eye examination. These are the most commonly used:
  • spherical diopters,
  • cylindrical diopters and axis of a cylinder (optional - used when one has astigmatism),
  • pupillary distance - the lenses which are delivered to the optician are much larger than the eyeglass frames and they must be prepared to fit into the frames. The raw lenses do not have the same physical properties on the whole surface. They are close to perfection in the certain area and they must be cut to fit your eyeglass frames and your pupils must be right behind that close to perfection surface. The farther from that sweet point the more the physical properties may be disturbed.
Fixed parameters are extremely important because they describe the physical properties of the lenses that are going to compensate your vision defect. It is worth mentioning that sometimes the oculist appointment may not be enough. In my experience the oculists are more focused on looking for eye diseases rather than determining the fixed parameters of your glasses really precisely - of course it is not a stereotype as your oculist may be meticulous. In my opinion the optometrist is more dedicated to choose the best combination of fixed parameters for you. The computer vision test is just a starting point for an optometrist whereas it may be treated like the final result by the oculist (again, your experience may be different, it all depends on the specialists). I realized that after buying my glasses. After some time I wanted to select the contact lenses and I went to the optometrist who also fine tuned the fixed parameters of my glasses. I did not believe that I could see better. The optometrist proved that a slight change of spherical diopters and axis of a cylinder made considerable difference in the clarity of my vision. He was using a phoropter all the time and he tested tens of different configurations.
2. Free choice parameters:
  • eyeglass frames - you may be limited by the thickness of the lenses. I noticed that new glasses  mean new eyeglass frames for many people and people seek for the most fancy eyeglass frames whereas the quality of the lenses is neglected by them, 
  • lens index (light refraction ability) - for an ordinary customer it determines the thickness of the glasses. Nowadays, people are free to choose high-index lenses. They are more elegant and lighter. However, it is necessary to find the balance because nothing comes for free. I noticed that the opticians try to propose high-index lenses because they are lighter, thinner but more expensive. I bought 1.67 index lenses according to the optician's proposal. They were really light and thin. However, after some time I realized that the vision quality is not as good as with my old lenses (1.50 index). The most disturbing optical defect was chromatic aberration. It was unacceptable to me as I saw some bluish glow around the letters in the corner of my screen. I started digging into the knowledge and I realized that 1.67 index lenses have a pretty low Abbe number equal to the value of 33. The lower the value the more probability that you will notice chromatic aberration and you will not be able to habituate. I ended up with Trivex lenses (1.53 index) - Abbe number equal to 43. They are almost chromatic aberration free and they are light and thin enough,
  • lens type - spherical or aspherical. Aspherical lenses tend to be thinner than the spherical ones. They also seem to have less optical deformations in the corners. However, aspherical lenses are not tolerated by some people. Spherical lenses are more safe to choose,
  • lens coatings - anti-reflective, anti-fog, scratch resistant. I suggest to choose the best coatings you can afford with one exception. There is a coating called blue control (the name may differ between the producers) that was designed to be used with LCD screens. However, the glasses with such a coating are not intended to be worn on a daily basis 24/7. They may be used at work only.
General tips:
  1. Make sure that the fixed parameters of the glasses are determined as precisely as possible. Do not rely on a computer vision test only. That should provide a point of reference only. The heavy lifting should be done by a qualified specialist and a phoropter.
  2. Treat the lenses at least as important as the eyeglass frames.
  3. Favor low-index lenses over high-index ones. Choose high-index lenses carefully. Check if you really need them (significant vision defect or perfect look at work). There is a pretty good online calculator that allows to compute the thickness of the lenses - click.
  4. Treat aspherical lenses carefully. Strive for spherical ones.
  5. Choose the best coatings you can afford (anti-reflective, anti-fog, scratch resistant). Treat a blue control layer carefully.
Thanks for reading!

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