Hehe...I never realized there was a specialized tool for that measurement - My opthamologist just uses the sharpie and ruler as you mentioned! Since every pair of frames she has in stock have a set of placeholder lenses in them, she can just send out the measurement as well as the glasses with marked lenses so the lab can make their own measurements as well
Yep, it's this thing.
Also, a little primer on glasses:
As far as glass frames go, you should be fine with any material unless you have a specific allergy. Beyond that, all you need to do is check how they fit, their overall solidity and their looks of course.
Concerning glass material:
- Mineral glass or organic glass. Mineral glass is nearly scratchproof, that's the only advantage they have. Organic glasses are unbreakable (nearly), thinner and accommodate for a larger variety of surface treatment etc.
- Brands: good brands are Essilor or Nikon in my country, there are others that I don't know about. Anyway, you should use a brand that provides guarantees about the type of material it uses.
- The more complex your eye correction is, the more expensive the glasses will be. A typical very expensive nightmare would be to progressive glasses designed to accommodate strong presbyopia and strong myopia with strong astigmatism. Simple myopia or presbyopia glasses should be a lot cheaper. The reason for that is that the more complex the shape of the glass is, the harder (and it must be tailored!) it will be to manufacture. Besides with several corrections to implement in the glass' shape, you can easily imagine it would be difficult to mass produce since no one would have the same correction.
- The refractive index, it basically determines how much the glass will "curve the light rays", the higher the refractive index, the thinner the glasses can be. I would wear very thick glasses but the problem was alleviated by using a high refractive index material. Another way to alleviate the thick glasses issue is having glasses with a smaller diameter.
- Surface treatments: they can make glasses a lot more expensive.
- Scratchproof treatment for organic glasses, normally it's present even on the basic offers. Not 100% effective but absolutely essential, otherwise your glasses won't last.
- Hydrophobic + anti fog. Looses 50%of its efficiency over a few months but even after, it's better than no treatment. Third party temporary renewal product exist for specific applications (sports in wet environments for example)
- Anti Glare: several versions exist, the basic version reflects in moderate amount blue and violet light, other colors are absorbed, it provides a very nice improvement and a lot more comfort over non treated glasses. Advanced versions reflect a low amount of green light, more comfort, but I consider it non essential.
- Polarization: Useful if you suffer a lot from sun reflections or do a lot of outdoor activity where vision is extra crucial, useless otherwise.
- Sun sensitive glasses with color transitions: If you want your glasses and sunglasses so be combined in one pair,. Awfully expensive but convenient.
That's about it.
- Last one, online or in a real shop? A lot of people have had good success online but a real shop with experienced staff will ensure that your glasses are adapted to your vision, they will usually change/replace the glasses if they don't fit; with glasses with multiple corrections this happens more often than believed, the staff can diagnose what the exact issues with the glasses are. Basically they offer a guarantee of results. You would still need to clarify that point with them, there has been a lot of crappy shops popping up these years.
