Optical quality is the most important point to consider when buying spectacles. Glass has the best optical quality, followed by plastic lens and polycarbonate lens. However, glass is not commonly sold by retailers since it breaks easily and is heavier if the power is higher e.g. more than +2 dioptres.
When CR39 was the only type of plastic lens available, polycarbonate had a couple of advantages. But with the advent of plastic lens like MR8, MR174 etc., polycarbonate is not required for low, mid or high power.
In a nutshell,
- Plastic lens has better optical quality.
- Plastic lens is strong and can be cracked or broken only with great force.
- CR39 plastic lens is good enough for most people.
- If the lens must be stronger than CR39, consider MR8 - it is as strong as polycarbonate.
- Polycarbonate lens is sold at a premium, which is why stores recommend it over CR39 for low power.
Buying spectacle lens
The following indication is general and simple, there are a few variables to consider. Armed with the details here and some research based on your prescription, you would be able to handle well the conversation with your optician.
- Lower power/index:
- Get CR39 plastic lens (or any alternative entry-level plastic lens)
- Get Ultraviolet and anti-reflective coating applied.
- Make the lens photochromic, if required.
- Higher power/index lens, high impact sports:
- Get stronger plastic lens like MR8.
- Frame:
- For half-rim frame, CR39 is fine (recommended)
- For rimless frame, get MR8 plastic lens since CR39 lens will crack when screws are drilled.
- Bifocals:
- The type with Line in the middle where bottom half is for reading. Especially useful if the glass is only for reading. May avoid tilting up head when reading. Check if this type of lens obstructs long distance (non-reading) vision.
- If the above type obstructs your long distance vision, go for D-shape or circular portion (Kryptok) at the bottom of lens.
- Special cases:
- Please consult an expert in optics. The MR series has other types of lens.
