Which lenses do I need?

Shops band it by the strongest sphere on the paper: up to 2.00 D standard 1.50, 2.00 to 4.00 D the 1.59 or 1.61 step, 4.00 to 6.00 D 1.67, past 6.00 D 1.74. Frame size shifts what you notice, because a smaller frame cuts away more of the thick edge.

Below about 3.00 D the difference is a millimetre or two of edge, which most frames hide. From 4.00 D on it changes how the glasses sit and weigh. The trade is colour fringing: 1.67 and 1.74 sit at Abbe 32 and 33 against CR-39's 58.

They win on different things. CR-39 is Abbe 58, the clearest common material, and heavier. Polycarbonate is Abbe 30, the most colour fringing, and impact-rated. For an everyday adult pair under about 3.00 D, CR-39. For sports, rimless mounts, or children, polycarbonate or Trivex.

A bifocal has two zones and a visible line: distance above, the reading add below. A progressive blends distance to near through a corridor, with no line and soft blur in the outer corners. Bifocals cost less and adjust in days; progressives take one to three weeks.

It depends on the distance number. With little or no distance correction, a separate reading pair covers the add and costs least. With a real distance number, people who dislike swapping pairs order bifocals or progressives; two single-vision pairs do the same job for less.

1.74 index plastic is the thinnest sold, with 1.67 close behind at lower cost. Both are glazed with anti-reflective coating, because the surface reflects more as the index rises. At high powers a smaller frame does more for edge thickness than one step of index.