Guides

Choosing lenses

The four decisions an optician asks you to make at the counter, with the arithmetic behind each one rather than the sales line.

• 1.67 vs 1.74 lenses

  Is the thinnest lens worth the money, or is 1.67 close enough?

  The step most people are asked to pay the most for, and the one that buys the least. Drawn to scale at every prescription strength.

• Polycarbonate vs Trivex

  Which impact-resistant lens material should I choose?

  The one comparison where the usual advice is wrong often enough to matter: nearly the same thickness, noticeably different optics.

• Bifocal vs progressive

  Should I get bifocals or varifocals?

  A visible line and no distortion, against an invisible transition and a corridor you have to learn. What each actually costs you.

• Short vs standard corridors

  Will a progressive lens fit in the frame I like?

  Why a frame you like may not take a varifocal at all, and what a short corridor gives up to fit one that it would not otherwise.

Reading what you were given

A prescription and an eye-test result are both written in notation nobody explains. These are the translations.

• SPH, CYL and axis

  What do the numbers on my glasses prescription mean?

  Every box on the card, what it does to the lens, and which of them actually change what you can see.

• logMAR, Snellen and decimal

  How do I convert between 20/20, 6/6, 1.0 and logMAR 0.0?

  Four notations for one measurement, the conversion between them, and why research uses the one nobody has heard of.

Faces and measurement

What the numbers behind the face tools mean, and what an ordinary face actually measures.

• The average face, measured

  How long and how wide is an average face?

  What adult faces actually measure in centimetres, where the figures come from, and why nearly every number online is one of three different measurements.

Or go straight to a tool

Every guide here exists because a tool raised the question. If you already know what you need measuring, the full list of 18 tools is the shorter route — all of them run in your browser, and none of them upload anything.