An optician's website that talks customers out of the most expensive lenses
High-index lenses carry the highest margin in an optical shop, which is why they are offered to everyone, and the customer has no way of checking whether they would change anything. Checking is easy, though: lens edge thickness follows from the sagitta formula and depends on prescription, index and frame size. The calculator on this page works it out for the visitor's prescription and draws four lens cross-sections to scale side by side — at minus two dioptres they look identical, because they are: the difference between the cheapest and the dearest pair is four tenths of a millimetre. The argument is shown rather than told, and ends with the sentence "the upgrade will change nothing" together with the amount the customer keeps. Only above roughly four dioptres do the drawings begin to differ, and the page says so just as plainly. The second edge of the same mechanism is the more interesting one: the verdict is computed from the difference in millimetres, not from the prescription alone, so the same minus six dioptres in a frame six millimetres narrower drops back from "worth paying for" to "a matter of taste". A smaller frame thins the lens by as much as jumping across all four indices, and costs nothing — the page states this under the drawing at every slider position. The contact form recomputes the verdict server-side and sends it to the shop with a tag in the subject line, so the promise made on the website does not depend on the memory of whoever is at the counter. In the photographic header sits a card with the next free appointment at the consulting room and the date it was last checked — the one piece of information on this page that cannot be computed from anything.
