What are bifocals, and do you need them?

The line, and the jump at it

The two zones are ground to different curves, so where they meet there is a step. Look down through it at a straight edge — a kerb, a stair nose — and the edge appears to shift sideways as your gaze crosses the line. Opticians call it image jump, and it is the reason the first week on bifocals involves looking at your feet more than usual.

Segment shapes have names: the flat-top D-seg is the common one, the round seg and the full-width executive are the others. The wider the segment, the more reading area and the more obvious the line.

The no-line progressive

A progressive removes the step by blending from the distance power at the top to the add at the bottom through a corridor down the middle of the lens. No line, no jump, and no visible sign from the outside that the lens holds two powers.

The blending has to go somewhere, and it goes into the outer lower corners of the lens, which stay soft however carefully the frame is fitted. That is the trade: bifocals give a hard edge and full sharpness on both sides of it, progressives give a smooth transition and two soft corners.

What the reading add actually is

The ADD on the paper is plus power added to the distance number for close work — a +2.00 add on a −3.00 distance number makes the reading zone −1.00. It is written once and applies to both eyes almost equally, and it is the only number on the paper that has to be combined with another number to mean anything.

An add appears when the eye's own focusing range no longer covers the distance you hold things at. That is why it shows up on papers from the mid-forties onward, and why it tends to be written larger at each subsequent visit.

Who ends up with them

Two groups: people who already wear glasses for distance and start needing help at reading distance, and people who dislike having two pairs on the desk. If the distance number is close to zero, most counters send you out with reading glasses instead, because there is no distance correction worth carrying at the top of the lens.

Bifocals are also what people are given when a shop expects a progressive not to be tolerated, which is a judgement made at the counter rather than something written on the paper.

What it costs, relative

Of the three multi-distance options, a bifocal is the cheapest, a progressive is the most expensive, and two single-vision pairs together still land below the progressive. Shops sell fewer bifocals each year, and the margin on a progressive plus its coatings is the reason the progressive is the default recommendation.

The two-pair alternative

Two single-vision pairs, one for the screen distance and one for far, is the cheapest of the three and the only one that lets each pair be exactly right for its distance. Every millimetre of both lenses holds one power, so there is no corridor to find, no corner to avoid and no line to cross.

The cost is that they live in two places and you swap them. That is the whole objection, and it is the reason the option is rarely offered at a counter that would rather write one order than two.