The Staircase in Your Glasses

Put on a new pair of progressive lenses and look straight ahead: the world is sharp. Now glance down and off to one side. Things go soft, and the floor may seem to swim a little when you turn your head.

Most people's first thought is that something is wrong with the lenses. Nothing is. Those soft areas are built into every progressive lens ever made, at every price, and the easiest way to see why is to think about stairs.

Your lens is a staircase

Somewhere in our forties, the eye loses the ability to focus up close by itself. Glasses take over by adding extra power for reading. A progressive lens adds that power gradually: your distance prescription at the top, a little more partway down for a screen at arm's length, and the full reading power at the bottom.

Think of those distances as floors in a building, with far away at the top and reading at the bottom. Moving your eye down the lens is walking down the stairs between them. Every step adds a little power and brings your focus a little closer.

One big step, two steps, many steps

There are several ways to get between floors. The bifocal, which dates back to the 1700s, uses one big step: distance above a line, reading below it. It works, but you can see the line, things jump as your eye crosses it, and nothing is clear at arm's length.

A trifocal adds a landing halfway, for the computer or the dashboard. The jumps are smaller, but now there are two lines. Keep adding steps and they get smaller and smaller, until there are so many that the edges disappear and the climb becomes a smooth slope. That is a progressive lens: no lines, no jumps, and a spot for every distance from the horizon to your phone.

The catch: the steps only line up in the middle

Look closely at the six-step lens in Figure 2. The edge of each step is flush in the middle of the lens and gets taller toward the sides.

That's because each step of a lens is a thin slice of a ball. More power means a more steeply curved surface, so each step down is cut from a slightly smaller ball. Down the middle of the lens the slices line up perfectly. But a smaller ball curves away faster, so toward the edges each step falls a little further behind the one above it.

Smoothing the steps into a progressive hides the edges, but it can't make the mismatch go away. The mismatch gets spread out, and the surface on either side of the middle has to twist, like a towel being gently wrung. A twisted surface focuses more strongly in one direction than another, and your eye sees that as blur: the soft zones at the sides of every progressive.

A staircase that has to turn

Stair builders run into the same problem whenever a staircase turns a corner.

On a straight staircase, every step is the same depth all the way across. On a turning staircase, the steps become wedges. Along the walking line, the path most people take, every step can still be just the right depth. Toward the inside of the turn the steps get shallower, toward the outside they get deeper, and the farther you are from the walking line, the worse it gets. Too shallow and your foot doesn't fit; too deep and your stride is off.

That is a progressive lens in a nutshell. The narrow corridor down the middle is the walking line, where the power changes exactly as it should. Step off it to either side and the steps are the wrong size for your eye.

A tighter turn makes it worse, and so does a lens that has to climb faster: a stronger reading prescription, or a climb squeezed into a shallow frame. Both narrow the clear path.

How designers deal with it

If the problem can't be removed, it can be shared out. Stair builders call this balancing the winders, or dancing steps. Each step pivots around its spot on the walking line, so the straight steps before and after the turn each take a small share of the shortfall. Instead of four dangerous steps you get ten slightly shallow ones. The walking line stays perfect, and the worst step at the inside edge goes from 5 cm deep to about 16.

Lens designers do the same thing with a computer. The power down the corridor stays exactly as prescribed, but every other point on the surface may be raised or lowered by a fraction of a millimetre, until the leftover blur is as mild as it can be and sits where it bothers you least: low and out to the sides, away from where you look for distance and for reading.

Designers still have choices. Some keep large clear areas and pack the blur into a few strong pockets at the sides, often called hard designs. Others accept slightly smaller clear areas so the blur is milder and changes more gently, called soft designs, which many people find easier to get used to. Newer lenses are calculated for your own prescription and frame, and for how the frame sits on your face. None of them removes the blur. The good ones put it where you will notice it least.

Living with your progressives

• Point your nose at what you want to see. The clear path runs down the middle of the lens, so turn your head rather than just your eyes, especially when reading.
• Give it a week or two. Your brain learns where the walking line is, and most people stop noticing the soft sides.
• Fit matters. The walking line has to sit right in front of your pupil. That is why your optician measures where your eyes sit in the frame you chose.
• Come back if it still feels wrong. Blur or swimming that lasts past a couple of weeks usually has a cause that can be fixed: an adjustment, a measurement, or a different design.