Everyday Health Tips

Your daily dose of wellness

Tuesday, a little after six, and the light was already going. You carried the big pumpkin out for Saturday and turned back down the porch steps for the second one. The bottom step was not there. Gray concrete on a gray path. Your foot found it a beat late: a jolt up through your knee, a hand on the cold railing. The progressives you picked up in spring read a menu fine. So how does a whole step go missing?

What's happening

That step was testing something your eye chart never checks.

Seeing has two separate jobs. One is sharpness: how small a letter you can read. The other is picking out a faint difference, like a gray edge against a slightly darker gray. That's contrast sensitivity, and it runs on its own track. You can score 20/20 and still lose it. Read that again.

The eye chart is black ink on bright white in a well-lit room. A step at dusk is the opposite. And with every decade, the lens inside your eye scatters more of the light passing through. Think of a thin fog drifting over the porch. Fog barely touches big black letters. It washes out edges.

Then there are the glasses themselves. Progressives and bifocals keep the reading zone in the lower half of the lens. That is exactly where you look when you step down. Through that part, the edge of a stair sits a little out of focus. Sound familiar?

An Australian team led by Stephen Lord tested more than 150 older adults. Those who wore multifocal glasses did worse at spotting edges and judging depth. Over the next year, they were more than twice as likely to fall, with stairs and outings among the trouble spots. They were older than you, but bifocals and progressives work the same way at any age.

There is one more layer. Researchers have found that the front of the eye explains only part of the contrast lost with age. The rest happens at the back, in the retina. That's the thin layer of cells that turns light into signals for your brain. Those cells run on a steady blood supply, and no bulb or new pair of glasses can reach them.

So which part matters most as the years go by? If you have not tapped your guess in the box above yet, see the retired doctor’s answer here. Then come back for the numbers.

The pulse: contrast and the vanishing step

Sources: Schefrin et al., Vision Research (1999); van den Berg et al., American Journal of Ophthalmology (2007); Owsley et al., Archives of Ophthalmology (2001); Haran et al., BMJ (2010); Lord et al., Journal of the American Geriatrics Society (2002)

What matters most

• Sharpness and contrast are separate skills. A good eye-chart score says little about a gray step at dusk.
• With age, the lens scatters more light and lets less of it through. Both wash out faint edges, and dim light makes the effect stronger.
• The reading zone of progressives and bifocals sits in the lower half of the lens, right where you look on stairs. Multifocal wearers in one study fell twice as often.
• Haze or glare that keeps worsening deserves an eye exam, since a clouding lens often dulls contrast before your eye-chart score changes.

Why this matters

This Sunday the clocks fall back. For months, the walk to the car, the school run and the trip for milk will happen in the gray hour. Curbs, stairs and dark coats on dark streets are all contrast tests. Knowing that changes what you fix: the light and the edge.

For more than 160 years, good eyesight has been defined by one chart. Herman Snellen designed it in 1862, and 20/20 became our shorthand for perfect sight. Yet the chart measures the easiest seeing there is: black on white, a bright room, standing still. Life after 45 is full of gray steps at dusk, and the chart never asked about them.

So if that jolt on the porch left you wondering whether you are getting unsteady, set the worry down. Your balance did its job and caught you. Your eyes met a gray-on-gray edge through the reading half of your glasses. Nobody told you that was a different test.

P.S. Still wondering which of the four suspects the retired specialist picks? Tap here for his answer. It takes less time than lighting the pumpkin.

Where does gray-on-gray catch you most: the stairs, the parking lot, or the drive home? Hit reply.