How binoculars bring birds in focus

Years ago, before I bought an actual good pair of binoculars, I had a very cheap pair. I eventually disassembled them, a science project, so to speak.

These were my family's around-the-house binoculars, rarely used, not so good. The barrels were out of alignment. The two images that were delivered required your eyes or brain to work very hard at creating a single, usable image.

It took my eyes actual seconds to focus on reality when I put those "bins" down.

In the 1980s I signed on with Duluth birding guide Kim Eckert for a trip to Texas. That merited decent glass. I bought Bausch & Lomb 8x42 binoculars. I don't remember the price. They were high-end.

Geez, were they good. Sharp image, lightened shadows, no eyestrain. They're now the bins I keep near windows overlooking our backyard.

With new binoculars in hand, I could take that old pair apart. How do these things work? I was introduced to the world of prisms.

Those original bins were of Porro prism design, named for their Italian inventor, Ignazio Porro. In the mid-1800s he solved a big — literally — binocular problem.

For an image to be sharply magnified the light must travel a specific distance between entry and exit lenses. If the optics were to be usable, say, aboard a ship, this produced bins with very long barrels.

Porro prisms were the solution to achieving binoculars of smaller size. Two prisms are used in each barrel. They create a shorter length by folding the light, one prism to the other, before sending it to your eyes.

Porro-prism binoculars can be recognized by the jog in the assembly of the lens barrels.

Roof prisms also create the proper distance, but by using a single prism of a different shape.

The Bausch & Lomb bins bought for the Texas trip have roof prisms. The barrels are 6¼ inches long. The pair weighs 2 pounds.

My newer pair, Swarovski 10x32s, also are roof prisms, but an inch shorter and half a pound lighter. Progress. And a higher price.

What does it mean, 8x42, 10x32? The first number refers to magnification, the second to light passage. The larger the number, the more light captured. Those bins will almost add light in shaded areas.

Binoculars with magnification of 8 are popular because, experts tell us, an image of lesser magnification is easier to hold steady. Steady, obviously, is good for detail. (I chose magnification with my newest pair.)

Roof-prism bins, generally speaking, are more expensive. Binoculars today, however, come in an extensive array of constructions, magnifications, light-capture qualities, weight differences, and protection from water and fogging. Oh, and price.

If you're in the market, be certain to look through several pairs before you buy. You'd look at the picture on more than one television set before the purchase.

The author of a book I recently read, a Brit named Simon Barnes ("The Meaning of Birds"), was about to take a birding trip. He figured the investment in the trip dictated better binoculars. (Right.)

He tried this and that, then a very expensive pair by Swarovski. He wrote that they "knocked his socks off."

He bought them. They were the best he could almost afford, he said.

Binoculars are likely to last the rest of your life. Knock your socks off.

Read Jim Williams' birding blog at startribune.com/wingnut.