In 1490, Leonardo da Vinci put an ear to a tube and stuck the tube into the water to listen for faraway ships. Today, the science of underwater acoustics has developed into a fine art called sonar.

Active sonar - equipment that sends out sound waves and receives them back after they bounce off objects - was refined during World War II.

Since World War II anglers have seen dramatic leaps in the technology. Japanese marine electronics manufacturer Furuno marketed the first practical fish finder in 1948.

Today, fish finders come in a wide array of configurations and prices. Understanding these tools that are used by and affordable to anglers and boaters worldwide can help when selecting one.

With all of the technology packed into them, today's fish finders can hold more features than an angler may ever need or use. An experienced user can help a potential buyer sort out the options.

One of those users is Phil Cable from Holly Springs, N.C., who has been guiding on Triangle reservoirs since 1992 and has extensive experience in finding deep-water bass.

"Give me one that will show the bottom good and give me good clarity and show my baitfish and give arches for my fish, that's what I'm really after. I want something simple," Cable said. "Some of the technology is just above the average fisherman.

"I can't tell you what every spot is on that depthfinder or every little mark, but I do know when I see baitfish, I do know when I see arches (which represent fish on the screen).

"You just have to get used to it. You have to put it on the water and catch the fish after you see them (on the screen) to really start knowing what you are actually looking at."

Most units today are very sophisticated, packing a lot of technology behind the screen that enables anglers and boaters to get a lot of information on the screen.

Virtually all fish finders start up in an automatic mode that will cover most freshwater and inshore saltwater fishing situations.

Their complex processors sample, verify and filter signals, removing unwanted clutter from the screen. This "noise" can be caused by electronic interference or by suspended debris and air bubbles in the water.

The basic components of a fish finder are the display unit and the transducer.

The display unit houses the transmitter, the processor, controls and an LCD display. The transducer sends and receives sound waves for the processor to display on an LCD screen.

The transducer emits sound waves that travel through the water in a cone shape from the bottom of the boat, like the beam from a flashlight. When the sound wave hits an object or the bottom, it bounces back to the transducer.

Because the speed of sound is near constant in water, the processor can measure the time it takes for the waves to reflect back from the object and calculate the distance from the boat.

The processor generates a vertical slice of the entire water column one pixel wide on the right-hand side of the screen. This process is continuous, with each new vertical slice displayed on the screen alongside the previous slice creating a scrolling action across the screen.

Display units and transducers use resolution, frequencies, power and cone angles working together for top performance. Knowing specifications helps evaluate different systems.

DISPLAYS

Vertical resolution is the single most important specification for any fish finder. The higher the resolution in pixels, the more detailed the information displayed on the screen.