Down-Imaging vs. Side-Imaging: Are You Getting the Most from Your Fishfinder?

Most mid-range and virtually all high-end fish finders are now equipped with side-scanning sonar, high-quality down-scanning tech, and standard 2D downward-facing sonar. 

As a result, it’s almost never a question of which one you want, but rather which one is better at this time and in this place.

And while plenty of anglers understand how to use and fish that 2D sonar, few really understand how the more technologically advanced side- and down-imaging options work or how to make the most of them.

Humminbird’s Saltwater Field Staff Manager, Bill Carson, says, “I see it every day… 90% of the anglers utilize 10% of their fishfinder. One of the most powerful capabilities is Down Imaging and this is by far the least used of the sonar technologies.”

Are you one of the 90% not getting all the performance you paid for? Or are you one of the savvy 10% that wring every dollar out of your fish finder?

If you’re not sure about the differences, advantages, and disadvantages of side-scanning versus down-scanning sonar, we’ll cover everything you need to know.

So keep reading!

What is Down-Scanning?

Down Imaging

Down-scanning sonar provides an exquisitely detailed view of what’s directly beneath your hull. 

To provide that detail, down-scanning sonar uses very high frequencies, typically in the 455kHz to 1.2MHz range, as they carry more information. Of course, the flip side of this coin is that high frequencies can’t penetrate the water column as well as lower frequencies can.

Charter captains and tournament pros use DI sonar more than you do.

That’s a simple fact.

And the reason is clear - literally.

Down-Imaging-2

Anglers who really understand their fish finders will run standard 2D sonar when moving, especially in deep water or over muddy or sandy bottoms where standard sonar can detect fish like they’re wearing a radar beacon. But when hunting fish over rock piles and other structure and cover, they’ll check the 2D return with the down-scanning sonar, as it provides better fish finding capability when fish are scattered among rocks or clustered very tightly together.

BJ Silvia, Flippin Out Charters’ Captain operating out of Newport, Rhode Island is one of the converted.

“I’ll admit, at first I resisted Down Imaging,” he confesses. “I felt I could do it all with 2D and Side Imaging. That all changed this spring. I happened to turn on DI in a rocky area and instantly began marking tog, something not done with normal 2D sonar.” 

“This summer we got on schools of striped bass which were glued to the bottom. I’m talking hundreds of fish packed together, 5-feet high. Other boats would get close to see what I was fishing, look at their screens and drive away not knowing what they saw. I know they went over the school and mistook it for bottom. If they knew what my Humminbird SOLIX showed, it’d make them sick!”

Keep in mind, the idea isn’t to run DI sonar constantly, at least not until you’re on the fish.

Instead, you use DI to check what the 2D sonar is telling you, especially when the bottom is cluttered and there’s plenty of structure and cover.

DI will absolutely pick out fish that 2D sonar misses.

Just ask Captain Dom Petrarca of Coastal Charters Sportfishing. 

“For years we’ve seen the giant clouds of bait on our fishfinders and assumed the tuna were in there,” he says. “With traditional sonar the bait shows up as a washed-out cloud, only showing gamefish when they’re separated from the pile. It was also difficult to distinguish the type of bait. Down Imaging changed all of this and is great for cutting through the bait, revealing giant tuna swimming in the middle. I can clearly identify the type of bait and know if the fish are on herring, mackerel or smaller sand eels so I can adjust my presentation accordingly.”

And that’s the reason to run DI while actively fishing: not only can you pick out the predators from the bait with ease, you can actively see what they’re eating, adjusting your bait choice to match.

That’s an advantage few anglers would knowingly pass up.

Advantages

  • Extremely detailed images
  • Spots fish that 2D sonar mistakes for the bottom
  • Identifies species of both bait and predator, allowing you to make informed choices

Disadvantages

  • Not useful while moving quickly
  • Limited range compared to 2D sonar
  • Like 2D sonar, only sees what’s directly below the hull

What is Side-Scanning?

Side Imaging

Side-scanning sonar uses multiple transducers placed to emit soundwaves directly to the side of your boat, typically with ranges up to 200 feet to each side.

And like down imaging, the detail SI sonar provides is created by very high frequencies, usually in the neighborhood of 455kHz to 1.2MHz.

side scanning under hull

The black stripe down the middle of the image is the area directly under your hull. To both sides, you get a view of the water column, picking up bottom details and fish with extreme detail.

Side scanning sonar

Side-scanning sonar is an awesome tool, and for most anglers, it’s absolutely game changing.

You can run side-scanning sonar while moving, spotting fish you’d miss on standard 2D mode. 

That makes it incredibly effective in open water or while trolling, and you can home in on schools you’d pass and never see in the old days. Just keep in mind that the faster you’re moving, the more lag you’ll generate, meaning that images you see will be seconds old, showing what’s behind you.

It’s also an awesome option in shallow water.

2D sonar’s low frequencies really punch through the water column, but as you’ve no doubt already experienced, in shallow water, the sonar cone is too small to be really useful, and interference with the bottom can cause the sonar to run amok.

As a shallow-water tool, SI sonar is unbeatable, and unlike DI, many anglers take the time to really learn the ins and outs of this tech because its advantages are immediately obvious.

But keep a few things in mind.

First, SI is completely blind to what’s beneath your hull. Once you find the spot you’ve been looking for, you’ll want to switch to 2D and DI to really get on the fish.

Second, it can’t penetrate the water column as well as 2D sonar, especially CHIRP.

And third, the images can be confusing or just plain hard to read on small screens. To get the most from side-imaging sonar, you really want the biggest screen you can afford.

Advantages

  • Extremely detailed images
  • Spots fish that 2D sonar would miss out to each side
  • Identifies species of both bait and predator, allowing you to make informed choices
  • Great for searching for fish while moving

Disadvantages

  • Limited depth when compared with 2D sonar
  • Blind to what’s directly beneath your hull
  • Images can be hard to read on smaller screens

How the Pros Use their Fish Finders

As you can see, there’s a time and a place for 2D, DI, and SI. Fish finders are definitely not a “set it and forget it” tool!

In deep water, charter captains and tournament pros will run 2D sonar looking for fish, typically in conjunction with GPS positions and predetermined hot spots like wrecks, humps, submerged islands, drop offs, weed beds, etc. - the stuff we all look for to find fish.

The more cluttered the bottom is, that is, the more variance it offers the sonar, the more likely it is that they’ll pause and switch to down imaging to look for fish that 2D sonar misses. That’s also true if they found fish with the standard sonar: they’ll switch to DI to get more detail, spot more big fish, and get a really good read on exactly what’s happening down there.

Are tuna slamming mackerel or squid? Are the bass holding tight to a submerged tree, or are they scattered nearby?

In shallower water, inshore or in lakes where the water isn’t much deeper than 50 feet or so, they’ll run side-scanning sonar, sweeping likely spots and looking for fish that aren’t where they predicted they might be. 

If the water is well and truly shallow, they might just stop, reducing lag, and using the SI to identify exactly where to cast. In slightly deeper water, they’ll switch to DI to get that killer detail.

Of course, mastering their fishfinders wasn’t an accident, and it certainly didn’t happen overnight. They took the time to learn to use the various features offered, practiced reading the returns, and got to know which sonar option to use and how they work together as a winning combination.

About The Author
Pete Danylewycz
Born and raised in Cleveland, Ohio, Pete grew up fishing on the Great Lakes. Whether he's casting a line in a quiet freshwater stream or battling a monster bass, fishing is his true passion.