The contrast setting can be increased or decreased to get the best image. (Brad Wiegmann Photo)
Optimizing Your Contrast and Sensitivity Settings
by Brad Wiegmann
You could be missing fish. Not because you have the wrong colored or sized lure on. No, it’s probably because you have not optimized your contrast and sensitivity settings when out catfishing.
Side-scanning sonar produces two razor-thin oblong fan-shaped beams on each side of your boat creating a photo-like image of bottom contour and structure. Down-scanning sonar, similar to side-scanning sonar, uses a single razor-thin oblong fan-shaped beam to create a photo-like image of the bottom and structure.
A contrast setting is available when adjusting imaging on side-scanning and down-scanning sonar. It is set to manufactured settings when you get a sonar unit, however, this is just recommended setting and can be adjusted to get the best image possible. Adjusting the contrast setting will not change the frequency setting, however, imaging resolution will change accordingly with 1.2 MHz having the highest resolution followed by 800 kHz and 455 kHz.
Increasing the contrast setting will brighten the ratio between the light and dark areas of the screen. In the manufactures auto setting this could mean you have it set too bright or dark to see cover or structure easily. Keep in mind, that the contrast setting has nothing to do with the backlighting of a sonar unit and how easy it is to actually see the screen.
Increasing the contrast setting will make branches in brush piles, stumps, road beds, boulders, ledges, or other objects brighter, allowing you to see more detail.
Suppose you have been catching fish on a hard bottom. By increasing the contrast setting correctly, a hard bottom will be bright enough to “pop” out and be identified quickly when idling around. On the other hand, you have increased the contrast setting too much when the image is blown out in brightness.
Traditional 2D sonar uses sensitivity to show detail below the boat. Similar to side-scanning or down-scanning the recommended factory setting can be tweaked to see fish, structure, or cover better. Increase the sensitivity to just before you start getting clutter in the imaging.
Several reasons influence how much you can increase the sensitivity setting with suspended sediments and nutrients being the main two. Most marine electronic units (Lowrance, Garmin, or Humminbird) have an auto sensitivity setting that allows the angler to adjust (+ or -) more or less while maintaining the auto sensitivity function.
Other options like noise rejection or surface clarity can be adjusted to optimize your screen imaging. Noise rejection settings can filter interference from engine vibration, air bubbles, and bilge pumps. Surface clarity settings reduce temperature inversion or wave action near the surface.