Garmin 400/500 Owner's Manual page 60

Using Sonar
Range—adjust the range of the depth scale on the right side of the
screen (auto, or to adjust manually, Manual Range).
gain—controls the sensitivity of the sonar receiver (auto, or to
adjust manually, Manual). To see more detail, increase the gain. If
the screen is cluttered, decrease the gain.
frequency—when using a dual frequency transducer, select how
the frequencies appear on-screen (200kHz, 50kHz, dual, or auto).
Zoom—zoom in to a section of the full screen. The zoom is off, or
set to No Zoom by default. Four options are available:
• No Zoom—turns zooming off.
• 2x Zoom—twice the magnification.
• 4x Zoom—four times the magnification.
• bottom Lock—locks the zoom window to the bottom.
• Split Zoom—displays the Split Zoom screen (page 51).
Noise Reject (available only when gain is set to Manual)—reduces
on-screen visual clutter (usually from electrical sources) on the
screen. Select the top slider bar to manually adjust the value of noise
rejection. Manually adjusting the amount of noise that is rejected
helps fine-tune the sonar to show the most detail with the least noise.
When frequency is set to dual (and gain is set to Manual), you
can individually adjust noise rejection for each frequency.
54 GPSMAP 400/500 Series Owner's Manual