Detection range (depth) of a fish finder depends upon the frequency used for ultrasound transmission. In principle, the higher the frequency of ultrasound, the shorter the propagation range can be. The wavelength of high frequency ultrasound is short, and its directivity angle is narrow, which enables detailed searches, but it attenuates significantly while travelling through water. Low frequency ultrasound is characterised by its long wavelength, wider directivity angle and high level of tolerance toward underwater attenuation, hence enabling wide-area-searches in deeper water. To summarize, you can choose low frequency if you are searching in deep water and high frequency if you are conducting a detailed search in shallow water. Further, when using low frequency ultrasound, you may be able to conduct more precise deep-water searches by adding an optional Power Adapter, which amplifies the transmitter power to a few kilowatts. Please note, however, that when using the Power Adapter, a dedicated transducer capable of handling higher transmitter power will be needed.

If I'm scouting a new area I won't know anything about what sort of structure is ahead until I paddle over it, blowing out any fish that may have been there. With this little guy, I could cast it to the spot, get a read on what's below and proceed. And, you have the luxury of retrieving it slowly to see bottom contours between you and that fishy spot. There have been a number of occasions when I've made numerous casts at a fishy looking location (without any luck), only to paddle over there and discover that it was 10" deep, nobody home.
The intensity of the sonar return from a hard bottom will be different to one from a hard bottom. Your fish finder uses colour to show this difference. In the standard colour palette of the Deeper display, the colour varies from dull brown (softest) to intense orange (hardest). In the day mode colour palette (see screen shot), the difference is even easier to see, with the colour ranging from purple (softest) to red to orange to yellow (hardest).
Look for equipment that will work with multiple frequency: higher for better detail but not as successful in deep water; lower for depth but less detail on your viewing screen. Of course, if you fish only in shallow water, you’ll be happier with high-frequency output. For deep-water work and for commercial fishing, you may want to use a lower frequency.
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