200 for shallower water as the higher frequency gives better definition but will not penetrate as deep into the water . Should get to 100 M easily. The 50 will punch deeper but also as the cone is going deeper it is wider. Think icecream cone upside down ,that is pretty much your sounders beam.

200 is a wider cone than 50khz (at the same depth).  I don't fish in over 100m but I sometimes use the 50khz - mostly use 200khz though,  You can put a split screen of 50khz and 200khz up at the same time and this will show you the differences in what each beam shows such as in the link below

http://fishwrecked.com/image/sounder-shot

Cheers

Paul will give that split screen a run next time out .. any idea Paul what the difference in cone size would be in say 30m ? I presume that info could be determined by using the A scope , which we are still trying to get a handle on . Quite often we'll catch fish that don't show up on the sounder at all even whilst anchored , with baits close to boat . I think the speed scroll is set to 1x presuming that would be right when not moving around too much ..    

Thanks for the info boys.

Interesting sounder shot there Paul. The 200kHz certainly shows more detail compared to the 50kHz. There seems to be conflicting information out there in regards to this subject as the below link explains the 50kHz has a wider cone angle and penetrates deeper than 200kHz and the 200kHz has a narrower angle however it picks up more detail in shallower water. At the end of the day Paul you have proven 200kHz is certainly the best choice for fishing shallower depths.

Chris - If the info in the below link is correct you would expect your sounder to pick up approximately 115ft (about 34m) of ground (Using 200kHz) in 100ft (about 30m) of water.

http://www.fishsniffer.com/forums/marine-electronics/94326-50-200khz-vs-83-200-khz-transducer.html

It's always interesting to learn more about our sounders because the fact is we've spent big bucks on them and many of us don't know how to use them properly - if at all!! :-)

What george jungle said is correct for typical transducers, if you want to know for sure you need to know what transducer you have 90% are made by airmar for most brands. Check their website for details regard your transducer specs it will provide the cone angle for each frequency and what the width is for each at a number of depth intervals

Paul here's some info for you

you can check it out for your self if you like

Typical specs for the very common P66 transducer

Cone angle @50khz  45degrees                                                      @200khz   11degrees

bottom coverage @ 9m or 30' =8m                                                 @9m = 2m

                         @ 30m or100'  =25m                                            @30m = 6m

                         @122m or 400' =26m                                           @122m =26m

                         @305m or1000' =252m                                        @305m = 64m

 thats the specs from the manufacturer

as already said transducers do vary in cone angles but in almost all cases 50khz is a wider

Thanks shortfuse - the difference in the return must be due to the differing frequency then not the width of the beam.

Paul 50 is wider than 200 trust me on this

 Looking at those beam widths and angles one would think the guys fishing super deep would have a hard time marking fish with a cone width off 250m @ 50 khz !!! You could be miles off the mark and not know it .. 

How ever no one runs a 600 w out deep and it makes you think about running a 1kw  200 kHz for super deep to put you closer to the mark ???

Cheers fishfish 

Yeah the cone is massive at that depth it is very hard to drop a bait 400M and hit the fish with the tide drift and wind as factors as well, 1 kw 200 khz wont hit the bottom at those depths