hey crip47,I was like everbody else I did not know if Iwanted to spend 2 grand on a sonnar unit ethier. So I did some serious reserch & found out they will do evrery thing it says that it will do.I have had mine for about 6 months. You will not beleive the detail the si shows,it does not matter if the water is muddy or clear.On the diffrence between the 797si & the 997si is the screen size you can see the screen on both units very well in bright sun light,the older 987 are the ones you can not see very well in bright sun light.Go to yahoo.com click on yahoo groups & then type in side imaging.there is a ton of info & pics. just on side imaging.