Tried making several and changing the height. The focus doesn't change. Without the object actually refracting the rays the light won't distort from the caustics. The only thing I can think of doing is to set your pool to gray and photograph the light through your caustic plane applied to a terrain with glass as a material. Do the second render with the tile material restored and receive shadows off. create a mask from the gray render and apply the light and shadow as a layer in postwork.