that is the total distance from the sea surface to the subject plus the distance from the subject to the camera should be more than 8 feet and less than 50 feet standard still or movie film balanced for use in daylight should be used the filter can have an in front of the lens format 10 shown in fig2 which is designed for installation upon a camera 11 and lens 12 as shown in fig3 this in front of the lens format 10 includes a ring 15 or other means for securing the filter 10 to the lens 12 here the ring 15 is provided with male threads 16 for mating with female threads 17 on the lens 12 known separate step up or step down rings not shown can be used to attach the filter 10 to the lens 12 if the camera is of the direct immersion non housed type the space 13 between the filter 10 and the lens front element 14 should be flooded that is the filter 10 should be attached to the lens 12 when both filter 10 and lens 12 are underwater this procedure will avoid the imbalance of water pressure only on the outside of the filter 10 such underwater pressures on the filter can easily exceed 25 psig if not balanced these pressures can break deform or otherwise damage the filter or camera less preferably flooding grooves not shown can be produced in the ring 15 to automatically admit water to the space 13.