Requirement for +4 Diopter

A lens behind any dome port will see a subject underwater as an apparent or imaginary image a few inches in front of the dome. A lens that can focus as close as twelve inches will NOT need a + diopter close-up lens when used with an Ikelite dome port. The + diopter close-up lens is only required if the lens can not focus as close as twelve inches. The apparent image position will vary depending on the water. The actual setting may range from 11.75 inches in Truk Lagoon to over 12.25 inches in the Caribbean. The Ikelite dome is a 3" radius which is sometimes referred to as 6" which would be the diameter of the complete sphere. This size was chosen to avoid the buoyancy and size disadvantage of a larger dome. Rather than utilize something like a compass sphere, the Ikelite dome is designed and molded specifically for underwater photography. Multiple focal length ("zoom") lenses usually will not focus close enough to provide full range operation behind any dome without adding a +4 diopter close-up lens. This close-up lens looks like a filter and is available from most camera stores.