Why do I have to use a close up lens if a domeport is used?
A glass dome underwater has the effect of a negative lens. It creates a virtual image which is much nearer to the port. How near is –besides other points – depends on the inner radius of the glass. To compensate this negative power a close up lens has to be used. For SUBAL domes you need a + 4 diopter for theDP-SWB/3 and a + 2 diopter for the DP-FE2.
Can I leave the housing after the dive somewhere in the sunlight?
Better not! This will produce a higher temperature inside the housing which could cause some problems. The humidity inside the housing will condense when you jump into the water again and will ‘fog’ the frontglass. If the housing was closed in a enviroment with high humidity it may happen that the water conenses to little drops which could damage the camera.
The best you should do is to store the housing between the dives in a humid enviroment – covered in a wet towel for example, or dry the frontglass with a soft towel and strore the gear somewhere in the shade.
Why do I need a dome port to use wide angle lenses?
Underwater, light rays are refracted (bent) when they pass from water to air (inside the camera housing). This is because water is denser than air. The result, when using a flat port, is a reduction in angle of coverage by approximately one third i.e. a 50mm lens becomes a 67mm.
For lenses longer than 35mm, this reduction in angle is accepted but for 28mm and wider angle lenses a dome port is needed to correct the angle of coverage and reduce edge distortion.
