If you have no idea what split photography is, please, read our article Secrets of Split Photography.
Now when you know some facts about split, let’s turn to the theory. As you should know from your physics lessons, as soon light gets into the sea water, it changes its nature. Sea water is almost 800 times denser than air, that is why sun rays are refracted, absorbed and are partly filtered. In air sun rays are spread equally and rectilinearly, but when they get in the environment which has a higher refractibility (air refractiblity is 1 point, water refractibility is 1,33 poit) light rays bend and create a magnifying effect.
As a result, even with optimal lighting conditions and high water transparency, the level of illumination under the water is one f-stop less intensive than above the water. Early in the morning and at the end of the day, when sun rays fell on the water surface at an angel, this difference is even higher, it reaches 3 f-stops. Thus the best time for split photography is from 10 am to 2 pm, when the sun is high in its zenith.
So we found out that the level of illumination of two parts of a split is different, even if the conditions are ideal. It means that it is impossible to set an ideal exposure – either the upper part will be overexposed, or the bottom part will be underexposed.
In conventional photography this problem was solved with the help of a graduated filter. This filter makes less visible the difference in illumination level of the two parts. The disadvantage of this method is that one has to divide the shot in the center, as it is impossible to change the position of the filter under the water. Read more about Neutral Density Filter
In digital photography the solution is much easier. RAW format helps you get good results. An ideal exposure for split photography in RAW format is when the upper part is a little underexposed and the bottom part is a bit overexposed. You will correct it in the post-processing.
There is one more secret of a split. When we shoot with a wide-angle lens, we use a spherical port as well. Under the water a spherical port forms a kind of additional ‘lens’ above the main one. As a result we can see a picture above the port (about 30-50 cm above), which is called a virtual picture.
Remember about this peculiarity of a spherical port when you choose a wide-angle lens. It should be able to focus closely; otherwise you won’t be able to focus underwater.
If a wide-angle lens focuses on a virtual picture underwater, then any subject above the water which is situated farther than 30-50 cm will be out-of-focus. To solve this conflict, use a magnifying attachment. Usually it is a filter put on the lens. Part of this filter is transparent, and the other part is a magnifying lens. In this case you also have to divide you image 50×50, to ‘hide’ the line of a magnifying lens and ordinary glass.
If you don’t want to divide your image 50×50, you’d better use a fish-eye lens for split photography. Use a small aperture and you will get a wide depth of field, which allows both the virtual picture and the above-the-water scape to be well in focus. Read more about fish-eye lenses.