|Theoritically it is 4/3, which means object at 4 meters seems to be at 3 meters.
It helps in close-up photography to get larger close-up (kind of extend the focal lenght) but it also have a negatif impact because for the same framing than on land you have to stand further away from the subject, and of course you increase water thickness between you and the subject. Increasing at the same time the number of suspended particules between you and your subject which reduce clearness.
This means you have to be very carefull when swiming not to move the sand on the bottom. You also need conditions where there is not too much current UW.
I was almost like this fish, laying down on my chest, breathing slowly, and kicking very slowly with my fins.
Flash exposure brings a lot of light for a lot of DOF, here again, care should be taken not to have suspended particules in water or it will look like a snow strom. Adjusting slow shutter setting will help to maintain shadow details in distant BG.
Now you are ready to reach the Cousteau team.
Lizardfish (76) *