Flash sync speed question

[AbyssalPlains]

Hi!
I'm afraid I don' t get the idea of flash sync. What does it mean to set a camera to a certain sync speed and why do I have to do that? On my Oly E-330 I set it to 160th of a second, but more because I felt I should set it to something rather than because I knew what I was doing. Now I have noticed that when I shoot on land, it won't let me choose the shutter speed anymore when I set it to "Fire flash" - it is locked at 160. I guess my question is three-fold:
1) Do I need to set my cam to a flash sync speed for uw photo?
2) What is it good for?
3) What should I set it to? (I'm using two DS-51s)

 

[gert7to3]

Having your flash synched means that for an image exposure to occur, the strobe must fire while the camera's shutter is open at its widest point. The strobe flash pulse is very short, 1/1000 of a second or faster. If the strobe is not synched, and the shutter is only partially open, then some of the image will be cutoff. It literally shades part of the film or sensor.

This was a real problem with curtain shutters in 35mm SLR cameras. They would only synch to 1/60 of a second or so. Leaf shutters, used in most digital cameras will synch at higher speeds. Some even as high as 1/500 of a second or more. This due to the shutter construction and the fact that digital sensors are typically much smaller than a frame of 35mm film.

The way you use this in underwater photography, is to set your shutter speed to obtain the exposure saturation for your background color. Then set your strobe output for your subject's exposure.

Setting higher shutter speeds, permits you to force the background color to be progressively underexposed. So instead of that washed out blue or turquoise, you can make it richer and warmer.

I use manual exposure; ISO at 100-200; preset my aperture, usually to ƒ5.6-ƒ8 for good depth of field; take a couple of background exposures, without the flash, so I can determine the best shutter speed for a pleasing color balance; then set my strobes' output to get the proper exposure for my subject. This method works for manual or TTL strobes.