Custom Function question on flash sync speed

[craigsexton]

hello, I'm curious to better understand the custom funtion on my camera that sets the flash sync speed to a "fixed" 1/200th shutter speed when in Av mode.

I mean I kinda understand what it does and from some testing indoors with a 430ex flash I can tell that the flash compensates for the faster shutter to expose correctly.

But, whay have it on a "fixed" setting? Is it basically just for indoors in a lower light settings and therefore you can have a faster shutter speed to stop movement and possible at the same time not shoot wide open? And then when outside in brighter light where the normal shutter speed should be above 1/30th then don't use it and let the flash just fill in only?

I know I have still have allot to learn with on camera flash and this is boggling my brain.

Also, when in M mode, just how far can you push the flash to compensate for an underexposed picture by your meter reading. I understand that distance will play a major role in this but just don't understand it fully either.

Thanks in advance.

[dougsmit]

I can't say I understand the reasoning behind the 1/200 option in AV mode since that really just gives you the same thing as M mode with 1/200 selected as the shutter speed. Many people (beginners) err by shooting flash in dark rooms and the caera in AV mode. This can result in a flash exposure handled well by the flash exposure system but placed on top of a time exposure with a lot of camera motion since the selected aperture might require several seconds exposure. By telling the camera to set 1/200 when shooting flash, one avoids this secondary blurred image and acknowledges that the entire exposure would be provided by the flash. One coud just as well set the camera on M mode and select the same exposure pair. You can 'push' the flash to be the entire exposure since this system would work in a totally dark cave as well as in a normally lighted room. As long as the light is dim enough not to have much effect on the exposure, the flash does it all.

The thing to remember is that you have two separate exposure systems that must work together to produce the final image. The flash automation has no idea what the regular exposure meter is doing and the ambient light meter does not know whether there will be a flash or not. The flash can be set to give underexposure and provide just a fill function or it can be told to blast away and assume there will be no help from ambient sources (....and anywhere inbetween). The camera does not know which to do without your input. The fixed 1/200 in AV is really just one way of telling the camera that the flash will be doing the work and the ambient light meter is not needed for this shot. The flash will do it all.

[craigsexton]

Thanks Doug. that's pretty much what I thought but just wanted to confirm it. I've been trying to explore all the custom functions of my camera lately and that one kinda stumped me.

Craig