synching flash with camera?

editing post for removal.

That's because your 580 has high speed sync capabilities. Great feature for capturing those "sun in the shot" pics. Also, studio strobes will sync at slower shutter speeds than speedlights.

Shutter is a mere moving slit rather than the full size of the sensor, when the shutter speed is fast. Therefore if the very fast (e.g. 1/1000 burst of light) flash emits all its light, it exposes the sensor only where the opening of the small slit happens to be. So normally the fastest X sync speed is the quickest shutter speed where it is the full sensor size rather than a mere slit.

HSS on the flash works by prolonging the time duration of the light, so that it fills enough time for even a fast shutter speed's slit to move across the full width of the sensor. It cannot output as intense a light, but puts out lower amount of light across a longer period of time.

tomdlgns wrote in post #6264209
when i am in ETTL mode with the 580ex II my camera shutter only goes to 1/250.

however, when i put the flash in M mode, i can put the shutter as high as i want on my camera.

You are incorrect. Your 580EX will not go above 1/250 in M mode. ETTL and M mode are both the same as far as flash sync is concerned.

If you have your flash attached to your camera, the only way to get your shutter speed faster than 1/250 is to go into the flash menu and select number 3 of the three flash sync options:

1st curtain
2nd curtain
Hi-Speed (HSS)

The ~H/>>> button on the flash can do this, or you can use the Flash control menu on your 40D.

If you have Hi-speed (HSS) enabled, it will revert back to normal sync if you change to Green Box mode or any of the other basic zone settings (portrait, landscape, etc.)

tomdlgns wrote in post #6264623
does the flash act the same way on the camera as it does on a hot shoe cable?

Yes, it will act exactly the same way if it a Canon compatible dedicated off camera shoe cord.

You must have the High Speed sync enabled, or your off camera shoe cord is not working properly. (or it is not a Canon compatible, dedicated off camera shoe cord)

Which "hot shoe cable" are you using?

Wilt wrote in post #6264405
Shutter is a mere moving slit rather than the full size of the sensor, when the shutter speed is fast. Therefore if the very fast (e.g. 1/1000 burst of light) flash emits all its light, it exposes the sensor only where the opening of the small slit happens to be. So normally the fastest X sync speed is the quickest shutter speed where it is the full sensor size rather than a mere slit.

This is correct. A bit more in depth... So, the shutter is made up of two 'curtains'. For exposures at or less that your flash sync speed the way the sensor gets exposed is this:

The mirror flips up, the first curtain slides open to fully expose the sensor, then the second curatin closes to block light from the sensor, then the mirror flips down.

Now this fully exposing the sensor with the sliding of the curtains becomes more difficult to do at higher speeds. So what they do at higher speeds is they start to close of the light with the second curtain before the first curtain is open all the way. So at any given time at higher shutter speeds the sensor is never fully exposed all at the same time. It always has part of the first or second curtain covering it.

What this means is if your flash goes off as a main light source the burst of light is so fast from the flash that you will 'catch' part of one of the curtains blocking the view of the sensor and only get a partial exposure. Part of the image will be under, or not exposed at all.

With high speed sync flash the flash will deliver multiple low power longer duration flashes to make sure it covers the sensor throughout the whole travel time of the curtains.

I hope I cleared things up more with this, sometimes I just muddy the water.

Sync. speed (limit) of a camera is the fastest shutter speed at which the entire sensor (film) is exposed at some point during the exposure.

tomdlgns wrote in post #6264648
then how am i able to adjust my shutter speed that high with the flash?

In manual mode it will 'let' you put the shutter higher than the sync speed.

If there is enough ambient/available light then that is what is contributing to your exposure. Try taking a shot with the exact same settings with the flash on and then with it off and compare the two. If they look the same then you know your flash is not a factor and your exposure is all from ambient... although i would be surprised to hear if you were not getting some kind of 'band' of darker or lighter areas in the image.... that or you have HSS enabled like others have said and that is getting you there.

tomdlgns wrote in post #6264623
i had my flash on and he ss was at 1/4000 and it was taking pics. they were dark, but i was taking pics.

Re-read paragraph #2, message #4.

If your flash does not have HSS enabled, you will not get a suitably exposed frame! Ambient light might expose the entire frame, but the non-HSS flash adds light only within a small slit across the frame!

tomdlgns wrote in post #6264733
i am still confused about flash syncing...

i based off of the posts posted, i am still failing to realize what it is.

So my post ( #10 ) made no sense? What part are you having trouble with? All of it?

tomdlgns wrote in post #6264761
you did a good job of explaining how it works, but i want to know what settings i need to set on my camera/flash to be able to 'flash sync'

if my camera shutter is set to 250, how do i flash sync it with 1 flash? how about 2 flashes?

As long as your shutter is at or slower than the max sync speed you will be fine. The camera will trigger the flash while the shutter is fully open.

If you want to use faster than the sync speed just flip the flash to HSS. But just be aware that this will yield lower power of the flash which will be fine for fill flash or closer camera to subject distances. It just won't deliver high power flashes.

When you put your 580EX II on the camera, or on a off camera shoe cord, it will automatically sync with the camera. There is nothing else you need to do.

tomdlgns wrote in post #6264761
if my camera shutter is set to 250, how do i flash sync it with 1 flash? how about 2 flashes?

For two or more flashes for the most part the same applies... but it depends on the way you are triggering the flashes. Some trigger methods will require you to be set a little below max sync speed.

Dermit wrote in post #6264779
As long as you shutter is at or slower than the max sync speed you will be fine. The camera will trigger the flash while the shutter is fully open.

If you want to use faster than the sync speed just flip the flash to HSS. But just be aware that this will yield lower power of the flash which will be fine for fill flash or closer camera to subject distances. It just won't deliver high power flashes.

ok, what is the sync speed?

on the flash unit, the only thing i am really altering is the power of my flash output 1/1, 1/2, 1/4, etc...and the zoom range.

where is the sync speed setting? i guess if i knew that, i wouldn't have a hard time understanding flash sync.

in the strobist DVDs many people were talking about flash syncing, i could be doing it w/o knowing it.