Both have specific jobs. The camera exposes or records the ambient or existing light and the flash exposes your subject. When you to separate those two processes in your mind things start to make more sense. This is probably the biggest hurdle but once you get it really helps if you want to get serious about learning about flash photography. When you have the flash on your camera the cameras light meter has nothing to do with the flash.
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Both have specific jobs. The camera exposes or records the ambient or existing light and the flash exposes your subject. When you to separate those two processes in your mind things start to make more sense. This is probably the biggest hurdle but once you get it really helps if you want to get serious about learning about flash photography.
When you have the flash on your camera the cameras light meter has nothing to do with the flash. The cameras light meter is still only metering only ambient light. I cannot predict flash output because that event has not happened yet. So if your camera is in manual and meter shows underexposed ignore it. The flash does use the cameras metering system but not the light meter itself.
When in ETTL the flash pre fires, light is reflected back to the exposure system through the lens , the flash output is determined and the flash fires when the shutter curtains are both opened after full depressing the shutter.
Correct flash power was predetermined by the per fire. Here is the kicker. You are probably aware that the cameras exposure system light meter can be fooled. Pure white snow will underexpose, pure black tar will overexpose and if there is an equal amount of snow and tar in the image the exposure will be good. Same goes for your flash.
I bride in a white dress, a groom in a black tux and the bride and groom will all reflect light back differently which can cause flash exposure inconsistencies. There is no way to determine the flash exposure unless you have a flash meter. So basically you chimp and then adjust the FEC on your flash to compensate for the correct flash exposure. This is of course when you are in ETTL. IN manual you just adjust flash output power. The histogram is an excellent tool to help with the flash exposure and if you get the whites right everything else falls into place.
Here is the white towel method. Scroll down to see the gent with the towel and the histogram. It discusses sync speed - something you need to know. It also talks about High Speed Sync. It does help to understand your flash completely. You can look at it as flash photography being two exposures in one. At around minute of the video it shows the first curtain opening and creating an ambient exposure. It may not be a great expose but is an exposure.
The flash fires illuminating the subject and then the second curtain closes. The flash firing is the second part of the exposure. The author calls it dragging the shutter which is just basically slowing the shutter speed down.
580EX Flash Tutorial Book?
The user interface is a bit confusing because it requires pushing several different buttons and turning the dials. First if you are using the ex on a camera or the TTL extension cable by itself with no other slave flash the three position "off-master-slave" switch must be in the "off" position. Seems counter intuitive until you realize that switch is for wireless multi-flash mode and when using a solo flash the wireless remote control signaling must be turned off. I suspect you may have that switch set to Master.
Canon Speedlite 580EX II Instruction Manual