A flash photo is really two separate exposures in one; the ambient light which would hit the sensor in that shooting situation if flash were not employed and the light contributed by the flash. This "split personality" is actually a very powerful creative tool outdoors once the cause and effect is understood.
My Camera Baseline Approach To Exposure
My first camera in 1969, an underwater Nikonos II, had no meter so I used "Sunny 16" and quickly learned to predict outdoor exposures. It was really not difficult. The sun shines the same way every day. Flash exposure was by the numbers: guide number / distance = f/stop. Dead on every time and actually very simple once you get the hang of it. The next year I bought Ansel Adams books and a Honeywell 1-degree spot meter and mastered obsessive compulsive spot metering, the technique where you can't take a photo of an old barn until measuring nook and cranny in it. By 1974 I was using incident metering for the lighting on my room-size 40" x 48" copy camera at photo labs of National Geographic. By 1975 I was teaching photography at a local college. Oh yeah, I forgot I also assisted Monte Zucker for a couple years from 1972 to 1974.
Controlling the Exposure - FEC vs EC
Note: E-TTL and E-TTL II cameras handle flash differently in Av and Tv modes. E-TTL cameras automatically change the amount of flash power based on the level of ambient light. When ambient light levels are low the flash is powered to expose the scene correctly with flash alone. But as the ambient light reaches EV9 (1300 Lux) the amount of flash is gradually reduced to a level -1 stop below the ambient light level, what is typically used for fill flash outdoors in sunlight. Cameras which reduce the fill automatically will have a C.Fn to override it. Both the automatic fill-flash reduction and the C.Fn to override disappeared from E-TTL II cameras, at least the 20D I own.
This section assumes that a E-TTL II camera or E-TTL auto fill flash reduction turn off is being used.
Exposure Compensation (EC) adjustments are only "in play" when P, Av, Tv or the other auto modes are in use. P and Tv are poor choices for flash because they control exposure by adjusting the aperture automatically. Aperture impacts the depth of field (DOF) and amount of flash which is needed and should be controlled by the photographer not the camera, so do not use P or Tv on the camera when using flash There are exceptions of course, but sticking with M and Av will simplify the learning process.
In M mode it does nothing at all because there is no EC in M mode (more on that later). In Av mode the photographer picks the aperture, then camera meters the ambient light using whatever metering mode is in play (i.e., evaluative, partial, center-weighted, spot) and picks the speed its programming determines is needed to expose the highlights in the scene correctly. The details of how the Canon system actually meters each zone is a proprietary secret, but the design of the metering system provides some clues.
In Av mode the photographer picks the aperture and the camera controls the shutter to keep the exposure constant. One of the best ways to separate a subject from a distracting background is with selective focus, and Av mode allows the photographer to control the blur (also called Bokeh) to achieve a specific creative goal. In Av mode the camera adjusts the shutter according to the AMBIENT light level, and the FLASH duration according to how the pre-flash reflects back off the scene. As the ambient light decreases the shutter times increase to keep the exposure constant, but the flash output stay the same and the slowing of the shutter does not affect flash exposure because the flash, in normal mode (non High Speed) mode fires when the sensor is completely exposed by the shutter curtains.
Av mode is good choice when shooting outdoors when ambient lighting is changing or you don't have time to set manual shutter speeds. But the TTL exposure system will "drag" the shutter slower that it is possible to hand hold, resulting in camera shake and blur trails in front of the action created by movement occurring after the flash fires but before the shutter closes. Canon cameras used in Av mode with flash will meter the flash about one-stop below the ambient level. Apparently Canon assumes everyone uses Av outdoors and wants - 1 stop fill flash.
In Av mode adjustments to Exposure Compensation (EC) change the shutter speed and exposure for the ambient lit background, but not the flash lit foreground because flash isn't affected by shutter speed (except the x-sync limit). Flash Exposure Compensation (FEC) is used to adjust the flash lit foreground. Being able to independently control the foreground and background tone gives the photographer more creative control. See my separate Fill Flash tutorial for more info.
Tv mode puts the camera in charge of the aperture, which makes it a terrible choice for flash photography because aperture affects flash exposure. M mode is a better choice.
M mode allows the photographer complete control of shutter and aperture. With flash photography the photographer would typically select the aperture desired for DOF and Bokeh and then adjust the shutter using the indicator in the viewfinder. Indoors with flash M mode is always better than Av indoors because the low light levels will cause the shutter to slow to very long exposure times.
On Canon cameras its the same indicator showing EC adjustment in Av mode so from a functional and results standpoint there is very little difference between M and Av; its just not automatic. FEC is used to independently control the flash lit foreground because although the camera shutter and aperture is set manually the TTL metering is still controlling the flash output if the flash is set to E-TTL mode. My suggestions for shooting with flash Understanding how the camera meters will result in more consistent predicable results. Start any new shooting situation by taking a "benchmark" test shot with EC and FEC set to zero to determine how the camera is evaluating what you are photographing. How the camera will guess exposure will vary depending on the metering mode you use and how you use it. Just remember that even with the camera in M mode the camera is still controlling the flash when its set to E-TTL. Starting from a zero baseline makes it easier to get an understanding of how the sophisticated metering sytem of a Canon camera evaluates a scene.
Having a standard exposure target also simplifies the task of determining when the exposure is correct. I suggest carrying a small white terry towel and gray card in your camera bag. Put them both in your test shots so there is always a known textured white highlight which can be used to evaluate the exposure. Looking at the texture of the towel in the enlarged view on the LCD will tell you if the file is overexposed and the spike it creates on the histogram on a dark background can also be used a gauge for exposure.
Using Flash Outdoors:
Make HS Flash your default setting: Outdoors put your flash into high speed E-TTL mode when you turn it on so it is available when needed regardless of what mode you are in. It will switch automatically from single to HS pulse as the x-sync limit for your camera is crossed. Set it and forget it.
Outdoors try Av first. After all, automatic control over exposure is why you bought such an expensive flash so its foolish not to use it whenever possible. But if you find Av isn't providing the control, such as when there is strong backlight or when shooting at night, switch to M mode in the camera.
When shooting in Av the camera will pick the shutter speed it thinks will expose correctly with the ambient light. If the ambient light is below the level needed for good exposure the camera will tell the flash to fire. The camera will tell the flash to illuminate the foreground about 1-stop darker than the ambient exposure. That was done because the most common use for Av and flash is fill for sunlit shots and fill one stop below the ambient light will produce a pleasing lighting ratio with good shadow detail. As noted above, keep the flash in HS mode outdoors so the x-sync limit of camera will not be a factor.
When in Av mode adjustments to EC will lighten or darken the ambient lit background by altering the shutter speed. For example a minus EC adjustment could be used to darken the sky and background for a more dramatic contrast of the flash lit subject in the foreground. As with the ambient exposure the camera metering may not get the exposure of some scenes correct. Adjustments to flash exposure compensation (FEC) will alter the amount of flash which is output and the illumination of the foreground. The ability to independently control background and foreground exposure gives the photographer a powerful tool for creative control. HS flash allows the use of wide apertures with narrow DOF and good Bokeh. Combined with a background darkened with minus EC a very effective contrasting of the subject from an otherwise distracting background can be achieved.
High Speed (FP): In bright sunlight the flash will usually switch to High Speed (FP) mode automatically if you open the aperture wider than f/8. Flash output will drop, but so will the need for flash as more ambient light contributes to the exposure and more of the reflected flash reaches to the sensor. So if the flash switches to High Speed (FP) mode and the photos become underexposed due to reduced flash output just open the aperture more!
Using Flash Indoors:
Indoors the low ambient light levels will cause a camera set to Av mode to slow down the shutter speed below the point were sharp images can be obtained. For this reason the camera should be switched to M mode for using flash indoors. Setting the shutter to 1/ focal length or 1/125th, whichever is faster is a good "rule of thumb" for indoor flash. Starting with an aperture of f/5.6 is a good compromise between DOF and the need for flash power. Adjust wider or smaller as creative needs dictate, being cognizant that the flash will need to work harder and will deplete its batteries flasher at smaller apertures.
If more of the ambience of the room lighting is desired a slower shutter speed can be used, but camera movement, subject movement, and color temperature differences between the 5500K flash and the room lighting may affect results. A tripod and cable release will solve the problem of camera movement, but fast moving subjects may appear blurred.
2nd Curtain Flash With long exposures the flash fires at the being if the exposure, so any blur trails will wind up in front of the moving subject which appears odd, as if they were moving backwards. Switching the camera or flash to 2nd curtain sync will fire the flash just before a long exposure ends, putting the blur trail behind the subject. Note: Setting 2nd curtain on the flash will override the C.Fn setting on the camera used to set 2nd curtain (C.Fn-15 on 20D). When more than one flash is used 2nd curtain is disabled because the flash control signals between the flashes would affect the image exposure.
White Balance: With some cameras (20D and later) the 580ex will automatically set white balance when the camera WB is set to either AWB or Flash. But as with all automatic functions the results are not always correct. It is better to do a custom WB using a Kodak Gray Card held by the subject being photographed. First set WB to Flash. Fill the center circle of the viewfinder with the card and photograph it. From the camera menu access the custom WB screen and select the image just photographed. Then set the camera WB to custom (\0/ symbol). If a modifier is used on the flash custom WB should always be used and set using it on the flash.
Mixing light sources: Indoors tungsten or fluorescent lighting may create color temp imbalances custom WB cannot remedy. If the desired effect is for the ambient light to dominate the color temp difference between ambient and flash can be eliminating by using a gel filters over the flash to match the ambient light: CTO orange for tungsten, "Tough Plus" CC30 green for fluorescent. For information on filters see the Cinegel filter information on the Rosco web site.
Manual Flash works better than E-TTL in some situations: The flash in E-TTL will vary in output based on the amount of light that is reflected back to the camera. Exactly how that happens is difficult to predict because 35 zones are evaluated. Manual (M) flash mode may work better than E-TTL when shooting a portrait session or an other situation where the background is the same in all the shots but the exposure is varying from shot-to-shot as the subject moves or different people in different toned clothing are photographed. When a situation like that is encountered its best to switch the flash to manual power and use the LCD and histogram to determining the manual power level needed for the flash.
How I get the most out of my 580ex:
I use Stroboframe "camera-flip" flash bracket and hotshoe TTL Extension Cord 2 to raise the flash 12" directly above the lens and add DIY reflector-diffuser like the one shown in my DIY Diffuser tutorial. I then add a second 580ex and diffuser on a rolling stand for precise light placement and of the lighting ratio. It is a simple but very effective and portable two-light solution. When I need separate control over the background I'll add a third 580ex:
Here is the Table of Contents for my other tutorials specific to Canon Flash
Holistic Concepts for Lighting