|Fill Flash Concepts
In the studio you use a low-key or high-key background to create contrast between the background and the face. Which key is most effective for the background is mostly influenced by the tone of the clothing, which needs to blend into the background so as not to overpower the face in the photo. Outdoors, by manipulating the balance of ambient-to-flash with EC and FEC or manual shutter/aperture and flash power, we can render the ambient-lit background normal (perfect match), darker and low-key (underexposed) or lighter and high-key (overexposed) while retaining normal exposure on the face of the subject
I illustrate why it is important to create strong contrast between the front of a face or other area of interest in this tongue-in-cheek tutorial COI for The Hard Of Understanding for those who can't grasp the importance of making the front of the face the dominant center of interest in a portrait. The grown-up version is Background and Clothing Considerations. What those two tutorials cover is really just plain ol' common sense about what attracts and distracts the eye in a photo and things don't change when you venture outdoors into the sunlight. There's still the need to contrast the face of your subject with everything else, namely background and clothing. That's were the fill flash comes into play.
Flash outdoors is two exposures in one
When flash is used outdoors there are two factors contributing to the exposure which can be controlled independently. Because the duration of a flash is so short the shutter speed of the camera has no effect on the flash* lit foreground of a photo taken outdoors with flash. Changes in shutter speed will however affect the ambient exposure of the background. That fact allows the background to be made lighter or darker with shutter speed while the foreground is keep the same.
* X-Sync Speed and High Speed (FP) Flash mode: Most DSLR cameras have what is called a "focal plane" or FP shutter which consists of two independent curtains which cover the sensor. The first is normally closed and lifts to reveal the sensor when the shutter is pressed. The second then follows the first to cover the sensor and end the exposure. A regular one-burst flash can only fire when the shutter is fully exposed; after the first curtain clears but before the second closes. The speed of the curtains and size of the sensor dictate the fastest allowable shutter speed, which is called x-sync. It varies between 1/200th sec on Rebels and the full-frame 5D, 1/250th on other 1.6 crop cameras such a the 20D and 30D, and 1/500th on the better engineered EOS1 pro bodies. The 1/250th limit severely limits the choice of aperture in bright sunlight to f/8 - f/11, which in turn requires a great deal of flash power for correct exposure.
Canon devised a way to use flash at speeds faster than x-sync. The flash is pulsed as the curtains travel so all parts of the sensor is evenly controlled. The advantage outdoors is that faster shutter speeds allow the used of wider apertures (e.g., 1/2000th @ f/2.8) for more creative options (DOF isolation and Bokeh). Total range is reduced, but wide apertures require much less power. If placed in High Speed (FP) Flash mode The EX series flashes will automatically detect when the shutter exceeds x-sync and switch it on and off as needed, so:
Outdoors just leave your flash set to High Speed (FP) Flash mode
If the camera is used in Av mode and flash in ETTL mode the exposure system will try to balance the flash with the background lighting (see yellow note below). To change the balance of the ambient background lighting an adjustment would be made to Exposure Compensation (EC) which will cause the shutter speed of the camera to change. Dialing in plus + EC will tell the camera to increase the exposure making the background lighter, while minus - EC will make it darker. The ETTL flash computer will make its best guess at exposing the foreground subject correctly. If the exposure isn't dead on a Flash Exposure Compensation (FEC) adjustment can be used to adjust it.
If the camera is used in M mode the only difference is that the photographer must adjust the shutter manually to change the tone of the background. We would start by adjusting the shutter until the exposure indicator is centered then take and evaluate a test shot. Shutter speed would then be adjusted faster to make the background darker (less exposure) or slower (more exposure) to make it lighter.
E-TTL vs E-TTLII and Flash: Note: older Canon DSLR models with Digic processors and E-TTL metering have a function called "auto fill reduction" which automatically reduces the flash output in bright outdoor conditions. For lighting conditions below EV9 the camera will assume the flash is the primary source of illumination. Between EV9 and EV13 the amount of flash will be gradually reduced on the assumption that the ambient light is the primary source and the fill is only needed to open the shadows. Above EV13 the flash will be 1-2 stops below what is needed to match the sunlight; about a 3:1 highlight:midtone ratio. Cameras with "auto fill reduction" will also have a Custom Function (C.Fn) which can disable it. E-TTL cameras fire and meter pre-flash when focus is locked and factor in the active AF point when calculating the flash exposure. Locking focus in one spot with the center AF point then recomposing to take the photo can result in inaccurate and inconsistent exposures.
The Canon 20D and later cameras utilize Digitc II processors and E-TTL II metering. There is no auto-fill reduction. Pre-flash is metered and compared with ambient across 35 metering zones on the viewfinder a split-second before the shutter opens, and focus distance (if available from a USM-type lens) is also factored into the flash exposure computation. AF points are not used in the exposure calculation so "focus/recompose is not a problem.
First decide on the role the sun will play
Using the sun for frontal lighting as you would in the studio is difficult for one simple reason; the bright light makes the subject squint. It's more effective to place the sun behind the subject like a hair light, then use one or more flashes in front for fill (one flash) or key and fill (two flashes).
With two flashes for frontal key and fill, with the sun providing the hairlight, you simply use the ambient / flash balance to control the tone of the background, ideally making it high- or low-key, For example with two frontal flashes in play, one on a bracket and the other 45 degrees off the subject's nose you could do a short-lit oblique low-key portrait as effectively as in the studio:
Those are only a few of the infinite creative possibilities, but they are the simplest to grasp conceptually and execute effectively. Using a TTL flash with High Speed (FP) sync greatly expands the creative possibilities by allowing the use of very high shutter speeds with wide apertures; just the combination needed to turn day into night using fill flash.
Holistic Concepts for Lighting