cgardner
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Stobist has an anti TTL bias because Mr. Hobby is frugal and only used cheap manual flashes. More recently he has tried ETTL with and discovered it works and is warming to the idea of using it. If you buy an external Canon flash it can operate in either ETTL or Manual mode as your shooting needs dictate. In general manual works best if the flash and subject are at a consistent distance, such as in portraiture. ETTL is a better choice if you or your subjects are moving around because the camera will adjust exposure automatically.
The basic things to realize about flash are:
1) The intensity decreases about 2 f/stops every time the distance doubles
2) Exposure is only correct at one distance from the flash.
In ETTL mode what your camera does when the shutter is pressed fully is: 1) meter the ambient light, then 2) fire a pre-flash and meter it. The metering is done off the viewfinder of the camera, not the sensor which is still hidden behind the mirror and shutter at that point. The ambient and pre-flash reflections are compared and then the camera makes an educated guess HOW FAR AWAY the most important stuff in the photo is and sets flash power to make the exposure correct at that distance.
The result of the camera's guess will:
A) put the point of correct exposure at the correct distance
B) Put it nearer or further away than you want
The odds of it being "A" are about the same as coin toss.
The camera can only guess what you want correctly exposed based on how you frame the shot in the viewfinder. If you put the back of Uncle Louie's white shirt in the foreground and Aunt Sophie's face 5ft further back the camera will think you want the shirt exposed correctly, not the face. So when composing a flash shot keep what you want correctly exposed nearest the flash.
If the camera doesn't put the point of correct exposure where you need it in the scene, we can override the camera meter with an FEC adjustment. The FEC adjustment is indicated in f/stops from -2 to +2 but actually what FEC does, functionally is change flash duration (which how power is modulated) and move the point of correct exposure closer (minus FEC) or further away (plus FEC).
Accept that exposure is never going to be completely automatic because the camera can't read your mind. Its very much like sighting in a new gun. Fire a shot, see where it lands, then correct the aim of the next shot with FEC.
An easy way to know what is correctly exposed is to turn on the camera's overexposure warning (OEW) which blacks out the over exposed places. Take a shot at FEC =0, look at the OEW and see where things like white shirts are blacking out, then adjust FEC until what you want correctly exposed is below where warning appears.
That is the basics... For more detailed tutorials click the WWW button below.
Chuck
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