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 p.1 #2 · 40d & 50d metering questions from a Nikon Guy! | |
The basic dilemma of exposure outdoors is that the scene range usually exceeds the range of the sensor. Exposing for highlight detail results in the middletones and shadows being rendered darker than normally seen by eye. Exposing perceptually for normal midtones will result in some blown highlights and lost shadow detail.
The only solution to the dilemma is to reduce the contrast of the scene, at least in the more important foreground with flash by:
1) Shooting into the shadows of the ambient
2) Exposing via shutter speed to keep the ambient highlights below clipping
3) Raising the level of illumination on the shadow side with one or two flashes.
The first flash added to the shadow side actually perform as key light, not fill because it creates highlights over the ambient fill from the sky. To create a highlight pattern and get lighter shadows (and creative control over ratio) its necessary to use two flashes - one for fill over the camera and one off axis as key light, just as with indoor lighting.
All TTL exposure control represents an educated guess on the part of the camera about what is important. Canon uses 35 zones on the viewfinder for metering and compares ambient and pre-flash in the time between the shutter button is fully pressed and the shutter opens. I find the overexposure warning in the playblack the most useful feedback because it shows when and where overexposure is occuring. EC is used to keep the ambient highlights below clipping an FEC the flash lit highlights below clipping.
See this thread on exposure: http://www.fredmiranda.com/forum/topic/821629/1#7660408
Also click the WWW button below and read my Canon flash tutorials. One on TTL includes a comparison of all the metering combinations of a 20D. The 40D adds spot metering. Also in the Concepts section there is a tutorial on shooting outdoors which you may find useful.
Chuck
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