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| p.1 #2 · Canon on Camera Flash Setup/tech |
I have tutorials at http://photo.nova.org/ you may find helpful.
Indoors flash controls exposure so its best to lock down the controls on the camera in manual mode because in P, Av, or Tv the camera metering will be changing shutter or aperture in ways you might not realize. On the camera you will want to set aperture based on the DOF you want in the shot, mindful that as you stop down the lens for more DOF the flash will need to work harder to produce correct exposure.
Shutter speed doesn't affect the flash part of the exposure, which allows you to control how light or dark the background beyond the range of the flash winds up in the photo. There you need to keep in mind that in most cases indoors the background color temp will not match the flash. Too much ambience can result in gnarly looking skin tones lit with a mix of ambient from the ceiling hitting the forehead and flash hitting the fronts of faces.
ISO is the third variable affecting overall exposure and image noise. Ideally you want it as low as possible to reduce noise but indoors you may want to raise it in situations where you want more background ambience or greater flash range.
I suggest starting baseline of ISO 200 - 400, f/5.6, and 1/125th for the camera. That will give you sufficient DOF without making the flash work harder as it much if you stop down the lens more. 1/125th should allow you to hand hold most lenses you will likely use with flash indoors without shake. Start there, adjust as needed.
On the flash set it into ETTL mode. Compose your flash shots so what you want correctly exposed is closest to the camera. Avoid situations where there something between the faces you want correctly exposed and the camera because the metering controlling flash output will influenced by the nearer object.
FEC = 0 represents the camera's "I think this is correct" guess about exposure. The problem is the camera doesn't know with certainty if the person reflecting the pre-flash is wearing a white dress or a black one so. So flash exposure in ETTL mode is a three step process: 1) let the camera guess, 2) evaluate the results, 3) correct the camera's guess using + / - FEC.
When flash is used directly what FEC does is change flash duration which is how output is controlled. If your first shot is underexposed add + FEC. You can do that on the camera body via the button on top, the camera menu, or using the wheel on the flash. Try all three and see which you find is more convenient. I prefer adjusting FEC using the wheel on the Flash.
The clipping warning in the playback will show you when and where any parts of the scene are clipping. The way I arrive at correct exposure easily is simply raising FEC until I see the highlights start to clip then back off FEC by one click (1/3 stop). After comparing the playback clipping with the same files on the computer a few times you should be able to understand how the highlights in the clipping warning on the camera corollate to the highlights in the RAW file.
That's all there is to it. Adjust camera settings to control the background and FEC to control the foreground.
If you bounce the flash off the ceiling or add diffusers you will find you need different amounts of FEC than when direct flash is used. That is due in part to the bounced lighting more uniformly illuminating the space, which causes the evaluative metering to interpret it differently. With experience you will be able to anticipate how much FEC you need as a starting baseline in different situations. As with direct flash its just a matter of shooting and adjusting FEC until the clipping warning reveals the highlights where you want correct exposure are just at the point or just below blacking out on the playback.