If I set my camera (canon 7d) to manual and the speedlight (600ex) to ETTL, shouldn’t the flash put out the necessary power to expose the image?
If the flash has enough power to properly expose the scene it should. But if your manual settings require more light from the flash than it can provide it will be unable to do so.
- Insure that the shutter speed is at or below your camera's flash sync speed so that both shutter curtains are completely open at the time the flash fires.
- Insure that there are no objects in the foreground that will cause the camera to reduce power to prevent blowing out the foreground objects. Things that are closer to the camera will be much brighter than things much further away. If the camera sets flash power based on near objects then far areas of the scene will be dark.
- Account for any modifiers you place between the flash and scene. This would include bouncing the flash off another surface. Any modifiers will reduce flash power to one degree or another. The more the modifier reduces flash power, the more power you need the flash to output to provide the same amount of light on the subject. If your flash can not provide enough light to overcome the amount lost via the modifier, then the scene will be underexposed.
- In cases were the flash is not bright enough at maximum power you can either increase the ISO sensitivity setting or open up the aperture (lower f-number). Changing shutter speed will not affect flash power since the duration of the flash is shorter than the camera's flash sync speed.