I'm experimenting with a shoe mounted flash at the moment. It's a TTL flash and yet the metering doesn't seem to account for the fact the flash will fire, making judging the correct exposure difficult. Any tips?
When using a TTL flash, camera's exposure metering will still aim for a proper exposure of ambient light so the background (unaffected by flash) would be nicely lit too; since ambient light is still the same, there shouldn't be much difference from photographing without flash; flash TTL metering is used to tune the power of additional light provided by flash. Each TTL system has its quirks here, though:
- Canon reduces flash power in bright ambient (to avoid clipping highlights);
- Nikon's newer version of iTTL (starting from D3 and D300) underexposes ambient (to avoid clipping highlights);
- Pentax caps shutter time (to prevent blur)
Judging exposure in (especially flash) digital photography should be performed by evaluating the histogram (preferrably of separate color channels) of a test shot, not by camera's anticipated exposure values. When you already know what the values should be (e.g. by using spot metering or a light meter), you'll be using manual exposure or exposure lock anyway.
If you have a scene where everything is equi-distant from the camera, TTL should attempt to properly expose. So set you shutter speed below the sync speed of your camera, and the aperture to whatever DOF you require, and let TTL take care of the burst of flash. Don't worry about the meter.
If you have subject nearer to the camera, then most systems will account for the fact you're focusing on a nearby subject and properly expose the subject, leaving the background dark. If you want to background to be lighter, drop the shutter speed (say from 1/200th to 1/60) because the shutter speed won't affect the flash exposure. This is called dragging the shutter.
So you are right, the camera's metering doesn't take into account the flash - so assume it will properly expose whatever you're focused on, and use the metering to adjust the "ambient" exposure of your background.