So you are surprised that your flash pictures always turn out differently and unpredictably, especially when using automatic (“TTL”) flash?

Then this may help:

A. First, worry about the background, ambient light:

1. First, decide “should the background light do any work?”. If you are using an automatic or semi-automatic mode, like P, Av/A or Tv/S, the camera will try to light the background well. so it will not just be the flash doing the lighting.
2. Realise that there are limits to the previous: on Canon always in P mode, and on Nikon in P and A when “Slow Flash” is disabled, the camera will limit shutter speed to avoid blur.
3. So if you want total predictability of the background, use manual, and set your meter to the desired ambient lighting level (I recommend you start at -2 stops, i.e. the light meter points to “-2”). See a recipe below.

In a typical room, a starting point might be 1/30th second, f/2.8, 400 ISO, and the flash pointed behind you. Auto ISO is not recommended!

B. Then, concern yourself with the flash:

1. The foreground is mainly lit by flash, not by your Av/Tv/ISO settings.
2. Canon cameras in particular try to avoid overexposing part of the picture, so even a small reflective object in the flash picture can result in a dark, mainly underexposed photo.
3. The flash exposure metering is, on most cameras, biased toward your focus points. So the camera looks mainly where you focus.
4. If you take a picture of something bright (a bride in the snow) the camera will underexpose it to give you a grey bride. If you take a picture of a dark object (a groom in a coalmine) the camera will overexpose it to give you a grey groom.
5. To fix this, you can turn the flash up and down using flash exposure compensation (“Flash Exp Comp”).
6. Play with the light: aim your flash at walls or ceilings if you can. and create a “virtual umbrella”.

Try it and see if you get more consistent!

Here’s a typical recent flash picture, of a nice photographer I met recently:

A flash photo - yes really.

Doesn’t look like your usual “deer in the headlights” snap? That’s because I was following my own suggestions above. Note I also used a Honl Photo 1/2 CTO gel, to make the flash light look a bit more like the background Tungsten light. I like warm backgrounds, but I often make them a tiny bit less warm this way.