How to create a flash photography effect
First create a spotlight. Change Decay Rate to Quadratic. This will give us a more realistic falloff to our light. To compensate for this falloff, increase the intensity to 50000. Increase the cone angle to around 110 and the dropoff to 1. Turn on Use Raytrace Shadows on the spotlight and slightly increase the light radius to 0.010. This will give us a soft edge to our raytraced shadows.
Create a single polygon plane and position it behind the spotlight. Scale the shape of it to make a rectangular shape.
Increase the incandescence to a high white value. You might want to increase the incandescence value above one in the colour chooser. This will give you an extra bright value for your flash that will show up well in the reflections in your scene..
Group the spotlight with the polygon reflector plane and scale them down to about 0.01. By doing this we are scaling them to the approximate size of a real camera flash. Use a transformation tool to better see the position of our newly scaled flash light
Position the spotlight and reflector group in the centre of the camera as shown in the image above. Middle drag the spotlight and reflector onto the camera in the outliner. This will group them to the camera ensuring that when you move the camera the light and reflector will follow.
Press 6 to activate Display Texture in Mayas perspective viewport. Press 7 to activate Display Light. You can now
interactively adjust the spotlights intensity and see the results in realtime. You might want to see the effect of increasing and decreasing the dropoff on the spotlight. When you are happy with the results hit render.
Flash Texture and Glow
If you want to go a stage further, you could map a bitmap of a real flash light into the colour and intensity channels of the spotlight. This would break up the emitted light and would produce more real world irregularities.
You could also add a lens effect shader glow so that the reflections of the reflector plane will flare.