Manipulating the flash’s light with a gel or filter is a key technique, when combining flash with fire or light trails.
Light Only the Foreground
Did you know you can adjust the on-camera flash’s intensity, such that it only illuminates what is immediately in front of the camera? This is a good trick to use if you want to light something that is in shade, but want to maintain the ambient light that is there in the rest of the frame.
Blow Out the Subject
Remember those bad party photographs where the person in the group who is closest to the camera looks like a ghost? Now, think of doing the same thing, but with a creative twist… blow out the subject deliberately.
This can be done by firing the onboard flash at full power (though it shouldn’t affect the rest of the shot) or by placing an external flashgun, such that it fires at the subject, but not elsewhere in the scene.
Subjects that receive flash will be comparitively sharper, while the background can be nicely blurred in frantic panning shots like these.
This can create a stark and disconcerting feeling, but can also make the image graphic and attractive. It can particularly look interesting in black and white. Australian photographer Trent Parke has used this technique several times too.
Colouring the Flash
The light emitted by the flash may look odd when combined with ambient light, so experiment with the use of gels or filters. If there is no gel around, use a coloured gelatin paper, or even a coloured-glass beverage bottle. These can give some interesting results!
Just a Hint of Sharpness
If there are a lot of moving subjects or you are moving the camera to pan or track, flash can momentarily freeze movement and give some much-needed sharpness.
If you are faced with a glass or reflective surface, either bounce the flash or change your shooting angle. The light reflecting off the surface will do so at an angle that is equal to the angle of incidence (basics of physics laws!).
Revisit Your Dreams
Long exposures with flash can look eerie. Also, if there is a lot of dust in the air, it shows up as surreal circles in the frame, if you fire the flash in the dark.