The trick is to balance the fill-in with the available light so that it doesn't look like you've used flash. This is done by making the flash output one or two stops less than the exposure for the ambient light (the harsher the light, the brighter the flash). Some system cameras and guns do this automatically. With an ordinary automatic gun, it's just a matter of setting it either to an aperture one or two stops wider than that set on the camera or changing the film speed on the gun to two or four times the speed of the film being used. Either way, the result is that the gun will give out half or a quarter the amount of light needed for a full exposure, just right for filling-in the shadows. For example, if the ambient light exposure is 1/100th at f8 using 400 ISO film, then set the gun to f5.6 or f4 at 400 ISO or to f8 at 800 or 1600 ISO. I prefer the second method. If the ambient light exposure changes I only have to set the same aperture on the gun and not think too much. With trannie, you'll probably find that you get better results by stopping down an extra half a stop, to counter the slight lift that the flash will give anything close to the camera. A hiccup occurs with cameras with focal-plane shutters. Since the aperture affects both ambient and flash exposures, it can't be set any smaller than the power of the gun will allow, causing the shutter speed to have to follow the aperture. This is no problem if the camera has a between-the-lens shutter, which synchs at all speeds, but a focal-plane shutter can only sync up to its sync speed, which puts a limit on the brightness of the ambient light that can be controlled. Modern 35mm SLRs sync at around the 1/250th mark, but older ones, and medium format cameras that use focal-planes, are much slower, limiting their flexibility.
Fill-in flash also lets you produce a more acceptable looking shot when faced with having to shoot a deep interior scene on colour film, particularly trannie, under fluorescent lighting. Rather than try to filter for the fluorescent, a problem at the best of times - worse if mixed colour tubes have been used - use flash to fill-in the foreground and set the shutter speed to allow the background to expose about half-a-stop under. Since the flash plus ambient exposure on the foreground will give approximately half-a-stop more than just the flash, the simplest thing to do is to close the aperture by half-a-stop; i.e. f4 becomes f41/2. Using second curtain sync is best, since any movement in the background which is caught by the flash looks more natural. Now, it goes without saying that you should test this technique before using it for something important. If you are using trannie film, you could shoot off half-a-dozen test frames at the start of a roll, then have the lab clip-test those frames to see how they look before having the rest of the film processed. (Clip-testing is where a few frames are cut (clipped) from the roll of film - usually the front of a 35mm and the end of 120/220, although reverse clips are possible - and processed. They can then be examined and any pushing or pulling of the film, to lighten or darken the exposures, can be decided on before the rest of the film(s) are processed.)
If you reverse the balance of fill-in flash, reducing the exposure for the ambient light and basing your exposure on that normal flash exposure, then day can be made to look like night. A foreground subject will stand out from an under-exposed background in a similar way to a night shot. This can be very effective on colour film when the subject is positioned against a blue sky.
Another effect on colour film possible with fill-in flash is obatained by using a filter of one colour, say magenta, on the camera lens, then its complementary - in this case green - on the flash. This will cause the subject, illumniated by the flash to register with the correct colours, but the background will have a magenta cast. In the example of using fill-in in fluorescent lighting, this is the way to correct for the ambient fluorescent, while keeping the foreground subject correctly balanced. (Assuming you can correct the fluorescent sufficently.) The strength of the filters can't be too strong otherwise too much of the light from the flash will be absorbed by the combination (hold them together and meter through them to get an idea of the effect they will have on the flash). There are filter kits avaliable which include exposure guidelines. However it is done, it's wise to test what effects you will get, and what exposures you will need, before using it on anything important.
© Barry Leighton FRPS