Flash is measured by the amount of TIME that the light from the flash is illuminating the subject.
This means that the smaller the aperture (say f22) the longer the flash needs to be illuminating the subject in order to obtain the correct exposure on the film. And of course the reverse is true when a larger aperture is used (say f4), i.e. a shorter flash time is needed in order to obtain correct exposure of the image.
Fill-in flash is used to reduce contrast in a scene that may contain a wide range of light levels in excess of the capacity of the film or digital sensor. Such occasions may be where a subject is back lit, or where shadow detail may be of importance in the presence of a high light level.
If you have a computer-controlled flash unit there is a way to reduce the contrast. Firstly, take a normal reading with the camera's exposure meter and set the aperture and shutter speed accordingly. Remember - some cameras can only use shutter speed below 1/100th sec (usually 1/60th sec). Others may indicate the shutter speed for electronic flash by a red dot on the shutter speed dial, or may have an X on the dial indicating the correct setting for flash. So you may have to use an aperture that corresponds with that speed setting to obtain correct exposure.
The trick now is to fool the FLASH unit into believing that you have a film in the camera that is FASTER than the one actually in the camera. Set the FILM SPEED on the flash to a setting twice the ISO of the film you are using. For example, say you are using 100 ISO film, set the flash unit to a film speed of 200 ISO. This will make the flash unit calculate a shorter duration time for the flash by believing that the lens on the camera was set to a larger aperture. Using this method reduces contrast and improves detail in shadow areas.
If you have a flash unit on which the film speed setting is fixed, what you will find is a scale indicating the aperture to set the camera to for each particular film speed. For example, most small camera flash units give a value of f4 for a film of 100 ISO. To use such a flash for fill-in flash may be possible by using the following method. Say you are using 100 ISO film. Set the lens aperture to f4 if that is the setting the flash indicates is the aperture for 100 ISO film. Meter the scene and determine the shutter speed for f4 lens aperture. You may find that the shutter speed calculated by the exposure meter will be greater than the shutter speed that your camera uses for flash. This problem can be overcome and I will deal with that in a moment. Now close the aperture down one stop - in this case to f5.6. This means that the level of the flash reflected from the subject will be reduced by one half compared to what it would have been if you were simply using the flash alone and not mixing it with ambient light. However, there will be enough relative illumination from the flash in combination with the level of ambient light to balance the highlights of the scene with shadow illumination to fill in the shadows.
As mentioned above, the shutter speed indicated by the exposure meter for correct exposure at the aperture the flash requires may be in excess (i.e. shorter than) the maximum speed for synchronisation of the camera's shutter with the flash (it may be 1/250th sec or more!) The only way to overcome this problem is to reduce the overall illumination reaching the film. This can be done by using a neutral density filter on the lens. ND filters are simply filters that restrict the passage of light through them by a known amount and are usually supplied having values increasing by factors of two. Hence a x2 filter will reduce light level by two times, x4 by four times etc. This means a x2 filter will reduce light level by one stop, x4 by two stops, x8 by three stops and so on. A polarising filter can also be used as a x4 ND filter, as it also works as a neutral density filter which reduces light level by two stops.
Many modern compact cameras and SLRs have small flash units built into them, and computer programs associated with them will work out the amount of fill-in flash needed to achieve even lighting when strong back light is encountered. However, it is my experience that the flash levels from these units are far to strong, probably because the designers believe that in most cases the unit will be used for snap shooting people and it's the face of the friend or beloved that is to be recorded, not so much the environment he or she is posing in!! In most cases you have to take what you are given as there will be no way, apart from seeking out a sympathetic camera technician, to set the camera for a less brilliant level of fill-in flash.
Fill-in flash is useful for photographing people outdoors. If you have the person facing the sun, the light may be so strong that the individual(s) will need to squint. Positioning the person(s) with their back to the sun alleviates this problem, but now requires some frontal lighting to balance the main light. Fill-in flash is useful in such situations. However, to be effective, fill-in flash should provide just enough illumination to balance the ambient light and therefore its use should not be discernible in the final image.