The Use of a Flash Gun

Hi Chris,

Are you sure your flash gun is not powerful enough? I would be surprised if it wasn't unless the distance between you and the subject is extraordinarily large or there is something else unusual happening. What you are describing is using your flash for fill light as opposed to key light.

You didn't mention how you use your flash gun, so just some tips to try (and for anyone else interested as well) in case you are not aware.

1. If you are using ETTL for your flash exposure then that could be one reason why the flash is not bright enough to light the faces. In ETTL mode your camera calculates an ambient and flash exposure to come up with an overall exposure for the scene. If you have a bright ambient conditions (eg. sunlight) then the amount of flash power the camera calculates might not be enough for what you want. You can compensate just the flash exposure using the flash exposure compensation adjustment in your camera, but that might not always be enough depending on your shooting circumstances.

2. This is my preferred option for fill flash if I have time to set up a shot first.

a) Switch both the camera and flash to manual mode.

b) Set up your ambient exposure (Aperture, shutter speed, ISO) using your preferred option for metering.

c) Take a test shot without any flash and adjust the ambient exposure if required.

d) Turn on your flash and set the flash power to a suitable starting point based on your experience. If not sure, you can set it to maximum power as a starting point.

e) Now take another test shot with the flash

f) Hopefully the flash exposure looks good or is too bright if you set flash power to maximum as your starting point.

g) If the flash exposure is not correct, just dial it up/down a bit on your flash gun (depending on what your flash power starting point was) and take another shot until the flash exposure is correct for the effect you want. It shouldn't take more than 2-3 shots to get the correct flash exposure.

3. The amount of light from the flash hitting your subject obeys the Inverse Square Law. For example, if you halve the distance from the flash to subject then the amount of light hitting the subject from the flash is now 4 times what it was previously. So moving just a small distance closer to your subject can greatly increase the amount of light from the flash hitting the subject, which just might be enough to lighten up faces.

4.I'm a firm believer of minimising the amount of time I spend post processing (but that's just me).If I have to spend too much time in post, then I didn't capture the image properly in the first place. No matter what post processing technique you use to "correct" for unwanted shadows, they will on most occasions not be as good as exposing correctly in the field. The danger in correcting shadows is that you will introduce noise and noise on faces is the last thing you would want.

Anyway, just some food for thought.

No problem Chris 

There are other options you can try if a similar situation comes up in the future.

But first, don't place too much emphasis on Guide Number.  It is mainly a marketing strategy and only a very rough guide to the available power in a flash because manufacturers determine their flash's guide number in controlled conditions in a lab to maximise the guide number they can get to put on spec sheets and advertising.

1.  If you have wireless connectivity between your camera and flash gun, perhaps take the flash off the camera and place it closer to the subject on the ground or wherever appropriate while still being out of the frame in your view finder.  As i mentioned in my earlier post, moving the flash closer to the subject by just a short distance can significantly increase the amount of light from the flash hitting the subject.

2.  If your flash has a High Speed Sync mode, you can use shutter speeds faster than the sync speed (1/200 sec on most cameras) with your flash gun.  With faster shutter speeds you can use a larger aperture and so increase the amount of light (flash + ambient) reaching the sensor.   

The sync speed is the fastest shutter speed you can set with the shutter's second curtain only beginning to move up to close the shutter after the first curtain has fully opened and exposed the incoming light to the sensor.  At faster shutter speeds the second curtain starts moving up before the first curtain is fully open and so trails the first curtain up to eventually close the shutter.  At these higher shutter speeds (faster than 1/200s) a narrow open slit, the gap between the first and second curtains, moves up across the sensor exposing just a portion of the sensor at the faster shutter speed (1/300s, 1/1000s, 1/2000s or whatever).   When the second curtain eventually closes the shutter, the whole sensor will have been exposed to incoming light for the duration of the shutter speed but just not at the same time as with shutter speeds at or slower than the sync speed.

At these higher shutter speeds, High Speed Sync mode on the flash synchronises the flash output  to the moving slit of the shutter.  The only down side of High Speed Sync mode is that it uses a lot more battery power in your flash gun compared to a shutter speed slower than the sync speed.