Flash fill is a very useful tool for photographers. In contrasty lighting situations our camera sometimes cannot record the range of contrast in a photo scene like our eyes can. In very bright situations, the perfect exposure will favor the highlights in the scene and often to the detriment of the shadows.
Flash fill can add light to the shadow areas reducing the harsh contrast created by bright sun or ambient light. It can also brighten up dull images that are taken in flat light. In fact I always recommend the use of a flash whenever shooting people and most subjects in full sun and in flat light.
Subjects outside when the sun is high overhead can suffer what is called ‘raccoon eyes’, dark eye sockets on a brightly lit face. Back-lit subjects can also benefit from flash fill by outputting flash into the front shadow side reducing the lighting contrast. Wildlife photographers are known to use flash simply to add a ‘catchlight’ to an animals’ eyes.
These backlit photos show the benefit of flash fill.
Flash vs. Ambient Light
Think of flash fill as the use of two light sources, the available light and the flash light. The available ambient light becomes the key light or main light while fill flash is the secondary light. By adjusting the output from the fill flash, the lighting contrast ratio is manipulated.
Key point to fill flash: Adjust your flashes exposure, not the cameras.
Here I was photographing canoeing and the early morning sun is about 2:00 o’clock from the camera at 6:00. This is a contrasty light that is almost back-lighting. I chose flash to open up the shadow side of the front subject. If you look at the guy in back he is not benefiting as much from the flash fill and this shows you the difference.
Applying Flash Fill
Flash fill can be applied in many ways and it depends on the contrast of the lighting on the subject as well as angle of the ambient key light. It can be applied at full output or any level below that.
For example, when you meter your subject for the ambient exposure and apply those settings on the camera, you have set a proper ambient exposure. Then you turn on the flash and can use it at full output level and this will work well in cases like back-lighting.
For side lighting, full output flash might bring the brightness level up to equal the ambient level on the subject. This creates flat lighting on the subject when the ambient light side is equal to the flash lit side.
So now you employ ‘flash exposure compensation’ (FEC) and you do that by lowering the flash output level so that it sends less light to the subject. In this situation you are putting less light into the shadows of the subject. This brightens dark shadows but not up to the level of the ambient exposure. For example, if your camera meter reading is 1/125 @ f/11 then a minus 1 flash fill would mean the flash outputs one stop less light or the equivalent of output for a proper flash exposure at f/8.
You can see it in these two images, taken on assignment to photograph a geologist, that the left image has flash equal to the ambient exposure. The aperture is f/11 and the flash is output is equal to f/11. On the right, using a -1 FEC the flash now outputs the equivalent amount of light for f/8.
You set FEC in two ways: if you are using TTL flash you can set output with the FEC button on the flash or camera, depending on make and model. If you are using manual flash, then you would change the output level of the flash.
Here is another example of using flash to fill with two hikers on a hot sunny day.
In this example, the light inside the dark forest is dark and muddy, so adding flash brightened the subjects in front nicely, but not the couple in the rear. This clearly shows the difference with and without.
This adventure lifestyle shot of a couple crossing the creek shows the flash fill benefit. Since they are back-lit their front side would be very dark without flash. There is no FEC here, just full output flash to bring the front up to normal brightness.
On this rafting trip, the rower was also backlit and flash at normal output brightened the subject perfectly.
Flash is a great tool for softening lighting contrast with your outdoor subjects. How much flash fill to use depends on your lighting contrast. I like a -1 FEC for most flash fill uses when in bright hard light, but as mentioned in back-lighting full flash fill becomes the main light source on the front of the subject and proper output often works best. Even on overcast days, flash fill at something like -1.33 or even -1.66 pops in just enough light to brighten darker areas on your subject.
If you have any thoughts please leave a comment.