Flash: Dark backgrounds with flash
Flash is good for illuminating nearby subjects, but gets weaker with distance. In doing so, it follows what is known as the ‘inverse square law’. This tells us that the brightness of the light will be inversely proportional to the square of the distance from the source of the light.
To put that in non-mathematical terms, it means that if we double the distance between the light source and the subject, the brightness of the light will decrease to one-quarter its original value.
From a practical point-of-view, it means that if you get the flash exposure right for the main subject, the background is likely to be underexposed. The amount of underexposure will depend on the distance between the subject and the background.
There are several solutions to the problem. One is to keep the subject close to the background − but watch out for unattractive shadows of the subject on light-toned backgrounds. Better solutions are to use bounce flash to spread the light, use multiple Speedlites to illuminate the subject and background separately, or use a long exposure time to pick up any ambient light from the background.
In setting 2 both the subject and the background appear well exposed. This is because the subject is relatively close to the background. Setting 1 has a darker background. That’s because the distance between the subject and the background has been increased (with the photographer also moving further back in the room). The camera was set to program (P) mode for both photographs, with the exposure automatically determined by the camera. In each case, the camera has given correct exposure to the subject nearest to the camera, leaving the rest of the scene to look after itself.
Using aperture-priority mode
If you want the background to your picture to be well exposed, but without the subject being close to the background, use aperture-priority (Av) shooting mode. This is where you set the aperture and the camera selects a shutter speed for correct exposure with the ambient light. That takes care of the background.
When you press the shutter release button, the flash fires, and its duration is automatically controlled to give correct exposure for the subject nearest to the camera. That takes care of the main subject.
Both exposures are captured at the same time, giving an image with a correctly exposed main subject and correctly exposed background.
With aperture-priority mode in low light, the camera is likely to select a slow shutter speed, which means that the image might be affected by camera shake if you do not use a tripod. You will also need to ask your subject to remain still during the exposure.
Long interiors are almost impossible to photograph with one Speedlite - the single light source cannot give good exposure to both near and far subjects (without Av). The answer is to shoot in aperture-priority (Av) mode, letting the flash look after the near subject and the ambient light handle the rest. Here, bright sunlight coming through the windows gives good detail to the distant tables (with Av).
Here is a dramatic demonstration of the Av mode flash technique. The first picture (Program) shows the main subject illuminated by flash in program mode. The exposure of 1/60 second at f/8 is too short to capture any background detail. Program mode does not allow the shutter speed to drop below 1/60 second with flash, to avoid camera shake blurring background subjects.
In the second picture (Combined), Av mode set an exposure of 4 seconds at f/8 to record the floodlit building, with the automatic flash giving good exposure to the foreground subject. In effect, two exposures have been made at the same time − one for ambient light, the other for flash. A tripod is essential for this type of night photography, to avoid blurring the background subject. Slight movement in the foreground subject is not a problem as it will be ‘frozen’ by the brief duration of the flash.