Digital photography may capture light on a sensor instead of a frame of film, but your lighting goal is the same: to use the correct ratio, or mix, of a flash and ambient light. You’ll know that you’ve found the right ratio when it reproduces the shadows in your images how you want them to look, with or without details, or somewhere within this range. To understand this concept and to use it correctly, don’t be limited in your thinking that a flash unit is always for the purpose of filling the shadows that the main light source (the sun outside) doesn’t. It’s better to think of them in terms of a proper mix because there may be occasions when the flash should be the main light source and sunlight secondary.
Mixing Flash and Ambient Light in an Interior Setting
For example, you want to photograph a person sitting in a room. Without knowing any better, many photographers would use an on-camera flash unit, set an aperture of f/8 and choose a shutter speed that matches with the camera’s fastest flash-sync speed. Such a photo will have plenty of depth of field, but the lighting will be terrible. A better option is to shoot without the flash; and a common exposure to use is an ISO of 400, f/4 aperture and a shutter speed of 1/60th of a second. You also have the option to adjust your exposure to 1/125th at f/2.8 for more speed or 1/30th at f/5.6 for more depth of field.
Now that you know a good exposure for just the ambient light in the room, it’s easy to mix this light source and a flash unit. It this situation, the flash is the primary light source and the ambient light will be the fill. To create the proper mix, you may want to mount a flash unit on a stand and bounce it off the ceiling directed at the subject. If you decide to change this angle, then you can maintain the proper exposure by making sure the new light angle is the same distance from the flash to the subject.
The next step in this process is to reduce the ambient exposure of 1/60th at f/4 by two stops to reveal shadows with details. This results in two exposure options if your camera’s sync speed is as much as 1/250th of a second. First, you can leave the shutter speed unchanged at 1/60th and choose an aperture of f/8, which will create more depth of field. Second, don’t change the aperture at f/4, but change the shutter speed to 1/250th. This will give you faster recycling, so you don’t miss an excellent expression on the person’s face, for example. You actually have a third option, which is between the first two, or 125th at f/5.6. The combination isn’t as important as achieving an underexposure of two stops, by starting with the correct ambient-light-only exposure. Once you’ve determined this ratio, you’ll simply manually adjust your flash unit until you have a well-lit subject.
As you use this process more, you’ll be able to evaluate the information your screen is providing, quickly and instinctively, and even begin to make very close guesses as to where to set your flash manually. In many cases, you only have a few minutes to photograph the person in the room, so this process allows you take as little of his or her time as possible and also capture images with excellent lighting balance.
As you become even more proficient, you’ll be able to fine-tune the manual setting of flash exposure in less than a full stop. Your flash manual should include instructions. You’ll want to learn how to make these smaller, incremental adjustments because it virtually guarantees the flash will produce consistent light hitting the subject exactly the same during each photo. This will also save you plenty of time.
Learning good lighting techniques always requires some practice and exploration. Ask someone to be a test subject the first time you use the process in this PhotographyTalk.com article to mix flash and ambient light for better photos.
Read these PhotographyTalk.com articles for more information about lighting and flash.