Harmonizing Flash with Ambient Light
By Steve Sint
Published by PixiqLearn how to set your flash and exposure to capture natural-looking images of people in mixed light with this excerpt from Steve Sint's Pixiq book Digital Wedding Photography: Art, Business & Style.
This excerpt from Digital Wedding Photography: Art, Business & Style is provided courtesy of Pixiq. To purchase the book, visit the Barnes & Noble Web site.
Many photographers already know that midday light is awful. A big, beautiful noonday sun beating down on subjects from directly overhead creates inky black shadows under our subjects’ noses and chins and makes their eyes recede into dark sockets under the shadows of their brows. Very often, such beautiful days (though they are what every couple hopes for) don’t lend themselves to making beautiful portraits at midday. But whether faced with soft light or glaring light, the wedding photographer must prevail! Every couple wants outdoor pictures, whether the light is cooperative or not. Against all odds, with only a flash unit in their DSLR’s hot shoe, a wedding photographer can still create beautiful pictures. Even though photographers new to the wedding game are often afraid to try flash outdoors, it is a tool that can save the day.
To a novice, mixing sunlight and flash is voodoo, but there really is no magic involved. You are just mixing two ingredients, and there are recipes you can follow. By understanding how the ingredients work, you can control how much of each ingredient to use in your recipe. Ingredient number one is ambient light (you can substitute the word daylight, sunlight, skylight, available light, or moonlight here), which is a continuous light source. You can control the ambient light’s exposure on your image by changing either the aperture or the shutter speed.
The second ingredient is the burst from your flash, which is an instantaneous light source. You can control flash exposure by changing the flash’s distance from the subject, its power output, or the lens aperture. Flash exposure, for the most part, is unaffected by your choice of shutter speed (as long as the shutter speed allows flash synchronization). This means that by changing the shutter speed you can affect the ambient light portion of the recipe without affecting the flash. And, by adjusting the flash unit’s output or changing its distance from the subject, you can affect the flash part of the recipe without changing the ambient light exposure settings. You can also choose a comparable set of f/stops and shutter speeds (for example 1/250 at f/5.6 and 1/125 at f/8) and the change in f/stop between the two will affect your flash exposure without affecting the ambient light exposure. This is where camera designs that offer high-speed flash sync really come into their own.
With flash and ambient daylight as your light sources, you must decide if you want one light to be dominant, and if so, which one. Or do you want the two sources to have equal value? Once you’ve made that choice, getting the two light sources to complement each other is the name of the game. The best way to understand this is by walking through a hypothetical scenario for each possibility.
In the first scenario, consider ambient light the main source and flash the fill light. We are outside at noon and we’re looking at a beautiful bride with black shadows under her brow, nose, and chin. We are in the middle of a field. If there was a nearby tree with leaves, I might suggest we work in its shade. But this scenario includes harsh sun and no shade, so we must make do. An incident meter reading of the ambient light falling on the subject calls for an exposure of 1/125 second at between f/11 and f/16. Set the camera to these settings. The next step is deciding how much light you need to fill in the shadows caused by noonday sun.
From hard-earned experience, that I openly share here, I’ve found that I can usually eliminate excessive shadows on the face with a fill-light exposure that is about 1-1/2 to 2 stops less intense than the main light. That means the flash has to be far enough away and have enough power to create an exposure of f/8 for the subject. (Why f/8 you ask? Because it creates an exposure that’s 1-1/2 stops less than the ambient exposure of 1/125 second at between f/11 and f/16.)
I know the intensity of light from my manual flash unit is f/8 when the unit is set to half power and is 10 feet (3 meters) from the subject. Perfect. So, using a long sync cord, or a radio slave, I can position the flash 10 feet (3 meters) from my subject, and voilà—I have a fill light 1-1/2 stops under the sunlit ambient exposure. Of course, you could dial the flash unit’s power up or down, as well. There is always more than one way to skin a cat.
Although this scenario may seem easy, maybe the ambient light intensity is something different, like 1/125 second at f/5.6. What should we do then? Well, remember that flash exposure is unaffected by a change of shutter speed as long the shutter and flash are synchronized. Then, remember that 1/125 at f/5.6 is equivalent to 1/60 second at f/8, which is equivalent to 1/30 second at f/11. We could shoot at 1/30 at f/11.
I know that my manual flash at half power puts out f/8 at 10 feet (3 meters). I also know that when a flash unit is 15 feet (4.6 meters) from the subject, the situation calls for f/5.6 (see the guide number chart on page 193). So, if I want an f/stop of either f/8 or f/5.6 (which would be 1 or 2 stops below my ambient f/stop of f/11), once again, I can put an off-camera flash unit (on a radio slave again) 10 feet (3 meters) from the subject and either choose 1/2 or 1/4 power, depending on whether I want my flash exposure to be f/8 or f/5.6. Now, the fill light will be 2 or 3 stops below the ambient light exposure.
Next, let’s look at a scenario in which we want the light from the flash to overpower the ambient light falling on the subject. This might be desirable when the subject is in shade but the background is a sunlit sky. If we expose for the light falling on the subject’s face, which is in the shade, the sky in all its azure glory will be overexposed to a very bland looking white. In this example, an incident reading of the subject might call for an exposure of 1/30 second at f/5.6, while a reflected reading of the sky might call for an exposure of 1/125 second at f/11. What do we do?
Because we want the sky to register deep blue, set the camera to 1/125 second at f/16. Then move the flash unit so that it is about 5 feet (1.5 meters) from the subject. Remember from our discussion about guide numbers (see page 193) that flash exposure from 5 feet (1.5 meters) calls for an exposure of f/16. Therefore, we are ready to shoot the portrait.
This picture will produce a properly exposed bride in front of a deep blue sky. You might want to take a second photograph without the flash, with the camera set to 1/30 second at f/5.6 (meter for an incident light exposure of the subject in the shade). The two pictures will appear wildly different, and you might get two sales from these two frames. The one (with flash) will be richly colored, while the other (ambient light only) will appear high key and airy.
Even under conditions that many photographers consider perfect, such as an overcast or cloudy bright sky, you can to use a tiny puff of light to brighten the subject’s eye sockets. For these situations, I carry a white handkerchief that I drape over my flash head and hold it in place with a rubber band. The white cotton cloth cuts the flash output by about 1 stop. I use this technique when the ambient light intensity is low so that light from the flash is not unnaturally harsh.
As mentioned in this book’s introduction, there is always more than one way to do anything photographic, and with that thought in mind, you can also accomplish all of the feats mentioned immediately above by using your shoe-mounted flash set to a TTL (through the lens) auto mode. Every DSLR I have seen or used has the ability to set the flash unit’s output relative to the ambient available light. That being the case, you can try setting your shoe-mounted flash to an auto mode minus 1.5, 1.7, or 2.0 stops and see if you like the results you get. Personally, I have not gotten consistent results using this method, so I prefer to work in a manual flash mode. Furthermore, if you want use a hot-shoe-mounted flash in manual mode instead of an off-camera flash, you can use the distance information outlined above and use a zoom lens to control your framing when working at the specific distances cited.
Category: How-ToClick for more articles of: Steve Sint