Artificial Light, Natural Light, and Your Exposure
In many situations, you’ll find yourself working with both artificial and natural available light sources. How you approach each situation is up to you. Whether you use your lights to overpower natural light or combine it with artificial lights depend on factors such as the look or style you’re going for, the type of subject you’re photographing, or the type of natural light that’s present.
How aperture affects natural and artificial light
Your aperture controls how much light comes in through the lens during a given period of time by adjusting the size of the opening in your lens. A larger aperture (determined by a lower f-stop number such as f/2.8) allows more light in than a smaller aperture (determined by a higher f-stop number such as f/22).
Changing your aperture affects how bright your artificial and natural light appear in an image equally. So, if you close down one stop (change your aperture setting from f/8 to f/11) you allow in one stop less light from both your artificial key light and your natural fill light. The relationship between the two remains the same in your image, but your overall exposure is reduced by one stop.
In a one-light setup using an artificial key light and the available, natural ambient light as a fill, you can adjust your aperture to change your overall exposure without affecting the intensity relationship between the two lights.
Notice how the exposure was affected in the photograph (taken with an artificial key light and a natural fill) as the aperture was changed between exposures:
In the first image, the scene was photographed with an aperture value of f/2.8 and overexposed it in the highlight areas.
To bring down the exposure, the aperture was set to f/5.6 in the middle image. Doing so causes both the shadow areas (whose brightness is determined by the natural fill light) and the highlight areas (whose brightness is determined by the artificial light) to come down in exposure equally.
In the last image, the aperture was set to f/11, and again, both the shadows and highlights were affected equally.
100mm, 1/15, f/4, 400100mm, 1/15, f/8, 400100mm, 1/15, f/11, 400
The role of shutter speed in manipulating your exposure
Your shutter speed affects your exposure differently than your aperture does, and this difference is especially important to understand when you use a one-light setup with natural fill.
Because a strobe and a battery-operated flash emit light only for an instant, the length of your exposure isn’t a factor in determining its brightness. The full impact of the strobe or flash is captured based on your aperture and ISO settings at the moment when the light is emitted. You can’t adjust the brightness level of this type of light by adjusting your shutter speed.
In a one-light setup, you use shutter speed mainly to change the intensity relationship between your key and fill lights.
The natural ambient light affects both your shadow and highlight areas in an exposure. To avoid over-exposing the highlights, you may need to lower the intensity of your artificial key light when the shutter speed is slowed down significantly.
Changing the shutter speed between each exposure altered the levels of contrast in this photograph of a screw. The first image has a high level of contrast because of its fast shutter speed. In the middle and last images, just slowing down the shutter speed reduced the contrast.
100mm, 1/160, f/2.8, 50 100mm, 1/30, f/2.8, 50 100mm, 1/5, f/2.8, 50