Stephen A. Dantzig gives a lesson on how to control exposure by understanding the quantity, quality, color temperature, and additive nature of light. Read the excerpt from his Amherst Media book.
This excerpt from Softbox Lighting Techniques for Professional Photographers is provided courtesy of Amherst Media. To purchase the book and learn more about the publisher, visit the Amherst Media Web site.
Light is governed by many laws of physics. These laws can be manipulated in ways that produce predictable results. Anyone can grab that perfect shot; however, understanding the laws and manipulating them accordingly will allow you to consistently create the perfect shot.
I promised in the introduction that I would try not to get too “techie,” but there are some technical issues that we need to address up front. I have chosen to look at four technical aspects of light that will influence each photograph you create and are particularly pertinent to the creative uses for a softbox. These aspects, presented in an arbitrary order, are: (1) quantity of light and how to measure it, (2) quality of light and how to manipulate it, (3) color temperature and how to play with it, and (4) the additive nature of light (or how do I add lights to my scene and still control how the image looks?).
Quantity of light simply refers to the amount of light that you are recording (photography is actually the process of recording light that is reflected off of your subject). Quantity of light is directly related to your exposure.
A “correct” exposure shows the full range of colors (or grays) available to the particular type of photography you are engaged in. If, for example, you are shooting black-and-white negative film, then you have an extremely wide range of grays between black and white that you can capture. The current state of digital photography, on the other hand, allows us to capture less than half of the tonal values available to some black-and-white film. As the exposure latitude becomes narrower, the images have more contrast, and the demand for exposure control become more exacting. This holds true for hot lights, sunshine, or strobes. It doesn’t matter what you are using to light your subject, you need to know how to record it accurately. Every image has a “proper” exposure. However, a “properly” exposed image will have a “neutral” section (usually defined by the main lights), a section that is underexposed in relation to the main exposure (known as the shadows), and a section that is overexposed compared to the main exposure (known as the highlights).
YOU decide what the most important part of your image is and meter accordingly. These two images of my friend Kathryn show the importance of critical metering. The difference between these two shots is dramatic even though she moved only about one foot from one image to the next. However, the handheld incident meter was positioned at the color card with the dome pointed at the lens of the camera. The color card is perfectly exposed in both images.
The discussion about exposure and metering is critical to understanding how to accurately expose an image. However, knowing these rules can also help you break them! Photography is an art form that lends itself to subjective creativity. This image of Kathryn is not properly exposed. In fact, it is overexposed and then manipulated further to enhance the overexposure. But for this image and the effect we were after, it is the correct exposure. The key is that the effect was created on purpose: we took the “rules” of how to properly expose a digital image and tweaked them to get this look. It must be noted that the image was not usable as shot. A Photoshop technique detailed on pages 34–35 was used to make the image work.
No, the only way to accurately and consistently judge your exposure is with a quality light meter. There are two types of light meters, and they both do the same thing. An incident meter reads the light that falls on your subject and tells you the exposure for the middle of your latitude. A reflective meter reads the light that bounces off your subject and provides the value for the midtones. I personally prefer the handheld incident meter for the majority of my work. Place the meter at the critical point of the image—for example, at your model’s cheek—and point the dome of the meter right at the camera, not the light! Use this technique for any light that strikes the front of any part of your set. Meter all other lights (lights that add highlights to your subject’s body or hair) by aiming the dome of the meter at the light source. Start by trying to keep all of your exposures within 1 stop of your main exposure. Do use your LCD to make sure that you are holding the highlights when your model is wearing white. Furthermore, many digital cameras are not true to their reported ISO (the digital equivalent to “film speed”). Some cameras are as much as a full stop under the rated ISO. It is extremely important to test your camera under controlled situations to determine your ISO! Understanding how to use your meter will help you previsualize the image and will help you place the shadows and highlights where you want them.
In contrast to the image of Kathryn, the photograph of Michelle was accidentally underexposed by about 1 stop. It took a couple of Curves adjustment layers and some specific color correction to get an acceptable image. The first Curves adjustment made the basic corrections and brought the flowers back to life. A second Curves adjustment brightened the whole image but blew out the highlights in the flowers. The flowers were brought back by “painting” them in using the adjustment layer mask. (To do this, select the mask in the Layers palette and use the Brush tool with the foreground color set to black to paint in the layer beneath the second Curves layer.) The Curves adjustments worked to bring the image back into an acceptable tonal range, but the underexposure created color shifts that needed correction. I used a Selective Color adjustment layer to fix the red shift. With the Red channel selected, I moved the Cyan slider to the right to add cyan to the whole image. I then made a second selective color adjustment layer and added a little yellow to the Red channel. Next, I used the mask and Brush tool to isolate the color correction to under Michelle’s neck. Did this underexposed image work? Yes, but it would have been much easier to get it right in the camera.
Quality of light refers to the effect that your light source creates on your subject. It determines how soft or harsh the light looks in your image. Soft light will have a slow and gradual shift from highlights, to neutrals, to shadows. The shadows will be light and smooth. Harsh light will have a sharp transition from highlights, to neutrals, to shadows. The shadows will be deep and dark.
Another way to think about quality of light is to think about how much contrast the image has. Photographers use lighting ratios to describe the amount of contrast in an image. These numbers describe the amount of light illuminating the highlight side of the image versus the shadow side. A 3:1 lighting ratio will have less contrast and will therefore have a softer quality of light than a portrait with a 5:1 ratio. A 1:1 ratio is very flat, and a 7:1 ratio is almost black and white. Your choice of capture medium will also influence your lighting ratio. Black-and-white film can “hold” much more information than digital capture, so you can play with more dramatic ratios with the former medium.
Quality of light is influenced by the interplay of two factors: the size of the light source in relation to your subject and the distance from the light source to your subject. Keeping the distance from your subject equal, a larger light source will provide a softer quality of light than a small light source. Small light sources are focused and only illuminate a small section of your subject, whereas large light sources, when used properly, will light the area targeted and wrap around the subject to add light to the shadow side as well. Another telltale sign about the quality of a light source was introduced earlier: each light will leave a specular highlight—basically, this is a reflection of the light off of your subject. Small light sources leave highly focused and sharp specular highlights. Large light sources leave specular highlights too, but they are broader, more spread out, and softer. Consider the examples below.
This image sequence shows the effect of the size of the light source on the quality of light.
Image 1—The spotlight is a harsh light source because of its relatively small size. The shadow off of Marissa’s nose is deep and dark. The transition from highlight to shadow is quick and sharp. The specular highlight is focused and bright.
Image 2—The image created with the umbrella is still fairly harsh, but the larger light spreads the specular highlight a little and illuminates the shadow side more.
Image 3—The small StripDome creates a distinctly different look from the spotlight and umbrella, but it creates a harsher quality of light when compared to the other softboxes in this sequence. Notice the difference that the softbox makes in the color of the image. This has to do with the color temperature of the light source. We’ll touch on color temperature issues later in this book.
Image 4—The medium StripDome begins to show more of the softer light that is characteristic of softboxes. The wraparound effect is beginning to become apparent as the shadows on the left side of Marissa’s face are less dense.
Image 5—The large LiteDome is one of my favorite modifiers. (Note: Different softbox manufacturers have different designations for their sizes. The softbox used here is about 30x40 inches.) It produces a beautifully soft light while maintaining a nice degree of contrast. The specular highlight is still apparent but is spread out. The shadow side of Marissa’s face is still defined, but the shadows are soft and gradual.
Image 6—The extra-large LiteDome is a beautiful light source. (Note: Some companies consider the 40x60-inch box large rather than extra large.) The light is smooth and even across Marissa’s face and into the shadows. The specular highlight blends into her skin. The extra-large softbox can be too soft for some applications but is perfect for other jobs. For example, the low-contrast light is great for photographing white garments against a white backdrop. We’ll explore ways to take advantage of the soft nature of light from this sized softbox and still provide a “pop” to your subject’s face.
If the size of the light source determines the quality of light, then why isn’t the sun a good light source for most of the day? The size of the light source is only one factor in the equation; the distance of the light source from the subject also plays a role in how harsh or soft your light will appear. The sun is a huge light source. It is also very far away. The extreme distance that the light travels to reach the Earth (and your subject) turns the huge light into a pinpoint light source that creates deep, ugly shadows for much of the day. The series of images just displayed shows the dramatic difference between a spotlight and a 40x60-inch softbox. However, if we had the room (and power!), we could keep backing the big box up until it produced the same quality of light as the spotlight.
Color temperature is important because selectively altering the color temperature of different light sources can be a creative field day! It is also very important to understand how color temperature works, because different types of lights will have different color temperatures. It was noted at the beginning of this book that some softboxes may be adapted for use with hot lights. Hot lights are simply lights that are on at all times—they may be floodlights, for example. Floodlights will have a different color temperature than either most strobes or high-noon sunlight (unless you get “blue” floodlights). Have you ever taken a photograph indoors with a roll of daylight-balanced film and the flash did not go off? The resulting image had an orange-amber cast to it because there was a mismatch between the color temperature of the film (or white balance) and the lights used to create the image. Most indoor incandescent lights are tungsten lights and have a lower color temperature than sunlight. Most floodlights are tungsten lights too. In contrast, most strobes are “daylight balanced,” meaning they have a higher color temperature than tungsten lights. Daylight at high noon has a color temperature of about 5500 to 6000 degrees Kelvin. Tungsten lights are about 3200 degrees Kelvin. Blue floodlights are close to 5500 degrees Kelvin. Daylight can have different color temperatures too. Early-morning and late-afternoon sunlight has a lower color temperature than high noon. The color temperature of light in open shade, in the mountains, or under a cloud cover is higher than high-noon light.
Go back and review the set of six images above, which showed the effects of different-sized light sources on the quality of light. The images taken with the softboxes were “softer” than the spotlight and umbrella, but they also have a different color. The spotlight was actually a strobe fitted with a 7-inch silver parabolic reflector. Lights that are reflected off of silver materials will have a higher color temperature. In contrast, light that is diffused by white materials (as is the case with softboxes) has a lower color temperature, resulting in a warmer image.
So, what do all of these numbers mean? Your capture medium (film or white balance on your digital camera) needs to be in sync with the color temperature of your light source or you will get a color cast to your images. The amber cast in the “flash didn’t fire” image is there because the film/white balance was expecting a higher color temperature than the lights provided. Lights that produce a lower color temperature than your film/white balance anticipates will result in images with a warm amber color cast. Lights that produce a higher color temperature than your film/white balance expects will result in a cool blue color cast.
The color cast that results from a color temperature mismatch is not necessarily a bad thing. Sunset beach shots can be beautiful because of the amber cast. Selectively placing a light with a higher color temperature can result in dramatic fashion images or add distinction to an executive portrait. We will talk about how to manipulate color temperature for effect later. See pages 98–99.
The additive nature of light is one of the more complicated aspects of light and lighting. It is mentioned here because we will be showing some fairly complex lighting schemes in this book. The exposure of each light has been meticulously determined to add a specific effect and to stay within the narrow exposure range of transparency film and digital capture. Simply stated, each light that is added to a scene will add to the overall exposure of the area that is lit by more than one light. You can figure out what each light will add to the scene because the amount of light added is directly proportionate to the other lights on the set. Light is measured in “stops,” a term that describes the unit of light referred to by a given aperture. The aperture of your lens is actually a ratio of the diameter of your lens opening to the focal length of the lens. The term f-stop refers to the diameter of the lens opening, or aperture. Therefore, in this sense, “stops” is a relative term. Adjacent f-stops are exactly 1/2 as large as the next larger aperture— or twice as large as the next smaller lens opening. For example, an f-stop with a value of f11 allows twice as much light into the camera as f16 and four times as much light as f22 because the lens opening is two times larger than f16 and four times larger than f22. Conversely, it is 1/2 as large as f8 and 1/4 the diameter of f5.6, so it lets in 1/2 and 1/4 as much light as the two larger openings respectively. This information helps you to understand and calculate how different lights interact and add to your exposure. For example, if you add a light that is 1 f-stop less than another light, then you will add 1/2 of a stop of light to the exposure where the lights overlap. If you added a light that was 2 stops less than another light, then it would add 1/4 of a stop to the exposure.
The additive nature of light also helps determine lighting ratios. When photographing people, you are concerned with controlling the range of detail from the highlight to shadow side of your subject’s face. The highlights are determined by your main light. The main light determines the base exposure for your image and establishes the direction of your lighting scheme. The amount of light that illuminates the shadow side of your subject’s face (the side opposite the main light that is not lit by your main light in a “traditional” portrait setup) depends upon whether you choose to add light to—or fill—the shadows. A very common lighting ratio is a 3:1 ratio. With a 3:1 ratio, there is three times more light on the highlight side than the shadow side. To use the f-stop examples listed in the previous paragraph, a fill light that is positioned to light both sides of your subject’s face equally and set at 1 stop less than the main light would add one unit of light to each side. The main light, in this case, would add two units of light to the highlight side (because it is twice as bright as the fill) and in theory (depending on your choice of main light), no unit of light to the shadows. Two units of light from the main light plus the one unit from the fill light on the highlight side, and the one unit from the fill in the shadows, results in a 3:1 ratio. You can play with other variations and combinations, but you won’t have much luck beyond a 5:1 ratio given the exposure latitude of digital capture.