All artists, including sculptors, painters, and photographers, use light as their medium to create images. Each of these types of creative work has its own challenges. Sculptors, for example, don't have control over the light used to illuminate their work, so they must create shapes using planes and curved surfaces so that the form envisioned by the artist comes to life from a variety of viewing and lighting angles. Painters, in contrast, have absolute control over both shape and light in their work, as well as the viewing angle, so they can use both the contours of their two-dimensional subjects and the qualities of the "light" they use to illuminate those subjects to evoke the image they want to produce. Photography is a third form of art. The picture-taker may have little or no control over the subject (other than posing human subjects) but can often adjust both viewing angle and the nature of the light source to create a particular image.
But, in all three cases, light, as it appears in the finished product that we see, is a main contributor to the mood, look, and effectiveness of the artwork. In photography, the direction and intensity of the light sources create the shapes and textures that we see. The distribution and proportions determine the contrast and tonal values: whether the image is stark or high key, or muted and low in contrast. The colors of the light (because even "white" light has a color balance that the sensor can detect), and how much of those colors the subject reflects or absorbs, paint the hues visible in the image.
As a Canon EOS 40D photographer, you must learn to be a painter and sculptor of light if you want to move from taking a picture to making a photograph. This chapter provides an introduction to using the two main types of illumination: continuous lighting (such as daylight, incandescent, or fluorescent sources) and the brief, but brilliant snippets of light we call electronic flash.
Continuous lighting is light that is available all the time during a shooting session, such as daylight or the artificial lighting encountered both indoors and outdoors. The latter sources embrace both the lights that are there already (such as lamps or overhead fluorescent lights indoors) and fixtures you supply yourself, including photoflood lamps or reflectors you supply to provide light for photography.
Electronic flash includes the flip-up flash unit built into your EOS 40D (see Figure 7.1) or any external flash you choose to couple with it, including those that mount on the accessory shoe on top of the camera and units that are used off-camera. Studio flash units count, too, and aren't limited to "professional" shooters, as there are economical "monolight" (one-piece flash/power supply) flash available in the $200 price range.
There are advantages and disadvantages to each type of illumination. Here's a quick checklist of pros and cons:
■ Lighting preview. With continuous lighting, you always know exactly what kind of lighting effect you're going to get and, if multiple lights are used, how they will interact with each other. With electronic flash, the general effect you're going to see may be a mystery until you've built some experience, and you may need to review a shot on the LCD, make some adjustments, and then reshoot to get the look you want. (In this sense, a digital camera's review capabilities replace the Polaroid test shots pro photographers relied on in decades past.) While the 580EX/580 EXII have a modeling light function
One form of light that's always available is the flip-up flash on your EOS 40D.
One form of light that's always available is the flip-up flash on your EOS 40D.
(consisting of a series of low-powered bursts that flash for a period of time), this feature is no substitute for an always-on modeling lamp or continuous illumination.
■ Exposure calculation. Your 40D has no problem calculating exposure for continuous lighting, because it remains constant. The amount of light available just before the exposure will, in almost all cases, be the same amount of light present when the shutter is released. You can even use a handheld light meter to measure the light yourself, say, when you want to compare the illumination falling on the highlight areas of a subject with the light reaching the shadows in order to calculate the exact amount of contrast to expect. Electronic flash illumination, on the other hand, doesn't exist until the flash fires, and so must be measured using a preflash an instant before the main flash, or measured during the actual exposure. In the case of a digital SLR, that is tricky to do in-camera because the mirror has flipped up and is blocking the metering system built into the camera; it's more common, when such a system is used at all, to measure the light by non-through-the-lens means using a light sensor in an external flash unit. If you have a do-it-yourself bent, there are handheld flash meters, too, including models that measure both flash and continuous light.
■ Evenness of illumination. Of continuous light sources, daylight, in particular, provides illumination that tends to fill an image completely, lighting up the foreground, background, and your subject almost equally. Shadows do come into play, of course, so you might need to use reflectors or fill-in light sources to even out the illumination further, but barring objects that block large sections of your image from daylight, the light is spread fairly evenly. Electronic flash units (as well as continuous light sources such as lamps that don't have the advantage of being 93 million miles from the subject) suffer from the effects of their proximity: the inverse square law dictates that as a light source's distance increases from the subject, the amount of light reaching the subject falls off proportionately to the square of the distance. In plain English, that means that a flash or lamp that's eight feet away from a subject provides only one-quarter as much illumination as a source that's four feet away (rather than half as much). This translates into relatively shallow "depth-of-light."
■ Action stopping. When it comes to the ability to freeze moving objects in their tracks, the advantage goes to electronic flash. As I explained in Chapter 4, the brief duration of electronic flash serves as a very high "shutter speed" when the flash is the main or only source of illumination for the photo. Your
EOS 40D's shutter speed may be set for 1/250th second during a flash exposure, but if the flash illumination predominates, the effective exposure time will be the 1/1,000th to 1/50,000th second or less duration of the flash, as you can see in Figure 7.2, because the flash unit reduces the amount of light released by reducing the duration of the flash. Action stopping with continuous light sources, on the other hand, is completely dependent on the shutter speed you've dialed in on the camera. And the speeds available are dependent on the amount of light available and your ISO sensitivity setting. For example, if you're shooting sports indoors, there probably won't be enough available light to allow you to use a 1/2,000th second shutter speed, but applying that effective speed by using a flash unit is no problem at all.
■ Cost. Incandescent lamps are generally much less expensive than electronic flash units, which can easily cost several hundred dollars. If you want to use more than one light source, the costs mount more quickly with flash.
■ Flexibility. Because incandescent lamps are not as bright as electronic flash, the slower shutter speeds required (see Action stopping, above) mean that you may have to use a tripod more often, especially when shooting portraits. Electronic flash's action-freezing power allows you to work without a tripod, adding flexibility and speed when choosing angles and positions.
Continuous Lighting Basics
While continuous lighting and its effects are generally much easier to visualize and use than electronic flash, there are some factors you need to take into account, particularly the color temperature of the light. (Color temperature concerns aren't exclusive to continuous light sources, of course, but the variations tend to be more extreme and less predictable than those of electronic flash.)
Color temperature, in practical terms, is how "bluish" or how "reddish" the light appears to be to the digital camera's sensor. Indoor illumination is quite warm, comparatively, and appears reddish to the sensor. Daylight, in contrast, seems much bluer to the sensor. Our eyes (our brains, actually) are quite adaptable to these variations, so white objects don't appear to have an orange tinge when viewed indoors, nor do they seem excessively blue outdoors in full daylight. Yet, these color temperature variations are real and the sensor is not fooled. To capture the most accurate colors, we need to take the color temperature into account in setting the color balance (or white balance) of the 40D—either automatically using the camera's smarts, or manually, using our own knowledge and experience.
The only time you need to think in terms of actual color temperature is when you're making adjustments using the Color temp. setting in the Shooting 2 menu (which allows you to dial in exact color temperatures, if known). So, those occasions are the only times you're likely to be confused by a seeming contradiction in how color temperatures are named: warmer (more reddish) color temperatures (measured in degrees Kelvin) are the lower numbers, while cooler (bluer) color temperatures are higher numbers. It might not make sense to say that 3,400K is warmer than 6,000K, but that's the way it is. If it helps, think of a glowing red ember contrasted with a white-hot welder's torch, rather than fire and ice.
The confusion comes from physics. Scientists calculate color temperature from the light emitted by a mythical object called a black body radiator, which absorbs all the radiant energy that strikes it, and reflects none at all. Such a black body not only absorbs light perfectly, but it emits it perfectly when heated (and since nothing in the universe is perfect, that makes it mythical).
At a particular physical temperature, this imaginary object always emits light of the same wavelength or color. That makes it possible to define color temperature in terms of actual temperature in degrees on the Kelvin scale that scientists use. Incandescent light, for example, typically has a color temperature of 3,200K to 3,400K. Daylight might range from 5,500K to 6,000K. Each type of illumination we use for photography has its own color temperature range—with some cautions. The next sections will summarize everything you need to know about the qualities of these light sources.
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