5.4 Working with light🔗⧉

A face beside a window can have plenty of light and still photograph badly if the eyes fall into shadow. Move the subject or the source before changing exposure. Exposure scales the measurement; lighting changes the relationships within the scene.

Describe a source by its direction, apparent size, intensity, spectrum and timing. Direction places the shadows. Apparent size controls their penumbra. Intensity sets the photon budget and the balance with other sources. Spectrum affects color, and timing determines whether a flash freezes motion or a lamp produces bands. These properties are related in practice, but none substitutes for the others.

A large source can soften a shadow without filling it. Shadow contrast also depends on light arriving from other directions and on the surroundings. A small source can be useful for revealing texture or making a crisp graphic shadow. Soft light is a useful choice for many portraits, not a requirement for a good photograph.

Move a light sideways and the shadows move; enlarge it without changing its central direction and their edges soften. These are different changes. A small frontal source can make texture difficult to see because its shadows lie behind the relief from the camera's viewpoint. Raking side light reveals the same relief with visible shadows. A large source spans a range of directions, so a point near a shadow boundary can see part of the source while another part is occluded. That partial visibility produces the penumbra. A reflector, diffuser, or softbox should therefore be judged from the subject's position: what luminous area does the subject actually see?

Highlights provide another way to inspect the source. On a glossy object, their shape depends on both the source and the material's reflection function. A small bright source can produce a concentrated highlight; a broad source can produce an extended reflection. On glassware or a polished object, moving or enlarging the source can change the visible form more than changing exposure. Lighting controls the reflected image of the environment as well as the shadows cast onto the subject.

5.4.1 Natural illumination🔗⧉

Outdoors, separate direct sunlight from sky illumination. Direct sun is a small, directional source. The sky supplies light over a broad range of directions and fills areas hidden from the sun. A shaded face and a sunlit background therefore receive different mixtures of illumination.

Low sun changes shadow direction and often warms direct illumination because its path through the atmosphere is longer. Photographers name the windows: the golden hour after sunrise and before sunset, when low, warm sun rakes across texture, and the blue hour just after sunset, when only the sky lights the scene, evenly, cool and dim enough to want a long exposure. Clouds turn the sky into a large diffuser that lowers contrast, and haze lifts the shadows and softens distant detail. It does not substantially enlarge the solar disk. A softer overall impression can come from the balance of direct sun and sky, or from clouds and haze. Under overcast light, inspect where the brightest part of the sky lies; the illumination can still have useful direction. A doorway or overhang can remove overhead light and create stronger modeling.

Direct sun and blue sky can produce different casts within one frame. White balance chosen for one region will not necessarily neutralize the other. A reflector, a change of position or added light can simplify the mixture at capture; Chapter 24.2, Mixed-lighting white balance treats spatial correction. Correlated color temperature summarizes some illuminants, but equal color temperature does not imply equal spectra or identical rendering of skin and fabrics.

The sky simulator makes the effects of sun direction and atmospheric conditions visible. Analytic daylight models, including Preetham et al. Preetham et al. 1999 (daylight model) and Hošek and Wilkie Hošek & Wilkie 2012 (sky-dome model), provide controlled examples; a particular cloudy sky can depart from those models.

5.4.2 Flash🔗⧉

A flash delivers light over a short interval. Electronic strobes store energy in a capacitor and discharge it through a lamp; the stored energy in joules is not the emitted optical energy. Beam shape, modifiers and efficiency affect the illumination of the subject.

A guide number summarizes direct-flash reach under specified conditions:

With a guide number of 40 m at ISO 100 and the stated head setting, a subject at 5 m calls for approximately f/8. Doubling distance needs two more stops of flash exposure. The estimate assumes direct illumination and the specified beam; it does not survive a bounce or softbox unchanged. Raising ISO by a factor of four doubles the nominal guide number but does not increase emitted light.

When ambient illumination contributes little, flash duration determines motion blur even if the shutter stays open longer. On many electronically quenched flashes, lowering output shortens the pulse; other flash designs have different duration curves. Compare durations under the same definition, because a short bright peak can be followed by a tail that records motion. Edgerton's photographs in Chapter 2.6, Motion blur show how controlling illumination time can reveal events that an ordinary shutter cannot resolve.

Electronic flash changed the workflow from consuming a light source to recharging one. Flash powder burned in the open; flashbulbs enclosed combustible material in a bulb used for one exposure. A capacitor-driven discharge lamp supplies a repeatable pulse whose timing and duration can be controlled. Edgerton's work made such control a photographic instrument for studying rapid events. The energy stored electrically, quoted in watt-seconds or joules, is different from the light delivered in a chosen direction.

The square-root scaling of guide number follows from the inverse-square law. Doubling flash-to-subject distance spreads the same light over four times the area. To retain the same exposure, we need four times the emitted energy in that direction or two stops more aperture area. At fixed beam shape and sensitivity, four times the flash energy doubles the guide number. Increasing ISO changes the nominal exposure setting for the same light; it does not supply that extra energy.

We can trace the measurement in photometric units. Let $Q$ be the luminous intensity integrated over the flash pulse, in candela-seconds. For a small source illuminating a normally oriented patch at distance $d$, its luminous exposure is $H_s=Q/d^2$ in lux-seconds. A Lambertian patch of reflectance $\rho$ then supplies a luminance-time integral $\rho H_s/\pi$. The lens and sensor determine how much of that light is recorded. Converting this photometric description to photons additionally requires the spectrum and spectral response; guide number alone does not uniquely determine the photon count.

5.4.3 Flash metering🔗⧉

Through-the-lens (TTL) flash automation commonly measures a preflash and estimates the main pulse. A reflective background or a change in framing can alter that estimate. Use flash exposure compensation to adjust the result, or manual flash output for repeatable lighting when subject distance stays fixed. A handheld flash meter measures the actual illumination at a selected location.

With a short pulse wholly inside the open shutter interval, the sensor receives ambient and flash contributions:

Here $E_a$ is ambient image-plane illuminance and $H_f$ the image-plane exposure delivered by the flash at the chosen aperture. Lengthening shutter time increases the ambient contribution while leaving that captured pulse approximately unchanged. Aperture and ISO affect the rendered flash as well as the ambient. This separation lets us keep a lit subject readable while deciding how much of the room appears behind it.

A focal-plane shutter has a sync limit: above it, a traveling slit may expose only part of the sensor during one short pulse. High-speed sync extends emission over the traverse and reduces effective reach. Leaf and global shutters have different timing constraints; check the actual camera and flash combination. For daylight fill at a wide aperture, an ND filter can permit a longer time within normal sync, while also attenuating flash light.

For a backlit portrait, expose for the background detail we want to retain, then add enough flash to bring the face into the intended range. Judge the face and catchlights. A fixed “minus two stops” recipe does not describe every TTL baseline, subject distance or desired balance.

The moving slit explains the daylight-fill problem. Above a focal-plane shutter's ordinary synchronization speed, one part of the sensor is already covered while another is still being uncovered. A short flash pulse therefore illuminates only the exposed strip. High-speed sync extends illumination across the curtains' travel, usually with a pulse train, so successive strips receive light. Much of that emitted light arrives while any particular strip is covered, reducing effective output.

For example, a wide aperture chosen for shallow depth of field in bright daylight can require a shutter speed above normal sync. High-speed sync preserves that aperture choice but costs flash range. A neutral-density filter can permit a slower shutter, but it attenuates the flash as well as ambient light. Leaf-shutter systems have different synchronization behavior; their supported speeds and the flash duration still need to be checked.

Flash metering has used two different measurement times. Some film cameras measured light reflected from the film during the exposure, terminating the flash once the integrated reading reached a target. The Olympus OM-2 history describes its direct off-the-film metering. The measurement used the actual taking path, but still depended on how the photographed subject reflected the light.

A preflash instead supplies a measurement before the main exposure. The camera estimates the main pulse from that return, possibly using scene regions and focus information. The subject can move or blink between the measurement and the capture, and a changed framing can alter the metered return. Manual output is useful when the setup is fixed because it removes that per-shot estimate. It does not remove the need to recheck exposure when subject distance or a modifier changes.

5.4.4 Indirect illumination and bounce flash🔗⧉

Direct on-camera flash can produce a small-source shadow and a strong frontal reflection, which may be exactly the desired appearance. For softer directional light, bounce off a suitable wall or ceiling. The illuminated area becomes the source seen by the subject. Its location, apparent size, color and distance matter; a distant dark ceiling can consume most of the available flash energy.

A small diffusion cap on a flash does little to enlarge the apparent source outdoors. Indoors it may spread light onto walls that then become larger sources. A softbox or scrim works through the illuminated area visible from the subject, so bringing it closer generally increases its angular size.

A white reflector redirects available light into shadows. A silver surface is more directional, and a colored surface tints the reflected illumination. A black panel removes fill. Move the panel while watching the face before adding another lamp.

Red-eye is more likely when the source lies close to the viewing axis and the pupil is large. Moving the flash away from the lens axis or changing the lighting geometry can reduce the return from the eye. Software can alter its appearance afterward, but it does not change the lighting recorded on the rest of the face. The geometry is a retroreflection: an on-camera flash a centimeter or two from the lens sends light through the pupil, the retina and the blood-rich choroid behind it return it red, and it comes back along nearly the same line into the lens. Moving the source off axis throws that return cone clear of the lens, and a pre-flash that contracts the pupil shrinks it. Many animals show green or white eyeshine instead, from the reflective tapetum behind their retinas (Chapter 3.6, Animal eyes).

5.4.5 Multiple-point lighting🔗⧉

Start with one source and decide where its shadow should fall. Add fill if that shadow is too dark for the intended picture. Add a rim or background light only if separation needs it. A good setup can contain one light; the names describe functions, not a minimum equipment list.

In a key-plus-fill setup, specify what a quoted lighting ratio compares. The ratio of lamp outputs is not the ratio of illumination on the two sides of a face: the bright side can receive both key and fill, and reflected light arrives from the room. Measure or inspect the subject under the combined illumination.

The portrait simulator lets us move the key and vary its extent while retaining the same face. Compare the shadow edge, eye sockets and specular highlights separately. “Butterfly,” “Rembrandt” and “split” are useful names for recurring shadow patterns, but a light placement should serve this person's face and the photograph's purpose.

The lighting names identify jobs:

Three-point lighting combines key, fill, and rim. It is a common arrangement, not a minimum requirement. A reflector can do the fill's job without a second powered source. Separating the jobs makes adjustment easier: a brighter background need not require a brighter face.

Portrait-pattern names describe shadows on the face. Butterfly lighting places a small nose shadow below the nose; loop lighting moves that shadow sideways; Rembrandt lighting produces a lit triangle on the shadowed cheek; split lighting divides the face more strongly into lit and shadowed halves. Clamshell lighting combines a source above with fill below. The exact placement depends on the face, pose, and source size. Use the name to describe the observed pattern rather than treating a fixed angle as a recipe.