smaller lens aperture (which lets in less light).
Your flash must be set for the film speed you are using in order to provide good
exposure. With a TTL autoflash, the ISO is set automatically by the camera and
flash. With non-TTL and manual modes or flash units, you usually have to set
the ISO yourself, often by turning a dial or pushing a button on the flash unit.
Using TTL autoflash. Automatic flash units combine all the above exposure factors
when calculating exposure. TTL autoflash is the most integrated; the
camera’s meter works in conjunction with the flash to determine how much
light the film needs for good exposure of a particular scene. The flash will
provide more power when needed, such as when the subject is farther away,
when the lens aperture is set at a small f-stop, and/or when you use a slow
speed film; it will provide less power with closer subjects, larger lens apertures,
and/or faster film.
If your TTL autoflash is built into the camera, simply set the camera on “P”
(program). If you are using a separate on-camera TTL autoflash in its default
(totally automatic) mode, set the camera on “P” and the flash on “TTL,” and
you are ready to take pictures without worrying about exposure.
Using non-TTL autoflash. Non-TTL autoflash has a light sensor located on the
flash unit, not in the camera. It varies the flash output to produce good exposure
of your subject, but not as automatically as TTL autoflash; first you have
to prompt it with the help of a scale located on the flash. The scale helps you
determine what lens aperture you can use when the flash is positioned at a specified
range of distances from the subject. For instance, it might indicate f/8
when you are 6–9 feet away and f/4 when you are 12–18 feet away; the scale
changes when you set different film speeds. Then you manually set the indicated
f-stop on your lens, and the flash will automatically provide enough
power for good exposure at that f-stop within the corresponding range of
distances. If you or your subject changes position and moves closer or further
away, you must consult the scale again.
With many non-TTL autoflash models, the scale offers more than one choice
of distance range. You manually choose the one you want to work with, set
the corresponding f-stop on your lens, and the flash provides the automatic
exposure.
Using manual flash. In an autoflash’s manual flash mode, or with a manual
flash unit, the output is constant. The only exposure factors to consider are flash
distance, lens aperture, and film speed. Since flash light diminishes with distance,
you have to set a relatively large f-stop if the flash is far away from the
subject and a relatively small f-stop if the flash is close.
To calculate flash exposure manually, you read a chart usually located on the
flash unit or in the flash’s instruction book. The chart will tell you what f-stop
to use when the flash is a certain distance away with a particular film speed. For
ISO 100 film, the chart might look something like this:
Flash-to-Subject Distance Lens Aperture
32' f/2.8
22' f/4
16' f/5.6
11' f/8
8' f/11
5.5' f/16
4' f/22
Thus, for a portrait of someone 4 feet away at a crowded party, set the f-stop
at f/22; for a performer 20 or more feet away at a club, set the f-stop at f/4.
Because of space limitations, the chart on the flash will not indicate all the
possible choices at every film speed; you may have to interpolate. For instance,
if the chart says use f/8 at 11 feet away with ISO 100 film, you should use f/16
with ISO 400 film, which is two stops faster than ISO 100.
Instead of using an exposure chart with manual flash, you can use the flash’s
guide number, a rating of its output at a specified film speed: The higher the
guide number, the more powerful the flash. To determine the lens aperture
needed for correct exposure, divide the guide number by the distance from the
flash to the subject. For instance, if your flash has a guide number of 40 with
ISO 100 film, and you are 10 feet from your subject, use f/4 (40 ÷ 10 = 4); if
you are 20 feet from your subject, use f/2 (40 ÷ 20 = 2). The guide number of
the same flash unit is twice as high with ISO 400 film (80), so use f/8 when your
flash is 10 feet away (80 ÷ 10 = 8) and f/4 at 20 feet (80 ÷ 20 = 4) accordingly.
Modifying the Flash
On-camera flash produces a distinctive look—harsh, flat, and usually brighter
in the foreground than in the background. This works well enough for many
pictures, but if you want a subtler look, there are many techniques to modify
and soften flash light.
Bounced/diffused flash. One reason on-camera flash looks the way it does is
because it points directly at the front of the subject. For a softer, more diffuse
light, you can bounce the light by aiming it where it will reflect off a surface,
such as a ceiling or wall, before it strikes the subject. To do this, you need a
white or light surface and a flash unit that can be adjusted to bounce light, such
as those that tilt upward for bouncing off the ceiling.
You can bounce in a variety of ways if the flash is not attached to the camera.
Take the unit off the camera and use a pc cord or cable to synch with the
camera’s shutter. You now have the freedom to move the flash in many directions
and at various angles to the subject. Point the flash at the ceiling so the
light bounces down to the subject, or aim it at a wall so it reflects obliquely to
the subject.
There also are various accessory reflector units that attach to the flash unit to
reflect light indirectly toward the subject or diffuse it somehow. One type
consists of a reflector card that sits in the back of the flash head; the burst of
light hits the card first and then bounces toward the subject. Another type
consists of diffusing material that fits around the flash head; the diffusing material
softens the flash light on its way to the subject, just as bouncing does.
When bouncing light, exposure depends on the distance the light travels—for
instance, from flash to ceiling to subject—which is generally much farther than
the direct flash-to-subject distance. You don’t have to worry about this with
TTL autoflash, which does all the calculations for you. With non-TTL autoflash,
you need to work with a longer distance range on the flash’s scale. And
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