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7.2
How
Flash Works
Just as with the operation of the
camera itself, it’s important to understand some basics of flash
technology in order to better appreciate the capabilities and
limitations of the flash unit when operating it in the field.
Although some of the material in this section may at first appear a bit
esoteric or arcane, its importance will become more apparent in later
sections of this chapter, and will help you to better understand why
particular techniques work (or don’t work) in particular scenarios.
Although it should be obvious to most camera owners,
it bears mentioning at the start that flash light is both very bright and very brief. Its brevity, in
particular, is worth keeping firmly in mind both as you read the
following sections and as you operate the camera in the field.
Because the flash’s pulse of light is typically much shorter in
duration than the time that the camera’s shutter is open, the effect of
shutter speed on overall exposure no longer follows the rules of reciprocity (introduced in section
6.1) when flash is in use.
Methods for dealing effectively with
this failure of reciprocity in flash photography will be discussed in
section 7.5.
In contrast to the brevity of a flash pulse, ambient light—from the sun—is
generally continuous in
duration. Note that ambient light needn’t come directly from the
sun. Photons of light arriving here from the sun will bounce off
of the first object they encounter, thus being re-routed in a new
direction. They may bounce off of any number of earthly objects
before eventually reaching the bird you’re watching. After
bouncing off of the bird, some light rays (beams of photons) will, just
by happenstance, be reflected in the direction of your camera’s imaging
sensor. These photons, which originated at the sun and may have
followed a rather contorted path before reflecting off the bird and
being registered by your camera’s sensor, comprise what is known as ambient light. Because
photons are continually being emitted by the sun, ambient light is
available during the entire interval that the shutter is open.
Flash light, on the other hand, has a very limited
duration—typically around 1/20000 sec, which is significantly faster
than the maximum shutter speed of even today’s pro DSLR cameras
(excluding those with electronic, rather than mechanical,
shutters).
Because flash is (typically) so much faster than the camera’s shutter,
changes in shutter speed often don’t have any effect whatsoever on the
amount of flash light that is
collected. The figure below illustrates this via a left-to-right
timeline (i.e., the horizontal axis represents time).
Fig. 7.2.1 :
Flash, ambient light, and shutter speed. The horizontal axis
represents
time. Since the flash pulse is typically far shorter than the
exposure duration, changes
in shutter
speed usually don't affect the amount of flash light received.
Doubling
the
shutter speed halves the ambient light received, but has no effect on
flash. As a
result, the ratio of flash to ambient light changes.
In this figure (above), the flash duration is seen to be far shorter
than the amount of time that the shutter is open. When the
shutter speed is doubled from 1/100 sec to 1/200 sec, the amount of
ambient light collected by the sensor obviously halves, but since the
flash emission is still wholely contained within the exposure interval,
the amount of flash light that is collected remains unchanged. In
this case we say
that the flash ratio—the
ratio of flash light collected, versus ambient light collected—has
doubled. Changing the flash ratio can have profound effects on
the captured image, most especially on the relation between the
lighting of the subject (the bird) and the background. Later in
this chapter we’ll see some examples of different flash ratios, and
consider methods for choosing the right ratio.
Differences in lighting between the subject and the
background obviously depend very strongly on their relative distances
to the camera. If the subject is much closer to the camera than
the “background” (i.e., any trees or bushes that
are visible behind the
bird), then the flash will have a disproportionately greater effect on
the subject than
the background. This is due to a fundamental law of physics
regarding electromagnetic radiation (of which light is one form),
referred to as an inverse power law:
For those not fond of mathematical notation, the above formula
indicates that if the distance (between the camera and any reflective
surface such as a feather or leaf) decreases at a certain rate, then
the gain in illumination due to flash will increase much faster. There are
several important consequences of this basic law of light. The
first is that in dim environments, any reduction in the
distance-to-subject will tend to result in an appreciable increase in
illumination of the subject due to flash. If the images you’re
getting are too dim even with flash at full power and exposure settings
at their practical limits, and you think you can get a few steps closer
to the bird without scaring it away, then it’s usually a good gamble to
try it.
A second consequence is the differential effect that
flash can have on a subject and its background, if the two are
sufficiently separated. The figure below illustrates the
situation schematically.
Fig.
7.2.2 :
The relative distances to the bird and the background have an
exaggerated
effect on the flash illumination of the two, due to the squared
distance term in the
inverse power
law. The further separated the bird and background are, the
greater
the difference in
flash illumination (in a nonlinear manner), thereby helping to make
the bird stand out more.
If most of the background elements
are positioned some distance behind the bird, then the bird can often
be made noticeably more prominent in the image by increasing the flash
ratio—i.e., by increasing the flash output and/or decreasing the amount
of ambient light that is collected (by increasing the shutter
speed). If, for example, the distance from the camera/flash to
the background is fully twice the distance of the camera/flash
to the bird, then the amount of flash light that reaches the bird
will be four
times (not just two times)
the amount that reaches the background. This
is simply due to the inverse power law. Thus, if by repositioning
the camera you can find an angle which places the background further
from the bird, then the effect of the flash should accentuate that
difference even more in the resulting image.
Now let’s return to the issue of shutter speed
versus flash duration. An important fact which many people don’t
realize is that the shutter doesn’t open instantaneously, nor does it
close instantaneously. There is an opening latency and a closing latency (which are
typically equal) that must be accounted for, as we
explain next. Let’s imagine what happens in slow motion.
Refer to Figure 7.2.3. (below) while considering the following chain of
events.
Fig.
7.2.3 :
The phases of shutter operation. At first, the shutter is still
closed (A). Then
the first curtain begins to open, exposing progressively more of the
sensor to light (B).
When it reachees the far side, the shutter is fully open (C).
Eventually the second curtain
begins to close (D). When it reaches the far side of the sensor,
exposure is finished (E).
To
begin with, the two curtains of the shutter are located together on one
side of the sensor (part A of the figure). When you push the
shutter-release button, the front
curtain of the shutter
mechanism begins to open. As its trailing edge traverses the sensor
window (part B), more and more of the sensor is exposed to the light
coming in
through the lens. Eventually, the front curtain (also called the first curtain) of the shutter
reaches the fully-open state. Now the sensor is completely
exposed (part C). When it’s nearing time for the exposure to end,
the rear curtain (or second curtain) begins to
close. As its leading edge traverses the sensor window, less and
less of the sensor is exposed (part D). Eventually, the rear
curtain
catches up to the front curtain (at the far edge of the sensor window)
and then the sensor is completely covered and exposure has fully
ended (part E).
Because the
front and rear curtains do take some
non-zero amount of time to travel from one edge of the sensor window to
the other, the above scenario has to be slightly modified when using
very short exposures. When the shutter speed
is set above a critical threshold called the maximum sync speed, the rear
curtain has to begin is traversal before the front curtain has even
reached the far edge of the sensor window—that is, the rear curtain
chases the front curtain across the sensor window. In the case of
very high shutter speeds, the rear curtain may be only just barely
behind the front curtain during their paired traversal, and in that
case the two curtains form a narrow slit that slides across the sensor,
allowing light to accumulate at each photosite while it’s exposed
within the slit. That exposure of individual photosites occurs at
slightly different times, of course, depending on when the traveling
slit reaches them. This scheme works fine for ambient light, but
for flash photography it complicates things, because a single flash
pulse would illuminate some parts of the sensor and not others
(depending on which parts are swept out by the chasing curtains during
the flash pulse).
The clever solution that camera engineers have come
up with
is to replace the single, strong pulse of flash light with a
near-continuous stream of weaker pulses that occur throughout the
duration of the curtain chase. If the pulses are fast enough (and
indeed, the engineers have ensured that they are), then all photosites
(i.e., “pixels”) will receive approximately the
same total amount of
flash light. That’s obviously a good thing. This latter “fix” has been dubbed high-speed sync
(HSS) by the camera
manufacturers, and is necessary whenever your shutter speed exceeds the
maximum sync speed (MSS) of
your camera. Most cameras have an MSS of about 1/200, with
higher-end bodies ranging up to 1/250 or 1/300.
Fig.
7.2.4 :
Normal flash mode versus high-speed sync (HSS). In part A, the
shutter speed is
below the maximum sync speed, so the flash can pulse just once.
In part B, the shutter speed
is faster than the max sync speed, so the first curtain doesn't have
time to completely reach
the far side before the second curtain has to begin to close. At
no time is the entire sensor
exposed, so a single brief flash pulse won't work. Instead, many
mini-pulses are emitted,
mimicking the effect of a continuous light source. Note that the
two intervals are not
drawn to the same scale: the interval in part B should be much shorter
than in part A.
Most name-brand flash units
support a high-speed sync mode, which you’d typically activate by
pressing one or more buttons on the external flash unit itself (though
sometimes it can be set remotely through the camera). There are
some negative consequences of high-speed sync, however. First,
there’s the obvious drawback that using faster shutter speeds results
in less exposure time for collecting light from the flash (as well as
ambient light). In addition, most flash units are limited to
providing a significantly reduced power output in high-speed sync mode,
due to the longer effective duration of the summed mini-pulses.
Because standard flash batteries (typically AA’s) can’t provide
current fast enough, the power for a flash pulse (or a stream of
mini-pulses) has to be stored in advance in a high-speed
capacitor. Because
the sum of the many mini-pulses of a high-speed-sync (HSS) flash
invocation can significantly exceed the power demands of an equivalent,
non-HSS pulse (at slow shutter speeds), the overall power output
typically has to be reduced in order to make the power from the
capacitor last for the full HSS interval. Keep in mind also that
in HSS mode, much of the flash output is wasted, since many of its
reflected rays that normally would strike the sensor (in a non-HSS
exposure) are instead reflected away by some portion of the front or
rear curtain as the narrow slit formed by the chasing curtains
traverses the sensor window. This isn’t just a loss of
efficiency: since the maximum amount of flash power available (which is
dictated by the flash unit’s capacitor rating) is limited, for higher
shutter speeds it’s simply not possible to compensate for the HSS “slit
effect” by increasing the flash’s output
power.
It’a also useful to know that for most flash units,
the “power level” that you set on the flash’s
control panel is actually
a duration setting. In other words, most flash units produce
light at the same (peak) intensity at all times, so what you’re really
changing when your dial in a different “power level” is the flash duration: higher
output levels are actually achieved by using the same
(peak) emission intensity over a longer time interval. As
mentioned
previously, these time intervals (in the non-HSS case) typically range
from about 1/1000 sec to 1/35000 sec. We’ll see later how this
fact can be used to effectively freeze birds’ wings by choosing an
appropriately brief flash duration.
The foregoing discussion of shutter curtains has
importance even outside of the high-speed-sync scenario. Many
flash units provide an additional mode allowing synchronization with
either the front curtain or the rear curtain. In part A of figure
7.2.4, we depicted the flash pulse occurring shortly after the opening
of
the shutter. This is the default synchronization mode for most
flash units, and is typically referred to as first curtain sync. The other
option is obviously second curtain
sync. A potential problem with first-curtain sync is that
if there’s motion in the scene and the ambient light isn’t completely
insignificant relative to the flash intensity, then objects in motion
can appear to be moving backward in the resulting image.
Fig. 7.2.5 :
First-curtain sync causes motion to appear backward. The
snowflakes were frozen in position by the flash pulse, and then ambient
light traced out the following interval of their trajectory.
Using second-
curtain sync would have fixed this problem by emitted the flash pulse
at the end of the ambient interval rather than at the beginning.
As you can see in the figure above, the snowflakes appear to be moving
up instead of down, because I made the mistake of using first-curtain
sync instead of second-curtain sync. In this case, the flash
fired as soon as the first curtain had cleared the sensor window,
freezing (no pun intended) the positions of the snowflakes at the
beginning of their trajectories; the continuous accumulation of ambient
light during the remainder of the exposure resulted in each snowflake
having a downward tail. Most people would interpret that downward
tail as indicating that the snowflakes were moving upward, when in fact
they were moving downward (as snowflakes normally do). A similar
effect can occur with the wingbeats of birds, so the issue of
first/second curtain sync isn’t limited to just snowflakes and
raindrops.
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