GHOSTING
“Ghosting” refers to a phenomenon that occurs when
photographing moving objects while using flash. There are
actually two kinds of ghosting and understanding the
difference and which one you are seeing will help you to
determine how to get rid of it: for the purpose of this article, ghosting will be treated as something to
eliminate from your hummingbird photos.
First, lets distinguish ghosting, a form of motion blur, from motion
blur generally. Motion blur
occurs as a result of the movement of the subject during the
time that your shutter is open and during the time that it is
illuminated by the primary light source. With hummingbirds,
the classic example is blurred wings. Your camera captures
these as a blur because it captures the image of the wings
during the course of movement. The blur will consist partly
of just the moving wings, and partly of a blend of wings and
the background that was partly visible and partly obscured
during the exposure.
Ghosting primarily occurs with flash so lets talk about
cameras and flash units for a moment. My camera, a Canon EOS
10D, has a
maximum flash sync speed of 1/200 second. It is an SLR. The
way that the shutter in most 35mm and digital SLR’s work is
through the use of two “curtains.” The first curtain
slides open across the film plane. Then the second curtain
follows it, closing the "curtain." Imagine a stage with two curtains
that are each long enough to stretch completely across a
stage. They meet on the left side of the stage so that only
one curtain is actually blocking your vision of the stage.
When the play or concert starts, that one curtain slides all
the way across opening from the left to the right. When the
curtains are "open," both curtains are bunched up at
either side of the stage. When the curtain closes, the curtain
on the left slides all the way across, following the
first curtain, and blocks the view of the stage. An SLR’s
shutter works this way, though it may actually travel
vertically. The first curtain opens and then, after an
appropriate delay the second curtain follows.
For very high shutter speeds, the second shutter curtain may
begin closing before the first shutter curtain has completely
opened. In that scenario, the entire film plane will never be
exposed all at once. This is what necessitates a maximum flash
sync speed, but lets talk about how flash units work first.
Flash output is fairly uniform. That is, a flash, while
“on,” shines at a fairly constant level of brightness. The
way that total flash output is controlled is by turning the
flash on and then turning it off when it has emitted enough
light to expose the image properly. This is done very quickly.
In fact, most flashes, even when used at “full power” are
only "on" for 1/1000 second or so. When at one half
(1/2) power, the flash is only turned on for half the time.
When at one quarter power, only for one fourth (1/4) the time,
etc. The brightness is always the same, the flash is just
allowed to shine for varying amounts of time. At the lowest
power settings, some flashes only shine for 1/30,000 second or
less.
Flash units are typically turned on at the point where the
first curtain of the shutter reaches the fully open position.
In order for the flash to expose the image properly and
evenly, the second curtain must not start to close before the
flash has turned off. Therefore, a maximum flash sync shutter
speed will be the highest shutter speed at which the delay
between the first curtain reaching the fully open position and
the second curtain starting to close is long enough to allow a
flash to complete its firing (turn on and turn off) at its
full power setting.
Ghosting, as opposed to general motion blur, is an effect caused by
the movement of the subject during the time that the shutter
is open but the flash is not on. The flash may perfectly
capture the bird, but if there is enough light on the bird, or
if the background is bright enough, the movement of the wings
after the flash is turned off, but before the shutter closes
will result in either the blurred image of the moving wings
(in the case where there was enough light on the bird to
register on the film or sensor) or a
shadow caused by the “dodging” of the background by the
wings or a combination of both.
The first type of ghosting that I will discuss is the form of
motion blur caused by the lighting of the hummingbird by a
source other than your flash. It is the image of the
hummingbird captured while the shutter was open but the flash
was off. The photograph below shows this form of ghosting:
Note
the ghosting in the tail and rump area.
When using flash to illuminate the hummingbird, the way to
avoid this form of ghosting to is to have the hummingbird as
shaded as possible and completely out of direct sunlight.
Additionally, the more the reliance on the flash for the
illumination of the bird, the less prominent this form of
ghosting will be. This means, the faster the shutter speed,
the smaller the aperture, the lower the ISO, the more flash
units you use, the higher the output of those flashes and the
lower the amount of ambient light, the less likely your image
will have ghosting
caused by ambient light.
As you can see, there are a lot of variables.
The best strategy is to take photos in as dark a shade as
possible and use as many flash units as you can so you will
have a relatively high light output from flash. This will
allow you to use a smaller aperture and a lower ISO.
The second type of ghosting is caused by the “dodging” of
a bright background by the wing movement after the flash has
turned off (or before if you are using second curtain sync).
With this form of ghosting, the hummingbird is not creating an
image on the sensor or film. What is happening is that the
hummingbird’s wings are blocking light coming from a
background that is much brighter than the bird during the time
that the flash is off. This creates a sort of shadow of the
wings against the background such as in the following image.
Notice
the dark ghosting around the wings.
There are basically three ways to avoid and/or minimize this
sort of ghosting when using flash. First, you can underexpose
the background. This is very effective but results in photos
where the background is black or very dark. Second, you can
use flash as the primary means of lighting the background.
This is also very effective, but can only be done if the
background is also shaded and, unless the background is very
close to the bird, might require a separate flash (see the
wide area flash method on my lighting methods page for an
approach of this sort). Finally, the third method is to use
the “high speed sync” function of a dedicated flash and
use a fast shutter speed. This works, but usually results in
wing blur, the avoidance of which may be the reason that you
wanted to use flash in the first place.
When the entire scene is primarily illuminated by multiple
flash units, ghosting of all types can be almost totally
avoided. In the following example, there is practically no
ghosting whatsoever.
Entire
scene illuminated by 4 flash units.
In the above image, everything in the image was illuminated by
flash. The drawback to this method is that images taken using
these settings will almost always look as if they were taken
at night. Parts of the background will be dark. You can avoid this
by having a close background that has no gaps or by using a backdrop of some
sort that is illuminated by flash, or you can use a
combination of both. Some photographers use a
blue mat-board or poster board behind a plant to make the background
gaps in a plant
seem to show blue sky. A separate flash usually is dedicated to
the blue backdrop.
In my experience, the second type of ghosting is the most
common and can be very difficult to avoid if you want a
background that is not black and you want to use flash. This
sort of ghosting can give black "borders" to the
hummingbird's body if long shutter speeds are used because of
slow max sync speeds or to properly expose the background.
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