Canon Flashes
‫מתאים גם למצלמות חדשות של קאנון וגם לפלשים חדשים של קאנון‬
WHAT HAPPENS WITH FLASH?
What happens when you use a flash? The Elan II and 380EX manuals are vague about this. Those
manuals provide a few hints, but don't describe precisely how having a flash present changes the
camera's shutter-speed and/or aperture settings, or how the flash exposure itself is determined. Chuck
Westfall, who works for Canon, has kindly provided much of this information, which has been
arranged in the FAQ below. In fact, Chuck has gone above and beyond the call of duty in that he's
provided a great amount of information.
SHUTTER SPEED AND APERTURE SETTINGS WITH FLASH
The EOS Elan II is consistent with other EOS cameras in the variety of exposure modes that can be
used for flash photography depending on the desired results. The following table shows how aperture
and shutter speeds are set according to the camera's shooting mode when used with most EOS
dedicated Speedlites or the camera's built-in flash:
P (Program AE)
Tv (Shutter-priority AE)
Av (Aperture-priority AE)
M (Manual)
X-sync shutter speed automatically set to 1/60~1/125 sec. based on ATTL or TTL program. Aperture value automatically set according to
A-TTL or TTL program.
X-sync shutter speed manually set to any shutter speed of 1/125 sec. or
slower.* Aperture value automatically set according to ambient light
level and shutter speed.
X-sync shutter speed automatically set between 30 sec. and 1/125
sec.** according to ambient light level and set aperture value.
Aperture value manually set to desired aperture
X-sync shutter speed manually set by main input dial to any shutter
speed of 1/125 sec. or slower.* Aperture value manually set by quick
control dial to desired aperture.
*The camera automatically resets the shutter speed to 1/125 sec. if a faster speed is set,
unless an EX flash is in FP mode.
**If an EX type flash is installed, and its mode switch is set to FP mode (high speed
sync), then Av mode will set a shutter-speed up to 1/4000 sec.
USING THE EXPOSURE MODES WITH FLASH
Now that you know how the camera behaves according to the selected camera exposure mode during
TTL and A-TTL flash photography, here's some additional background as to which exposure mode is
best according to the situation at hand:
P (Program AE):
With the camera set for fully automatic operation, the EOS and Speedlite
work together while you concentrate on picture-taking. In daylight or
brightly lit indoor situations, the background will always be exposed
correctly and the camera will control the fill-flash ratio for optimum
results. Indoors or at night, the Speedlite becomes the main source of
illumination and the shutter speed will automatically be kept high
enough to permit hand-held exposure.
Av (Aperture-priority AE):
Selecting Aperture-priority AE mode with flash gives you maximum
control over depth of field when it is a concern. The camera will
automatically set a shutter speed to provide adequate background
exposure, day or night. Outdoors, or in relatively bright indoor lighting,
if the selected aperture is unusable, the shutter speed indicators in the
camera's viewfinder and external LCD panel will blink. Just select a
smaller aperture, and shoot. Indoors or at night, slow shutter speeds are
likely, so it's best to use a tripod or pick a different camera exposure
mode.
Tv (Shutter-priority AE):
Shooting in Shutter-priority AE mode with flash lets you select the
shutter speed while the camera selects the aperture to give correct
background exposure. High shutter speeds up to 1/125th of a second (or
up to 1/4000 if an EX flash is used in FP mode) can be used in bright
light, while slower speeds down to 30 seconds are more appropriate in
dark conditions or for special effects. In low light situations, the
maximum aperture value of the lens may blink in the viewfinder and
external display if the selected shutter speed is too fast to produce a good
exposure of the background. Just set a slower shutter speed and shoot, or
switch to manual exposure mode on the camera.
M (Manual):
Manual exposure mode lets you control both the shutter speed and
aperture. This option is important in low light situations when you want
to combine small apertures with high shutter speeds. Keep in mind that
manual exposure mode on the camera can be combined with fully
automatic flash exposure, since the EOS Elan II's metering systems for
flash and existing light are independently controlled.
FLASH METERING MODES
To understand the TTL, A-TTL and E-TTL metering modes, one must first understand the function of
the *two* light sensors contained in EOS cameras:
The Evaluative metering sensor (positioned above the camera's focusing screen) is used
Evaluative
for ambient metering with all EOS cameras. E-TTL flash readings are also taken with
Metering
this sensor. E-TTL is currently available with EOS Elan II/IIE, Rebel G and EOS IX
Sensor:
when used with Speedlites 380EX or 220EX. Note that the Evaluative-metering sensor
is blocked during exposure by the camera's reflex mirror.
TTL Flash
Metering
Sensor:
The TTL flash metering sensor (positioned in the camera's mirror chamber and aimed at
the film plane) is used for A-TTL and TTL flash readings off the film plane during
exposure. All current EOS cameras feature segmented TTL flash sensors, designed to
bias flash exposure to the area of the picture identified by the AF system as the main
subject. Note that only the TTL flash metering sensor is active during exposure, and
then only in combination with an A-TTL or TTL Speedlite.
With that background, here's what the flash metering modes do:
Evaluative Through-the-Lens Mode (E-TTL)
E-TTL (pre-stored evaluative through-the-lens flash exposure control) is a new flash mode that reads
through the lens, but not off the film. Introduced for the first time in 1995, E-TTL is currently
available with the EOS Elan II/IIE, Rebel G and EOS IX SLR cameras exclusively when used with
Speedlites 380EX or 220EX. Utilizing a pre-flash fired after the shutter button has been fully
depressed but before the camera's reflex mirror goes up, E-TTL uses the camera's evaluative metering
sensor to analyze and compare ambient light exposure values with illumination reflected from the
subject by the pre-flash. This data is used to calculate and store the flash output required for optimum
exposure of the main subject (identified by the auto-focus system), while maintaining a subtle balance
between foreground and background. Though transparent to the user, E-TTL represents Canon's most
advanced flash exposure control system to date.
Advanced Through-the-Lens Mode (A-TTL)
The predecessor to E-TTL, A-TTL (Advanced Through-the-Lens) is used with Speedlite 540 EZ when
set for direct flash in the camera's Program mode. (TTL flash exposure control is available at other
times.) Like E-TTL, A-TTL reads through the lens and concentrates its sensitivity on the area of the
picture covered by the active focusing point. Unlike E-TTL, A-TTL controls flash exposure with a
dedicated sensor that reads flash illumination reflected from the surface of the film during exposure.
A-TTL also uses a pre-flash, but unlike E-TTL, the pre-flash is used to calculate an aperture value
based on the distance that light must travel from the flash to the subject. In Program mode, the camera
compares the distance-based aperture data to the ambient-based aperture data calculated by the
camera's normal metering system, and selects the smaller aperture value of the two. This method
ensures accurate exposure of the subject in any lighting condition. Speedlites equipped with A-TTL
are fully compatible with all EOS cameras.
Through-the-Lens Mode (TTL)
TTL (Through-The-Lens) is the automatic flash exposure control mode used by the small pop-up flash
units built into some EOS cameras. It is also available with Speedlites 380EX and 220EX when used
with EOS cameras other than the Elan II/IIE, Rebel G and EOS IX. Additionally, TTL is the standard
flash mode for Macro Ring Lite ML-3, Speedlite 200E, Speedlite 480EG and Speedlite 540EZ (except
when set for direct flash in the camera's Program mode as outlined above). TTL is identical to A-TTL
in almost every way, except there is no pre-flash. It can be used in every camera exposure mode. In
Program mode, TTL always selects an aperture value based on the level of available light.
HIGH-SPEED SYNCHRONIZATION (FP FLASH)
The FP flash mode of the EX series of Speedlites (220EX, 380EX) allows the EOS Elan II/IIe, Rebel G
and EOS IX SLRs to synchronize E-TTL or FEL flash exposure at *any* shutter speed. Therefore,
even in bright daylight conditions, a fast lens can be used at a wide aperture to blur the background and
emphasize the subject in portraiture, using the flash to provide shadow- fill. FP flash is automatic and
simple to use. It is available in Intelligent Program, Shutter-priority AE, Aperture-priority AE, and
Manual modes.
Be careful when using flash for shadow-fill in sunlight. Flash shadow-fill also creates flash shadows
of its own. The Elan II drops flash exposure 1.5 stops below normal for shadow-fill. You can reduce
flash fill (and thus reduce flash shadows) by setting Flash Exposure Compensation to a negative
number.
I think of FP mode as an abbreviation for "Fast Pulse mode", although it actually stands for "Focal
Plane mode". Only EX flashes can do this. The flash emits a series of brief flashes at high frequency
(around 50 kHz or higher) to uniformly expose film while the shutter's slit is moving across the film.
Although you can do flash photography at any shutter-speed, the flash's guide number is higher
at/below the body's flash sync speed.
The guide number drops as you increase shutter-speed, with the net result that the maximum distance
you can shoot remains approximately constant, regardless of shutter-speed (and regardless of film
speed; think about it). For the 380EX, that maximum useful shadow-fill distance (in noon sunlight) is
about 2.5 or 3 meters. The manual for your flash has a table of the guide numbers for all shutter speeds
and focal lengths from 28 to 105 mm.
There is a FAQ about FP-mode for those interested in the details.
MORE FLASH FEATURES
Flash Exposure Lock (FEL)
FE Lock (Flash Exposure Lock) provides an AE lock function for flash photography. Available
exclusively with the Elan II/Elan IIe, Rebel G or EOS IX equipped with Speedlites 380EX or 220EX,
FE Lock takes advantage of the camera's AIM system and partial metering capability to isolate flash
readings to a small part of the picture area. With FE Lock, the Speedlite 380EX or 220EX pre-flash
fires when the camera's AE lock button is depressed, storing AF and metering data for up to 16
seconds. This provides enough time for adjustments: Not only can the shot be recomposed, but the
background exposure can also be altered for maximum creative control. FE lock is extremely useful
when you wish to place the main subject in a part of the picture area that is not covered by one of the
three focusing points. It can also eliminate potential exposure errors caused by unwanted reflections
from highly reflective surfaces like windows or mirrors.
Flash Exposure Compensation
Flash exposure compensation adjusts the level of illumination provided by the flash, and is therefore an
important method of creative control for all kinds of flash photography. It's particularly effective for
fine-tuning the balance between foreground and background exposure during fill-in flash, but it can
also be effective to compensate for extremely bright or dark tones in the subject. Built into Speedlite
540EZ (up to +/- 3 steps in 1/3 step increments), flash exposure compensation control is also built into
the EOS-1N & RS, A2/A2E, and Elan II/IIe.
Second-Curtain Synchronization
Instead of firing the instant the shutter opens, Second-curtain Sync fires the flash at the end of the
exposure, allowing streaks of light to flow naturally behind the movement of the subject. Originally
developed by Canon in 1986 with the T90 and Speedlite 300TL, Second-curtain Sync is currently
available with Speedlite 380EX when used with the Elan II/IIe or EOS IX, as well as with Speedlite
540EZ when used with any EOS camera.
DETAILS OF FLASH EXPOSURE
First, a couple of definitions:
1. Subject: The subject is assumed to be centered around the selected focus point(s).
2. Background: This is everything *except* the subject.
The camera's decision of speed/aperture (based on ambient light) is independent of the camera's
decision of flash fire-time (based on reflected flash light). That's important to remember. However,
the way flash exposure is decided is affected by the ambient light level. In bright light, flash is used for
shadow-fill. In dim light, it's used to normally expose the subject. The following table shows how
ambient exposure and flash exposure are decided, based on dim/bright ambient light:
If ambient light
is:
Dim (<= 10 EV)
Then speed/aperture choice is:
Normal exposure based on
ambient light level, but with less
weight on the subject (exposing
for background)
Bright (>= 13 EV) Normal exposure based on
ambient light level (exposing for
everything)
Exception 1
Exception 2
Exception 3
And flash fire-time (flash exposure)
is:
Normal exposure of subject using TTL,
A-TTL or E-TTL (exposing for subject)
1.5 stops below the above normal flash
exposure (doing shadow-fill)
In M mode, the speed/aperture is whatever you set.
In Av mode, when CF 09 is 1, and the flash is on, the shutter-speed is always
1/125, and the aperture is whatever you set.
In P mode, when the flash is on, the shutter-speed stays high enough for handholding, even if this underexposes the background
The above table can be summarized as:
Dim light
Bright
light
Flash correctly exposes the subject (at selected focus point); Speed/aperture are set
to correctly expose the background
Flash is for shadow-fill, and is 1.5 stops below normal flash; Speed/aperture are
set to correctly expose everything.
Between EV 10 and 13, the camera uniformly drops the flash exposure, starting at 0 stops at EV 10,
and ending at 1.5 stops of drop at EV 13. Therefore, at EV 11, the drop is 0.5 stop. At EV 12, the
drop is 1.0 stop. Note that this exposure drop is in *addition* to any flash compensation the user has
set.
However, some EOS cameras are equipped with a Custom Function (CF16 on the EOS A2/A2E, CF14
on the EOS-1N & RS) which shuts off the above flash exposure level reduction algorithm, thus
applying a standard flash exposure level in any lighting condition. The use of CF16 on the A2 or A2E
or CF14 on the 1N or RS is recommended for anyone who plans on applying their own flash exposure
compensation settings, since it allows greater control of the results. Note that this Custom Function is
not available with the Elan II series.
A common problem occurs in Av mode in dim light: You set the aperture to something, and the
camera sets a slow shutter-speed -- much slower than you expected. This is because the camera is
selecting a shutter-speed which will correctly expose the dark background. To avoid this problem, use
one of the three exceptions above to underexpose the background: boost the shutter-speed by using M
mode, or use Av mode with CF09=1, or use P mode.
Exposure compensation only affects background exposure, and *not* the flash exposure. Flash
exposure compensation only affects flash exposure. Thus, you can independently control the exposure
of subject and background. And in bright light, you can change the fill-ratio using flash exposure
compensation. For example, setting flash exposure compensation to -2 EV will provide negligible
shadow-fill (due to the -3.5 EV flash exposure drop), but will put catch-lights in the subject's eyes.
Chuck Westfall: I don't have my notes in front of me right now, so I can't guarantee that the Elan II's
A-TTL/TTL algorithm is exactly the same as this, but if it isn't, it's close. (BTW, note that I am only
speaking of A-TTL and TTL here. The camera calculates flash exposure differently in E-TTL, and the
concept of an arbitrary flash-fill ratio does not apply).
METERING PATTERNS WITH FLASH
Ambient Metering Patterns
For non-flash photography, the 6 metering segments are weighted according to the user-set metering
pattern (center-weight, partial, or evaluative). The pattern is more heavily weighted around the subject.
But when a flash is turned on (internal or shoe-mount) in dim light, the camera wants to set shutterspeed/aperture based on the background, and *not* the subject. So the metering pattern changes.
The way it changes depends on the flash unit in question. For TTL flash units like the built-in flash, or
A-TTL/TTL units like E-Series or EZ-Series Speedlites, the ambient metering pattern switches to an
overall average (all segments weighted the same) with the Elan II or IIe, regardless of the user-set
ambient metering pattern. For E-TTL units like the 220EX and 380EX, the camera switches to
evaluative metering regardless of the user-set ambient metering pattern.
[I tried the above, and my Elan II always seemed to use the user-set metering pattern for ambient light.
And this pattern did *not* appear to change when the internal flash was pushed down. -- Mark
Overton]
The way the ambient metering patterns change for other EOS bodies used during dedicated flash
photography vary according to the camera model, Speedlite and focusing mode (AF or manual focus)
in question. (No time for all the details now.)
Flash Metering Patterns
E-TTL: Utilizing a pre-flash fired after the shutter button has been fully depressed but before the
camera's reflex mirror goes up, E-TTL uses the camera's evaluative metering sensor to analyze and
compare ambient light exposure values with illumination reflected by the pre-flash from the subject (as
identified by the AF system). When the lens is set to manual focus, E-TTL is carried out using the
central focusing point by default.
A-TTL, TTL: Flash metering is carried out during exposure by the TTL flash metering sensor. The
EOS-1N, RS, and A2/A2E feature a 3-point, 3-zone TTL flash metering sensor, whereas the EOS Elan
II series, EOS IX and Rebel G cameras use a 4-point, 3-zone TTL flash metering sensor. When the
lens is set for AF, all zones see flash illumination, but the zone covering the main subject (as identified
by the AF system) has greater sensitivity. When the lens is set for manual focus, all TTL flash
metering zones are equally weighted.