Abstract: The Canon EOS system

Photography - Canon EOS, digital, nature


Contibuted to the EOS FAQ by Mark Overton mark@sdd.hp.com


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 shutterspeed 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.


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:
(Program AE): X-sync shutter speed automatically set to 1/60~1/125 sec. based on A-TTL or TTL program. Aperture value automatically set according to A-TTL or TTL program.

Tv (Shutter-priority AE): 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.

Av (Aperture-priority AE): 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.

M (Manual): 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 shutterspeed up to 1/4000 sec.


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.


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:
Evaluative Metering Sensor: The Evaluative metering sensor (positioned above the camera's focusing screen) is used for ambient metering with all EOS cameras. E-TTL flash readings are also taken with this sensor. E-TTL is currently available with EOS Elan II/IIE, Rebel G and EOS IX 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 (prestored 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 preflash 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 preflash. This data is used to calculate and store the flash output required for optimum exposure of the main subject (identified by the autofocus 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 preflash, but unlike E-TTL, the preflash 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 preflash. 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.


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 shutterspeed, the flash's guide number is higher at/below the body's flash sync speed.

The guide number drops as you increase shutterspeed, with the net result that the maximum distance you can shoot remains approximately constant, regardless of shutterspeed (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 metres. 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.


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 preflash 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.


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: Then Speed/Aperture choice is: And Flash Fire-time flash exposure) is:
Dim (<= 10 EV) normal exposure based on ambient light level,but with less weight on the subject(exposing for background) normal exposure of subject using TTL, A-TTL or E-TTL(exposing for subject)
Bright (>= 13 EV) normal exposure based on ambient light level (exposing for everything) 1.5 stops below the above normal flash exposure (doing shadow-fill)

Exception 1: In M mode, the speed/aperture is whatever you set.

Exception 2: In Av mode, when CF 09 is 1, and the flash is on, the shutterspeed is always 1/125, and the aperture is whatever you set.

Exception 3: In P mode, when the flash is on, the shutterspeed stays high enough for hand-holding, even if this underexposes the background.

The above table can be summarized as:

Dim light: Flash correctly exposes the *subject* (at selected focus point); Speed/aperture are set to correctly expose the *background*.

Bright light: 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 sloooow shutterspeed -- much slower than you expected. This is because the camera is selecting a shutterspeed which will correctly expose the dark background. To avoid this problem, use one of the three exceptions above to underexpose the background: boost the shutterspeed 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 catchlights 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).


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 preflash 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 preflash 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.