Appendix E: Sequence of Operation

E-TTL (film and digital) sequence of operation

The usual E-TTL sequence of operation, not counting the optional flash exposure lock (FE lock) feature or wireless operation, is as follows:

• When the shutter release is pressed halfway, the current ambient light levels are metered by the camera as usual. Shutter speed and aperture are set by the camera or user depending on the current mode: PIC (icon) modes or P, Av, Tv, or M.
• When the shutter release is pressed all the way, the flash unit immediately fires a low-power preflash from the main flash tube (i.e., white light).
• The reflected light from this preflash is analyzed by the same evaluative metering system that the camera uses for metering ambient light levels. The appropriate power output (i.e., flash duration) of the flash is determined by an internal computer which subtracts the ambient light level. The entire sensor area is evaluated and compared to the ambient metering, and the area around the active focus point is emphasized. The value of the required power output is stored in memory. If manual focus mode is used, then either the central focus point or averaged metering is used.
• If the photo is being taken under bright lighting conditions (10 EV or brighter), auto fill reduction is applied (unless it has been disabled by a custom function, as is possible on some bodies), and the flash output is decreased by anywhere from 0.5 to 2 stops. Canon has not published details of the E-TTL auto fill reduction algorithms, and they vary from model to model.
• The mirror flips up and the shutter opens, exposing the film or sensor chip.
• The flash tube is then fired at the previously determined power level to illuminate the scene. Start time of the flash burst depends on whether first or second curtain sync has been set. The OTF sensor in the camera, in the case of most EOS film cameras, is not used in E-TTL mode.
• The shutter stays open for the full duration of the shutter speed time.
• The shutter closes and the mirror flips back down. If the flash unit has a flash exposure confirmation light, and if the flash metering was deemed sufficient, the light glows.

Table 17.1: Basic E-TTL II sequence, not including user-selectable options such as sync timing (first, second, high speed), metering types, and flash exposure compensation.

E-TTL II sequence of operation

E-TTL II is similar to E-TTL in terms of the basic sequence. The primary differences are as follows:

• Exposure is not biased to any focus point.
• Distance data from the lens is factored into the calculations if the flash unit is not in bounce mode or wireless.
• The user has the ability to specify whether evaluative or averaged metering is used for the ambient light.
• The camera actually performs ambient metering twice. The first reading occurs when the shutter release is half-pressed and is used for automatic exposure settings as normal. The second reading occurs when the button is pressed all the way and is used as a point of comparison for the preflash reading. Table 17.1

Wireless E-TTL sequence of operation

The wireless E-TTL control sequence is as follows:

• Photographer presses the shutter release button halfway.
• Ambient light metering of the scene is conducted.
• Photographer presses the shutter release all the way.
• The master flash unit sends a wireless signal–optical or radio is appropriate–to all slave units in group A, instructing them to issue a low-power preflash.
• Any slave units in group A fire a preflash, and the camera records this light output using its evaluative meter.
• The master flash instructs group B slaves to issue a preflash.
• Any slave units in group B fire a preflash, and the camera records this light output.
• The master flash instructs group C slaves to issue a preflash.
• Any slave units in group C fire a preflash, and the camera records this light output.
• In the case of radio wireless E-TTL, and if groups are enabled, the camera may instruct slave groups D and E to issue preflashes as well.
• The camera calculates what the final flash output for the scene should be, based on both the preflash data from each slave group (if any) and the user-defined group ratios / flash exposure compensation settings.
• The camera flips up the mirror and opens the shutter.
• The master flash instructs all slave units to fire simultaneously.
• All slave units fire at whatever level the master unit has told them to. If the master flash unit is flash-capable and is configured to fire, then it too will do so.
• The camera flips down the mirror and closes the shutter.

Table 17.2: Wireless E-TTL sequence with three slave groups in use.

There are, of course, differences in the timing of some of these events if AE lock or flash exposure lock (FE lock) are used, but this is the basic work flowchart. The command pulses and prefires occur at an extremely rapid rate. Optical wireless commands will register with a human observer but occur far too quickly to be interpreted as much more than a flash burst. They also occur prior to the shutter opening and so should not light the scene. Radio wireless commands are, of course, invisible.

TTL (film only) sequence of operation

• When the shutter release is pressed halfway, the ambient light levels are metered by the camera. Shutter speed and aperture are set depending on the current mode: P, Av, Tv, M, or icon mode. In P mode and most icon modes, the camera sets the shutter speed to between 1/60 and X-sync. In the other modes it meters normally (except cameras with a custom function that locks to X-sync in Av mode).
• When the shutter release is pressed fully, the mirror flips up and the shutter opens, exposing the film.
• If second curtain sync is used, the shutter stays open for nearly the full duration of the shutter speed time.
• The flash unit illuminates the scene. The start time depends on whether first or second curtain sync has been set. First curtain: immediately after the shutter opens. Second curtain: shortly before the shutter closes.
• Duration of the flash pulse is determined by the off-the-film (OTF) flash sensor. Under bright lighting conditions (10 EV or brighter), auto fill reduction is applied, unless disabled by a custom function. This reduces the output by 0.5 to 1.5 stops.
• When the foreground subject has been adequately lit (by real-time measurement of flash reflecting off the surface of the film), the flash tube is turned off. Light from the flash unit is quenched.
• The shutter remains open for the rest of the shutter speed time in the case of first curtain sync.
• The shutter closes and the mirror flips back down.
• If the flash unit has a flash exposure confirmation LED, and if flash metering was deemed adequate, then the light glows.

A-TTL (film only) sequence of operation

The A-TTL sequence of operation is as follows:

• When the shutter release is depressed halfway, the current ambient light levels are metered by the camera. In P and Tv modes, the ambient aperture value is determined and stored, but not set. In Av and M modes, the ambient aperture value is user-set.
• In order to determine the approximate distance from the flash to the main subject, the flash unit fires a preflash (either near-infrared light from a front-mounted secondary flash tube, or white light from the main flash tube, depending on the flash unit and operating mode) in conjunction with the ambient light metering. In P mode only, the correct aperture value to expose the main subject is then calculated.
• In P mode only, the two aperture values (ambient and flash) are compared when the shutter release is fully depressed. The camera typically sets the smaller of the two apertures, particularly if the distance to the subject is determined to be fairly close. In Av and M modes the aperture is determined by the user setting, and in Tv mode the aperture is determined by the ambient light meter settings.
• If the photo is being taken under bright lighting conditions (10 EV or brighter), auto fill reduction is applied, unless it has been disabled by a custom function. This can reduce the flash output by anywhere from 0.5 to 1.5 stops.
• The camera flips up the mirror and opens the shutter, exposing the film.
• The flash unit then sends out the actual scene-illuminating flash. The start time of the flash pulse depends on whether first or second curtain sync has been set. Duration of the flash pulse is determined by the standard OTF sensor, which is exactly the same as TTL flash.
• The shutter stays open for the full duration of the shutter speed time.
• The shutter closes and the mirror flips back down. If the flash unit has a flash exposure confirmation light, and if the flash metering was deemed adequate, then the light glows.