Canon EOS 50 E - Page 66

Part 2: Technical Information
Table 2-10 Flash Aperture
EOS 50/55/Elan II Exposure Mode
Flash Aperture
Programmed Image Control
The flash aperture is set automatically by the EOS 50/55/Elan ll's
E-TTL program.
Program AE (P)
The flash aperture is set automatically by the EOS 50/55/Elan ll's
E-TTL program.
Shutter-priority AE (Tv)
The aperture set automatically by Shutter-priority AE becomes the
flash aperture.
Aperture-priority AE (Av)
The aperture set manually becomes the flash aperture.
Manual (M)
The aperture set manually becomes the flash aperture.
Depth-of-field AE (DEP) Same as with Program AE (P).
SW1
SW2
6) Real-time metering for framing change detection
To detect any changes in framing while the shutter button is being pressed
halfway, real-time metering is executed. After ambient light is metered in step 2),
real-time metering continues up to the moment the shutter button is pressed
completely. The E-TTL Auto Flash Exposure System obtains a correct exposure
with a mixture of ambient light and flash. Real-time metering is necessary so
that the algorithm can take the ambient light into account when calculating the
correct exposure. See 11). Framing change detection is described in 10).
7) Instantaneous metering of ambient light before preflash.
8) Preflash metering
The light reflected off the subject by the preflash is calculated by subtracting the
ambient light reading taken before the preflash (7) from the subject brightness
metered during the preflash (8). The metering for 7) and 8) are executed instanta-
neously ( 1 ms) to prevent flicker from affecting the result.
9) Correction for highly reflective subjects
Correction is executed for highly reflective subjects such as glass. If the SO, Sl,
or S2 sensor attached to the current focusing point senses an abnormally strong
light reflection, the sensor controls the flash to prevent underexposure of the
subject.
10) Framing change detection
EOS cameras equipped with multiple focusing points have had the focusing point
linked to the flash exposure through multiple-zone flash metering. However, if
the framing is changed after focus is achieved in the One-Shot AF mode, the
focusing point may no longer cover the subject. In such a case, the correct flash
exposure may not be obtained. To resolve this potential problem, the E-TTL sys-
tem compares the ambient light reading obtained during the halfway pressing of
the shutter button (step 2) and the real-time reading obtained immediately before
the shutter button is pressed completely (step 6). If the brightness distribution of
the two readings differs beyond a stipulated degree, the camera deems that the
framing has been changed. The flash exposure linkage to the original focusing
point is then canceled, and the correct flash exposure is based on one of the 6-
zone metering sensor's five sensors (Sl, S2, S3, S4, S5) that senses the greatest
reflected light (whose distance is deemed to be shortest). The flash is conse-
quently controlled to obtain the correct exposure. Any change in the framing can
be detected only in the One-Shot AF mode (or AI Focus mode set to One-Shot AF).
If the shutter button is pressed completely without being pressed halfway first,
the camera will deem that there has been no change in the framing.
2-22