Back button AF lock / Half-Press AE lock prevents \"focus - recompose\" errors in Av/Tv mode when also using ETTL-II flash with Evaluative (zone) flash metering in Single-Shot AF mode.
Canon ETTL-II flash metering makes a second \"baseline\" ambient reading, separate from the one used to set shutter / aperture in Av/Tv mode, immediately after FULL shutter press. It is subtracted from the preflash+ambient \"zone maps\" for each group\'s pre-flash which allows the metering to logically deduce what each flash group is hitting and which are overlapping for setting ratios.
When AF/AE lock are both on the shutter (half-press) if the shutter is half pressed, then the viewfinder is recomposed, when the shutter is fully pressed the flash metering will be performed on a different scene. That can lead to incorrect exposure if there\'s a difference between the before / after content in the viewfinder, particularly in the center metering zones.
With AF lock moved to a back button via CF.n it is possible to lock AF with the back button, recompose, half-press to lock AE, then fully press to initiate the flash metering and make the exposure. AE metering is also done when the AF button is pressed, but not locked until after recomposing with the half-press of shutter. Since AE lock and flash metering are done on the same scene results are more predicable.
AF lock + metering with back button (Programmed via CF.n)
Recompose
AE lock with half press of shutter
Full press of shutter
-- second ambient baseline exposure reading is made for flash calculation
-- preflash group A
-- preflash group B
-- preflash group C
-- Flash calculation
-- Coded command signal to flashes to set % power level sent
Shutter opens- First Curtain Clears Sensor (normal flash mode)*
-- Coded signal to fire slaves sent
Flashes Fire
Shutter closes
* It takes about 1/300th sec. for first curtain of shutter to move across and fully expose the sensor so flash can be fired in normal single burst mode. in HSS mode the signal to start pulsing the flashes is sent just before the first curtain opens.
Feb 23, 2013 at 06:57 AM
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