Post by USM on Nov 24, 2006 13:45:23 GMT 8
TTL and A-TTL flash metering patterns
The flash metering pattern is determined by the type of flash sensors built into the camera. If the camera has only one focusing point then it will have a single zone flash sensor. Flash metering is conducted using this sensor in a centre-weighted averaging pattern.
If the camera has multiple focusing points then it will have multiple zones; what Canon call their AIM system. The number of flash metering zones depends on the camera model. For instance, the EOS 10 has three focusing points and three flash metering zones, and flash metering uses whichever corresponding auto focusing point or points are active.
However, the EOS 5 uses the same sensor as the 10 so it has 3 flash metering zones even though it has 5 AF points.
These multiple zone flash sensors let the camera bias the flash exposure to the currently selected AF point. When you focus manually the camera does not bias any flash zone but chooses the central zone instead.
Note that EOS 5 is somewhat different from other multiple AF point cameras in that it will only bias flash exposure correctly to the nearest AF point if that point was manually selected. In automatic and ECF modes it apparently always chooses the centre zone.
E-TTL flash metering pattern
The camera uses its evaluative metering system to meter the flash output, based upon the preflash. When in auto focus mode, most EOS bodies which do not use E-TTL II bias flash metering toward the currently selected AF point, but always in an evaluative mode pattern - they don’t use spot or partial metering patterns. When in manual focus mode it appears that at least some EOS bodies switch to centre-weighted averaging.
Note that this biasing of E-TTL metering to the active point is potentially problematic, since it means that the flash metering is done in almost a spot-metering fashion. Many user complaints regarding flash metering problems in E-TTL mode appear to be linked to this issue. If the camera happens to be over a dark object, for example, flash metering can be considerably overexposed, and vice-versa.
The standard answer to this problem is
1) Use FEL and meter off something mid-toned, but this is clearly not a solution for rapid-shooting situations such as weddings and sports.
2) Set the camera lens to manual focus, since the body apparently switches to centre-weighted average metering in that mode, but that’s obviously not a useful answer much of the time either.
Users of the EOS D30 and D60 have been particularly unhappy with E-TTL flash metering. EOS 10D apparently reduces this problem by defaulting to a centre-weighted averaging metering pattern in E-TTL, even when the lens is set to auto focus.
E-TTL II addresses this problem by altering flash metering considerably. It examines each evaluative metering zone before and after the E-TTL preflash. It then calculates the weighting for each zone independently, biasing against those zones with high reflectivity in the preflash. This means that E-TTL II does not have a flash metering pattern as such, since it is calculated dynamically.
The fact that the camera biases flash exposure to the nearest focus point. If the camera has multiple focus points, it is important to keep in mind if you have the habit of using the old “focus, lock AE and recompose image” technique. Please do not do this when you are taking flash photos.
Flash metering occurs after ambient light metering, so in this case you’re locking AE but not flash metering. Therefore, recomposing messes up your flash metering. Instead, select the focus point that’s closest to your subject in order to bias flash exposure to that area.
The flash metering pattern is determined by the type of flash sensors built into the camera. If the camera has only one focusing point then it will have a single zone flash sensor. Flash metering is conducted using this sensor in a centre-weighted averaging pattern.
If the camera has multiple focusing points then it will have multiple zones; what Canon call their AIM system. The number of flash metering zones depends on the camera model. For instance, the EOS 10 has three focusing points and three flash metering zones, and flash metering uses whichever corresponding auto focusing point or points are active.
However, the EOS 5 uses the same sensor as the 10 so it has 3 flash metering zones even though it has 5 AF points.
These multiple zone flash sensors let the camera bias the flash exposure to the currently selected AF point. When you focus manually the camera does not bias any flash zone but chooses the central zone instead.
Note that EOS 5 is somewhat different from other multiple AF point cameras in that it will only bias flash exposure correctly to the nearest AF point if that point was manually selected. In automatic and ECF modes it apparently always chooses the centre zone.
E-TTL flash metering pattern
The camera uses its evaluative metering system to meter the flash output, based upon the preflash. When in auto focus mode, most EOS bodies which do not use E-TTL II bias flash metering toward the currently selected AF point, but always in an evaluative mode pattern - they don’t use spot or partial metering patterns. When in manual focus mode it appears that at least some EOS bodies switch to centre-weighted averaging.
Note that this biasing of E-TTL metering to the active point is potentially problematic, since it means that the flash metering is done in almost a spot-metering fashion. Many user complaints regarding flash metering problems in E-TTL mode appear to be linked to this issue. If the camera happens to be over a dark object, for example, flash metering can be considerably overexposed, and vice-versa.
The standard answer to this problem is
1) Use FEL and meter off something mid-toned, but this is clearly not a solution for rapid-shooting situations such as weddings and sports.
2) Set the camera lens to manual focus, since the body apparently switches to centre-weighted average metering in that mode, but that’s obviously not a useful answer much of the time either.
Users of the EOS D30 and D60 have been particularly unhappy with E-TTL flash metering. EOS 10D apparently reduces this problem by defaulting to a centre-weighted averaging metering pattern in E-TTL, even when the lens is set to auto focus.
E-TTL II addresses this problem by altering flash metering considerably. It examines each evaluative metering zone before and after the E-TTL preflash. It then calculates the weighting for each zone independently, biasing against those zones with high reflectivity in the preflash. This means that E-TTL II does not have a flash metering pattern as such, since it is calculated dynamically.
The fact that the camera biases flash exposure to the nearest focus point. If the camera has multiple focus points, it is important to keep in mind if you have the habit of using the old “focus, lock AE and recompose image” technique. Please do not do this when you are taking flash photos.
Flash metering occurs after ambient light metering, so in this case you’re locking AE but not flash metering. Therefore, recomposing messes up your flash metering. Instead, select the focus point that’s closest to your subject in order to bias flash exposure to that area.
