Monday, March 15, 2010
TTL/BL and metering modes .
In my video on how TTL/BL responds to a change in the meter someone suggested that it was not the movement of the meter that caused this change but rather the fact that the change from Centre Weighted metering to Matrix metering caused the flash system to meter differently . This is not so . TTL/BL uses the focus point diamond for its metering regardless of whether you choose matrix or CW metering - these modes do not change how the flash system meters .
The reason for the change in output is that TTL/BL is trying to balance the flash with the ambient already on the subject . If you are in CW metering mode with the meter centered while shooting manual , and you change to matrix metering which decides the subject is underexposed and causes the meter to suddenly register "-1" then TTL/BL will increase its output - not because of the different metering mode but rather because it noticed the meter change . It decides that since the ambient is underexposed it may as well take over the exposure by increasing flash output .
My thought on the fact that you can't use TTL/BL with spot metering is that perhaps spot metering 'switches off' the rest of the metering pattern so TTL/BL can't do its calculations and therefore reverts back to TTL metering with the centre of the frame .
Either way I see no reason to suspect that the different metering modes [ CW vs matrix ] cause TTL/BL to meter with a different pattern - it still uses the focus point diamond - as with wireless CLS .