#11. "RE: SB-700 TTL Selection Using Spot Metering?" In response to In response to 10 Sat 24-Nov-12 03:07 PM by Arkayem
Richmond Hill, GA (Savannah), US
>>>When your camera is set to spot meter and the SB-700 >is >>in >>>TTL, the camera will meter on the focus point and >send >>that >>>info to the flash. >> >>Actually, in regular TTL mode, no camera metering >information >>is sent to the flash. In regular TTL mode, the flash >power >>setting is calculated strictly from the monitor preflash. >The >>metering value or mode on the camera will make no >difference >>to the flash power. > >Hi >Damn it, I thought I understood this but now I dont. I know >that I have to select Spot Metering mode to force the gun off >TTL-BL and into TTL mode. But what metering mode is is being >used to work out info from the pre-flash?
When using flash in Regular TTL mode, the reflected energy from the preflash is always measured center-weighted across the entire frame, regardless of the metering mode of the camera. In Regular TTL mode, the metering mode in the camera is only used for determining the aperture/shutter combination and has no impact on the flash metering.
There is no communication between the flash metering and the camera metering when using Regular TTL mode. The flash sets the entire exposure based on the preflash, and ambient light has no impact on this measurement. This is why it is best to use the camera in Manual mode when using regular TTL and set the ambient exposure to be about 3 stops underexposed based on the camera meter. Then, the flash will effectively make the entire exposure on the subject (the background will be mostly ambient).
This works entirely differently when using TTL-BL, and the approach you use should also be entirely different.