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| p.1 #6 · Shooting TTL on an SB-910 |
I'm a Canon user but see the same thing in high ISO flash shots. At high ISO the flash metering, which also includes a baseline ambient reading for comparison, cuts back flash because to avoid overpowering the ambient set shutter/aperture/ISO for.
All TTL metering is at best a logical evaluation of what is in the metering zones and the camera settings to try to guess the intent of the photographer. The logic appears to work like this. When low ISOs are used the flash metering figures the photographer wants the flash to dominate and the ambient to be a secondary concern and will bias exposure on the flash lit foreground. The opposite is true in high ISO situations. Why are you using high ISO? Not for the foreground, but for the background ambience. So the flash metering cuts flash so not to overpower the ambient.
The fact the dress was shiny satin and bouncing light back into the camera like a mirror and there is a closer guy in the white shirt on the left may also have been a factor in the under-exposure in that shot. Evaluating in Levels I found that the shiny white dress in the foreground started clipping with only about 1/3 stop more exposure. By the time I corrected exposure to the point the face was correctly exposed (starting to clip in red in specular highlights) the dress was blown out in all channels:
The flash metering, seeing and reacting to the light reflecting off the shiny dress underexposed what you want to be correct, the face. The camera metering can't know that the face is more important, it can only react to the light it sees. Had she been wearing less reflective black dress the metering seeing less pre-flash coming back would have metered out more main flash. Had you better understood how TTL works and cropped the guy in the white shirt out of the photo the metering also would have seen and reacted to less pre-flash coming back and exposed it better, but you still would have needed to override with + flash compensation to handle the brighter than average white dress biasing the metering.
When composing flash shots, both in Manual or TTL, you need constant situation awareness of subject distance to the flash. In TTL things like closer guy in the white shirt or shiny satin dress, or even a mirror on the wall in the background throw off the metering because the algorithms assume the photographer knows to avoid situations like than or will compensate for them. The laws of physics being what they are had you been using manual flash as set it via guide number for correct exposure on the woman's face her closer lighter dress would be overexposed and clipping as in the Levels corrected edit. Flash can only be correct in the highlights at one distance from the flash. With TTL the metering finds brightest metering zone in photo and sets flash power so as not to clip it. Everything further way gets less flash because it falls off per the inverse-square law regardless of how it's metered. The metering worked correctly per it's programming, you simply composed the shot poorly for flash by including something highly reflective in the foreground closer than the face you wanted to have "normal" exposure. But had you used flash compensation to override the metering's best guess to the point of exposing the face of the woman correctly her dress. The solution when people are wearing white clothing? Compose the shot in way that the face winds up closer to the flash than the clothing.
Different metering modes might help improve results but the biggest problem in this shot were the composition decisions (white dress and white shirt in foreground) and unrealistic expectations of what the camera metering is capable of. With flash you need to learn to compose shots so what you want correctly exposed is closest to the flash. With TTL you need to compensate manually with lighter and darker than average clothing. If you see a white dress filling the center of the frame you can anticipate the metering seeing more than average pre-flash coming back will underexpose the flash will need + flash compensation. If you a subject is wearing a long sleeved black dress you can anticipate you will need to dial in minus flash compensation to avoid overexposing the face.