Lately there have been several comments made by experienced photographers stating that selecting different focus areas in the camera will change the place in the image that the flash meters from. I knew that this wasn't true based on my knowledge gained by designing flash and camera integrated circuits. I knew that the flash meters the entire frame based on center weighted brightness. However, I needed some proof, so I took the following series of images.
Firts, I arranged a white panel and a black panel so that they split the frame exactly in two. These images are unedited, directly out of my D3, low ambient conditions, f/5.6, 1/200th, ISO 400, regular TTL.
In image 1 (below), , the focus point was moved to the far left in the center of the white panel (the panel appears gray, because the flash system has averaged the center weighted frame and chosen a power that balances the entire image to Nikon’s meter 12% gray reflectance.
In image 2, I moved the focus spot all the way to the right side in the middle of the black panel. Notice that this image is exactly the same exposure as Image 1) showing that the position of the focus point is not used for the regular TTL metering system.
In the next few images, I investigate the effects of the focus system on TTL compared to TTL-BL.
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