In these multiple exposure modes, the bightness or darkness of each pixel in the base image is compared to the brightness or darkness of each pixel in the subsequent image or images in same position. In Bright mode, the lighter pixels replace the darker, and in Dark mode, the darker pixels replace the lighter.
Here are three images of the same scene. The first is an ordinary image, the second and third each consist of nine hand-held exposures, with slight variations in camera position from one exposure to the next.
First, an ordinary “base image”:
Image number two: a multiple exposure in Bright mode. The lighter pixels have replaced the darker pixels with each additional exposure.
Image number three, a multiple exposure in Dark mode. The darker pixels have replaced the lighter pixels with each additional exposure.
In this case, creating image number two can be thought of as a process of subtraction or elimination, since the object of interest is dark compared to the background. The exposures were hand-held, therefore slight variations in camera position caused each additional exposure to result in the gradual cancellation of darker pixels. The tree thus appears to be shriveled.
Creating the third image, however, was more a process of addition, as the dark pixels annexed the lighter pixels around them, more and more with each additional hand-held exposure, giving the tree a fuller appearance.
The creative posibilities of these exposure modes are very unlike mutliple exposures created with photographic film. Chris Friel has already elevated their use to an art form, a reminder that photography continues to evolve, an evolution which is driven directly by changes in technology.