A question came up in a Flickr group that I used to moderate. A photographer had taken two pictures of a beautifully lit Christmas tree. Both photos were taken at the same exposure; but one was defocused so that the lights were visible as circular bokeh. In the focused image, the lights appeared to be white or very nearly so. In the defocused image, the colors of the bokeh were deeply saturated. The photographer why this should be so. He expected the saturation of the in-focus lights to be the same as the colored bokeh saturation and was surprised to find it wasn’t.
Several other people in the group did point out that saturation increases with less exposure but they all seemed stumped as to why the colors weren’t similarly saturated despite the photos being taken at the same exposure. While they were on the right track, they were missing a crucial piece to the puzzle.
Exposure is defined as the amount of light per unit area that reaches our sensor or film. It’s an average. When we think of the exposure we shoot a frame at, we are considering the total amount of light coming through our lens. We rarely need to think about it, but that light isn’t evenly distributed across our sensor or film when we take a picture. If it were, our pictures would all be totally flat, completely lacking in contrast. Photos are only interesting because some parts of the sensor get less light and others get more.
Obvious, right?! But how does that complete the puzzle? How does that explain the difference between the colored bokeh saturation and the saturation of the Christmas tree lights when they were in focus? Well now that we are thinking of exposure as both a global quantity (how much light did we allow through the lens?) and a local quantity (how much light hit this part of the sensor?) it’s easy to see how the rule that greater exposure results in less saturation actually applies here.
Even though the exposure remained the same between the photographer’s two photographs, the local exposure received by different parts of his sensor changed dramatically between the in-focus and out-of-focus shots. When the Christmas tree was in focus, the light from each bulb was concentrated on smaller parts of the sensor. Consequently, the exposure on those parts of the sensor was higher and the colors were therefore less saturated. When the tree was out of focus, the light from each bulb was spread over a greater area on the sensor. The same amount of light over a greater area means less light per unit area, i.e. less exposure. And the decrease in exposure resulted in more deeply saturated colors.
To sum it up: local exposure affects colored bokeh saturation.