Doug Kerr
Well-known member
On of the beloved bedtime stories about the Canon evaluative exposure metering systems (Jerome Marot just recently invoked it in another thread) is that of the "scene database".
The story is usually told like this:
But not only don't I know "exactly, for sure" how that works, I don't even have in hand a fanciful story, reaching from end to end, about what might actually happen.
Maybe somebody out there has a credible story, which I'd be glad to hear. I don't look for it to be "the real story". Just a whole story.
For now, let me think a little about the matter, as If I were inventing the scheme from scratch.
At the very top of the thought chain, what would we like to know about the actual scene if we could determine it directly in some practical way?
Perhaps we would like to know the maximum luminance anywhere in the scene, so we could plan an "expose to the right" shot.
Or perhaps we would like to know the maximum luminance and the minimum luminance that is of any importance in capturing all the scene detail, if possible, so we can plan sort of a "zone system" exposure.
Or something else, but it has to be something that could be described in technical terms - it can't be the "mellowness of the scene" or such.
And, once we had defined it, it could be determined directly if we knew the apparent luminance of every pixel spot in the image.
But (in an EOS camera of traditional form) we do not examine, during metering "every pixel spot" in the image. For example, the maximum luminance in our scene necessarily fall at any of the metering "segments". So we can't determine the needed properties directly. And that's why we need to have recourse to the magic database.
In effect, we say, "for scenes like this one, what is most likely the maximum luminance (as a ratio to, for example, the average of the measured luminances)?
"Like this one" means perhaps "having these ratios between 10 of the 30 luminances we measured" (the "search vector").
Then when a match is found, the entry "returns" that ratio, the metering system multiplies that by the average of the values in the search vector, and gets the "equivalent scene luminance" which it enters into the metering equation to determine the photographic exposure (actual Ev) that is best for the shot. (We know that for the special case of a uniform-luminance scene, that equivalent scene luminance is the same as the average measured luminance. That's probably one of the scene models: "all search vector values equal".)
Well just some thoughts to fill space until somebody with a real story can bring it to mind.
Best regards,
Doug
The story is usually told like this:
After the scene is examined by the exposure metering system, a data base of "scene models" (held in the camera) is searched for one that most closely matches the properties of the scene apparently before the camera. That data base entry returns something that the exposure metering system uses to complete teh process by which it chooses the "most likely best" photographic exposure for the shot.
But not only don't I know "exactly, for sure" how that works, I don't even have in hand a fanciful story, reaching from end to end, about what might actually happen.
Maybe somebody out there has a credible story, which I'd be glad to hear. I don't look for it to be "the real story". Just a whole story.
For now, let me think a little about the matter, as If I were inventing the scheme from scratch.
At the very top of the thought chain, what would we like to know about the actual scene if we could determine it directly in some practical way?
Perhaps we would like to know the maximum luminance anywhere in the scene, so we could plan an "expose to the right" shot.
Or perhaps we would like to know the maximum luminance and the minimum luminance that is of any importance in capturing all the scene detail, if possible, so we can plan sort of a "zone system" exposure.
Or something else, but it has to be something that could be described in technical terms - it can't be the "mellowness of the scene" or such.
And, once we had defined it, it could be determined directly if we knew the apparent luminance of every pixel spot in the image.
But (in an EOS camera of traditional form) we do not examine, during metering "every pixel spot" in the image. For example, the maximum luminance in our scene necessarily fall at any of the metering "segments". So we can't determine the needed properties directly. And that's why we need to have recourse to the magic database.
In effect, we say, "for scenes like this one, what is most likely the maximum luminance (as a ratio to, for example, the average of the measured luminances)?
"Like this one" means perhaps "having these ratios between 10 of the 30 luminances we measured" (the "search vector").
Then when a match is found, the entry "returns" that ratio, the metering system multiplies that by the average of the values in the search vector, and gets the "equivalent scene luminance" which it enters into the metering equation to determine the photographic exposure (actual Ev) that is best for the shot. (We know that for the special case of a uniform-luminance scene, that equivalent scene luminance is the same as the average measured luminance. That's probably one of the scene models: "all search vector values equal".)
Well just some thoughts to fill space until somebody with a real story can bring it to mind.
Best regards,
Doug