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DAN: thank you so much for the helpful info, especially the point about the red channel - very interesting. But I still have this one question: if the in-camera histogram is based on a camera generated JPEG, then how much discrepancy is there between that JPEG reading and the actual RAW image I ultimate care about? In other words, I'm not getting a histogram of my RAW image, but a camera generated JPEG, so how accurate is that?!
You bring up an interesting question. I could answer several ways. One would be to go look up which picture setting is the one that supposedly provides the most accurate histogram display since, as you mention, it is based on (or at least used to be based on) a jpg version of the image.
But... in reality the histogram image, no matter how "accurate" it is, must still be interpreted by the photographer. The red channel issue is a good example. Here is a little list of issues in this particular situation:
1. Some don't realize that they can display a multiple channel histogram that separately displays the red, green, and blue channel histograms. The combined ("white") histogram display averages the RGB values. This works fine for many sorts of scenes, but in those that are strong in one channel but weak in the others, that strong channel can be over-exposed with no indication from the white histogram, since it averages that over-exposed channel's value with the less intense values of the other two channels.
2. With this in mind, at least with subjects that are very strong in one of the color channels (but why not all the time?), it is better to use the three channel display and watch for the strongest channel to determine your ETTL ("expose to the right") based exposure settings.
3. However, this isn't enough in all cases. There are certain subjects in which you might still need to drop your exposure by a stop or so. Here I commit the sacrilege of actually looking at the image in the LCD, where I have learned to recognize how it responds to near-blown or blown channels by changing the image color or saturation in those areas. This is not an objective measure nor an accurate view, but it tells me that I'm pushing it in situations where the camera says I'm not. Another situation - which I think I mentioned in my earlier post - is when the image is very high key and I want to retain sufficient detail in the near white tones. Here I will not push all the way to the right, especially if there are few or no dark tones in the scene. I find that I'll have better luck working those high key tones in post if I "underexpose" a bit from the supposed ideal ETTR setting.
Among the many themes that I would like to see more forum photographers understand is that there is no such thing as a camera-determined ideal or perfect exposure. There are many ways to decide to expose a thing, and we still must use our experience and exercise judgment (and occasionally make an educated guess) about things. Some, especially new photographers and double-especially new "digital" photographers hope that the camera will make the right decisions in all cases. This simply is not the case.