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Can you post a histogram image of a correctly exposed scene?
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First, let me start by saying that there may be no such thing as "correctly exposed", or at least one can imagine a range of exposure levels that might be considered such. There are images with large areas of overexposure that could actually be considered "correctly exposed" - for example the image of the inside of a room including a window to the bright outdoors.
In short, one MUST consider the image content when evaluating the histogram.
Your request for examples is reasonable, so I'll give it a try. Please note that I'll be showing you Photoshop histograms here. These are essentially the same as those you'll see on the back of the 10D, in that the column heights represent the relative numbers of pixels at each luminance value, darkest on the left and brighter to the right.
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Consider this image:
Here's the histogram (from Photoshop) for the above image.
Note that the image does not contain luminance values that extend all the way to the right side of the histogram. This indicates that even the brightest parts of the image do not occupy the maximum possible luminance values. In short, it's a little underexposed. Also note that there are two narrow areas on the histogram where a large number of pixels fall. These represent the luminance of the sand and water. Last but not least, note that there are no pixels running all the way to the left - though parts of the dog look pretty black, this is telling us there's real data there, and not just complete black-out. This image can be brightened up in Photoshop to be quite good.
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Consider this high contrast image:
Here's the histogram (from Photoshop) for the above image.
In this histogram you can see that data DOES extend all the way to the right side, in a thin line of pixels across the bottom, then a bunch of them jammed up against the side. This is typical of an image with bright highlights, such as the reflections of the sunlight near the butterfly and on the leaves. A little of this may not indicate a problem, if you expect white highlights, though in this particular image the brightest portions of the red flowers and leaves are actually a bit too blown out. Also note that the deep gray background has contributed to some large bumps toward the left.
You also often see data jammed up against the right side if you capture a bright sky in the background.
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Here's a third example, an image most would consider pretty much correctly exposed:
Here's the histogram (from Photoshop) for the above image.
Note that the data pretty much falls off just a short way from the right side, with a thin line of pixels running the rest of the way up. Also note that there is no data stacked up against the right side. No overexposure, with highlights right up to but not exceeding max luminance. There's nothing perfectly black in this image either, though there's a fair amount of dark data. What to do with the space on the left? One can punch up the contrast some by using Image-Adjust Levels and making the darkest data run all the way to perfectly black. You end up with something like this:
Hope this helps!
-Noel
Linear Mode
