viewed small histograms

Read an item over on stack exchange where a user said one could improve DSLR focus by using the histogram. Huh? I read that again: "...may be able..." And then, "in theory." Finally: "I haven't tried this, so I am not completely sure if it would work." I see.

Anyway, it got me thinking. What effect does focus have on the histogram? His argument was not too crazy: an unfocused bright image would be spread out. So, time to do some research and experimenting.

After hooking up the 40D to the computer, I activated the Live View in EOS Utility. There it is, the little distribution graph, off to the right. It did shift a bit as I changed focus—I think. Interesting. But the display is tiny. Wondered if I could make it larger... Opened the Zoom View panel. Hey! No histogram. Boo. Could the histogram be made much larger? Could it be shown in the zoomed views? Could the histogram use only the data in the zoomed view, like a spot meter?

I opened the EOS Utility instruction manual. And was disappointed to find absolutely nothing on the histogram. The camera instruction manual had one page on the graph feature, split between the brightness, combined view and the RGB view. Right side, the highlights; left side, the shadow. Yada yada. Uh huh.

Finally, after some quick experiments, I've come to a few conclusions. The histogram chart, either on the back LCD, or on the computer, is simply too small to use as an accurate gauge of focus. And, not surprisingly, the histogram is the display of all the data reaching the entire chip, not a selected area.