To really use histograms effectively when evaluating exposure, you need to use three-color-histograms. If you understand what I've explained in Part 1, Part 2, and Part 3, this isn't too complicated. We must simply realize that our cameras aren't recording different levels of grey, but actually different levels of red light, green light and blue light. So instead of thinking of brightness levels as the variation between black and white, we must think of them as the variations between dark red and light red, dark green and light green, and dark blue and light blue. If that doesn't make sense, come back a read this paragraph again after you read the rest of this post.
When we look at the three color histogram from the picture in Part 3, we see this:
from this picture
So what you are looking at in the graph above is the variation in brightness of red light, green light, and blue light in the picture. So that's how digital cameras work - they record the variation in red, green, and blue light and then overlay the three one-color images to produce color images. So when we take a picture, we need to make sure that all three graphs stay away from the edges.
Next time we will take all the theory and make it practical and usable.
Read Part 5 next.
