Histogram Part 5

Now that we've been working with the Histogram Adjustment, we should take a moment to review the function of the tool again.

We use it to adjust the lightness and darkness of an image, or a selected section of an image. We've seen that we can expand the range of luminance or even create a non-linear (make the response curve bend) range. We can vary the start and stop points of the response curve to limit the white and black levels, we can set breakpoints where white and black are located on the input span of luminance.

There one more thing we can do with the curve between the white and black breakpoints. We can either Compress or Expand the response curve. Later on, we'll see what they do, but for now- Since these are opposite functions, you cannot have both. You'll have one or the other, but not both.

Look at HistogramTutPic8.

HistogramTutPic8.jpg

Look that the slider control on the right side of the display window. We've had the control set to 0 up to this point. You can see that moving the slider up add the curve effect shown on the left of the slider bar to the response curve between the breakpoints. I've used a value of about 28 to make the compression effect really appear on the response curve.

Remember that a way to visualize the effect of this tool is to imagine a range of luminance coming in on the horizontal of the spectrum window, Either TOP or Bottom (doesn't really make a difference) and moving down or up the graph until the luminance line hits the response curve, then that individual value is immediately output on the side (defined as the output)

Yes, I keep driving this home, since it's the very key to understanding several of the tools! When this point is understood then several new vistas are going to open to you!

Now, apply that knowledge to the picture. I've added some compression to the response curve. Look at the input/output spectra displays. The output now has been compressed, that is, the range between the dark areas of our image and the light areas has been decreased. Since this curve is non-linear the compression is also non-linear. Luminance in the middle of the curve isn't changed as much as those near the breakpoints of white and black. The picture itself starts appearing gray. Now, let's see what the opposite effect is:

Look at HistogramTutPic9.

histogramTutPic9

While the center of the curve is still relatively linear, the light and dark areas have been stretched out, and along a gently changing curve, at that! Notice that the individual channels of dark luminance are spread out enough to start showing as spikes. Remember that should this become too obvious, then our luminance will start showing as 'Posterized'! Recall that image that had a low amount of luminance values?

There are no 'Magic' values or settings for the Histogram. It's totally up to you to decide how you want the luminance of your image to appear! Use the Histogram Adjustment to tame your image! It's a powerful tool, but it's up to you to apply it to an image and make it work of art.

Just one more section to cover in the Histogram, then a general wrap up. You're almost there. Congrats on making it this far! Join me for Part 6 next.