Mid-tone Contrast
As we've adjusted our exposure as described in Part 4 and our shadows and blacks in Part 5, it is now time to turn our attention to adjusting the mid-tone contrast of the image.
The image of a cloud deck over farm fields was taken from an airplane just after take off from my home in Minneapolis. The image, having been adjusted up 1/4 stop to expose the whites properly and having had the blacks or shadows pulled down using a RAW Converters' Black Control, is now ready for mid-tone evaluation.
For me the tool of choice is the Tone Curve. If you are a Corel Paint Shop Pro Photo or a Adobe Photoshop user, the tool is called Curves. Today, I am working in Corel PhotoPaint so we will call it the Tone Curve. Here is an example of the dialog.
The dialog is pretty simple. Your black and your white are anchored on the null line that represents an unadjusted image. You have the ability to adjust the entire image at once or to split the channels into red, green and blue and adjust each individually. One of the first things I do is give myself a finer grid with which to work. Working in 1/4 divisions doesn't give me the precision I feel I need so I <ALT><Click> in the grid to make it into a 10x10. Then, I establish anchor points at the 10,10 and 90,90 markers to hold my blacks and whites in place because I have already worked them to the value I prefer.
Now that I have that done, I can begin to work on adjusting my mid-tone contrast. The curve tool is a subtle device. A little change can go a long way which is one of the reasons I like to be in the 10x10 grid mode. I seldom move more than 10% from the neutral line in any adjustment. Adjusting the curve is as simple as clicking and holding on a spot and dragging it up or down to increase or decrease exposure in the mid-tonal range.
Here is a grid adjustment where I have anchored my end points 10% in. Looking at the 3rd, 4th and 5th points that were added, I have deepened the shadows slightly, brightened the middle range and then pulled back some of the highlights.
Note that no point was moved more than a few percentage points from the neutral slope line shown in the default.
Play with the adjustments and note what they do. If you are moving an area you don't wish to move, click on the curve in that area to anchor it and then go back to the area you wish to adjust. Be careful. A lot of points or a lot of adjustment makes for posterization in your image.
The difference in the adjusted image is subtle as desired. There are better tones in the white areas with softer shadows on the tops of the clouds. There is also more detail in the dark areas immediately adjacent to the shadows. The real gain is in the detail in the patchwork farmland below. You can now see definition in the fields and the rays of sunlight more clearly.
Our blacks, whites and mid-tones are now just where we wanted them. Oh no! Our shadows have turned purple. That is why I adjust contrast before color. Looking at the initial image it wasn't terribly obvious that we had a purple cast in the shadow areas of the clouds. Now that we've adjusted contrasts, that color cast is very obvious and we can go after it.
Next time.
Rikk Flohr © 2008
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