To really show the process of using curves I think it will be easiest if I take a couple photos and go through the process with example images and an explanation of each step. The first image I’m going to work with is a color image and I will work through it using Photoshop curves, and the second will be a black & white example although I will show the curves adjustments performed in Capture One instead.
Color image
 The original image looked a little flat to me so I wanted to enhance it some with the curves tool. Normally I would perform these curve adjustments in my RAW converter but since many people will probably be using Photoshop for curves I will show these modifications using the Photoshop curves tool. |
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The figure in the upper left corner is from the Photoshop CS3 curves tool and shows the curve applied overlaid on the original histogram. The figure to the right of that shows the output histogram that is the result of the curves applied to the image. You can see that the histogram starts to get jagged and has gaps, this is because I did these adjustments on a 24 bit RGB image (8 bits per channel). Normally I would only use the curves tool in Photoshop on a 16 bit image but I did this for demonstration. With only 256 shades available in each color channel, as you stretch out pixels across a broader range of tones you wind up creating gaps because there is no data with those shades to fill in the histogram. If you do this enough you can see posterization in your image where there are gradients (gradual changes in brightness) because those gaps in the histogram mean the image doesn’t have data to fill in the tonal transitions. |
 At that point the whole image was lighter and it had reduced contrast in some of the lower tones so I created a control point right near the bottom left end to pull the shadows down. This makes the curve much steeper between this control point and the one I created previously which gave me some of that contrast I wanted. It also has the unwanted effect of pushing the curve up even higher to the right of the original control point I created in the previous step. |
 To counteract the boost to the highlights that the previous adjustment caused I added a third control point to the right of the first one I used and pulled the curve down some which reduced contrast in the highlights and lowered their brightness. This is a matter of taste, some would probably not mind the image as it was before. |
B&W image
 I chose this image to work with because I happened to show it on the previous page. The curves dialogs shown are actually from Capture One however the process would be similar if you chose to apply the curves in Photoshop or any other program. The workflow in this particular example is a little abnormal but the process of adjusting the curves remains the same. |
 This second image is simply the original with the channel mixer settings applied to the image. The settings used for this conversion can be seen in a pop-up by clicking on the photo to the left. Instead of using the channel mixer there is also a Black & White option in the image adjustments menu in Photoshop which is similar but gives you control over more colors rather than simply the red, green and blue color channels. |
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The figure in the very lower left is again the output histogram which is the result of the applied curves. The figure just above that is the curve tool in Capture One which is overlaid on the input histogram. Since these curve adjustments were performed on 16 bit data there is a much lower chance of gaps in the histogram which means posterization is far less likely to be a problem. This is why you should always do your manipulations with the highest quality versions of your images you have! The closer to the original (RAW for digital capture) the better. |
 The previous step corrects the overall brightness of the image but the highlights have been made too bright, so I add another control point to the right of the first that allows me to pull down the entire curve to the right of the first point. By doing this I’ve reduced the contrast of the highlights some in areas, however overall that’s OK because the subject matter I’m most interested in (the people) is darker, so reducing highlight contrast a little isn’t an issue. |
 Finally I need to pull the shadows back down so I add a third control point to the curve close to the bottom left and pull it down some. This forces a steeper curve across a very high part of the histogram ensuring that I am increasing contrast across part of the image where there is a lot of data. This also improves the depth of the shadows on the jacket the person in the middle of the frame is wearing. |
 The final step with this image when I first processed it last year was to apply layer masks for local burning of the bright background so I have included that here to just show the final image produced last year. |
I think I will try to recommend this post to my friends and family, cuz it’s really helpful.