Histograms. They tell you about the data contained in your picture. With it you can understand the retouching requirements and potential problems. In this article we look at before and after pictures relating their histograms to the actual images. The text contained between the two images describes some of the retouching done to the picture. The objective is to help you relate to the histogram and how retouching reflects on the histogram. We'll take a look at two pictures before and after retouching from the point of view of the histogram. We see what the histogram tells us about our picture and also how the histogram reflects our retouching efforts.
1. Old Bridge
Interesting picture, but unfortunately it is not very usable. There seems to be too many dark density pixels on the left of the histogram. They translate into very dark areas of the picture that are carrying virtually no detail. Depending on how your monitor is set, you may see more or less detail in these areas. When retouching, you have to take this into account. If you give your viewer a very dark picture, their monitors may be making it even darker. The result is a black glob that holds no image. But what if the viewer likes it that way? Well, then by artificially darkening the pictures they view they are expressing a preference. And they like it that way. Others may artificially lighten their views, again, because they like it that way. In retouching we must present an even density, not too light or dark. We must preserve near highlight and near shadow details. The last thing we need is a picture that is too dark and cannot be lightened on anyone's monitor.
Moving densities on your histogram around and redistributing the densities is a task for Curves. At left is the curve we are applying to try and improve this image. Notice that the highlight densities at the 1/4 tones are left as-is (lower left) because there really is no serious problem according to the right side of the histogram.
But it is the shadows that need work. The upper right dot (not the corner one) on is pushed very far to the right. This lightens the shadows improving the near shadow details. Along the 75% tones, the dot is lowered making the 3/4 tones lighter. We have to because you cannot have a flat part to a curve. The 1/2 midtone is also lowered. This improves contrast and is done while viewing the results on the picture itself. After a bit of tinkering and visually evaluating the results the curve at left is acceptable.
The finished picture is shown below at left. Notice the improved shadow details under the bridge and rocks. The highlights remain relatively unchanged. Notice the difference in the histograms. The highlights and midtones remain about the same. It is the shadows that are moved noticeably to the right (toward highlights and thus becoming lighter). They are also somewhat lower and this is compensated by a slight increase in the height of the midtones. We have effectively made the heavy shadows into detail carrying midtones.
Success? Not quite. If you look very closely at the newly adjusted shadows we see a problem common with digital cameras, especially the Sony. Video noise. Is this an indication of the camera's inability to separate close shadow tones. These near shadows are critical to the success of the picture. So we are faced with a problem. Do we want to present our viewer with areas of solid black or video noise of strange colors they will know do not belong? This is obviously a serious problem that is going to require more thought!
2. Ski Area
Here is a picture of a ski area during the summer. Not much going on with so much green and so little white. This picture is a bit flat, lacking contrast. According to the histogram, there are no shadows whatsoever. Although the picture looks light, there aren't that many highlights either. That little glob on the very end at right? That's the small chunk of sky in the upper left hand corner.
Usable picture? Certainly, there's a lot to work with here. Since there is a range of tones we can use Curves to stretch the densities out creating the illusion of highlights and shadows that aren't in the actual picture. Notice how the dot on the 3/4 tone is raised very high. This shifts all the tones toward the left of the histogram artificially making them darker. The highlights are left almost untouched, not because there are no near highlight densities on the original picture but because there really aren't no white areas of the picture other than the sky and cream colored building. There is no sense building highlight densities into a picture if they really don't belong. Shadows are another problem. We would assume that there would be a lot of shadows because we see darker gray areas in the original picture, most notably on the bottom of trees. The midtones are also darkened. This shifts a lot of the midtone densities toward the shadows. This will improve the contrast giving us a better looking picture while getting rid of the flat look. Could the quarter tones be reduced? This would improve contrast further but it was decided that it would be too high a contrast. This is the advantage of looking at the picture while you make your curves adjustments. Give it a lot of thought as you work, slowly evolving a curve that will produce as perfect a picture as possible.
Notice the change in the histogram that now gives us tones well into the shadow range. Not much for the highlights, but it really isn't a white picture. We could have made it a bit lighter by lowering the dot on the 1/4 tones but artist's discretion prevailed for better or worse. We could have darkened the shadows even more but that might have been at the sacrifice of near shadow details.
Where did the bluish haze go? In Photoshop Curves you actually have a curve for each of the three primary colors plus the overall. We have been working with the overall in these examples but in this case the Blue channel was reduced eliminating the excess blue haze. Hope you don't mind, but this picture really needed it!
Conclusion
Two entirely different pictures. One that is flat with no highlight or shadow details, the other with too many dark shadow densities. Which one came out better? From a usable standpoint, the ski area proved to have more possibilities because there are more midtone densities to work with and less densities at the extreme ends of the histogram. Midtones can be stretched or moved at will up or down the histogram.
But if the densities are at the ends, highlight or shadow, there is little that curves can do for them. True, solid blacks can be dropped down in density but they will then be added to the other densities already there forcing near shadows int dark grays. Separation of the near shadow densities is critical for the success of any picture. The same holds true for highlights. White carries no image except for the contrast next to other darker pixels. A large area of white carries no detail and no detail can possibly be added. When we adjust with curves, we alter the densities between adjacent tones improving the contrast bring out detail.
Our last point is the relationship between the curves, histogram and picture. Notice that there really is no pattern because each picture is unique. We can slowly build up an understanding of certain observances in histograms and what they may mean to our picture. From then we can use the histogram to help guide us in our curves adjustments. Experience is going to be necessary before you learn how to read them. Go ahead and fix a bunch of pictures. But as you work, watch the histogram change. That is an important lesson, an important part of getting good results from poor pictures.
