POSTERIZATION of Photography
Have you ever come home from a shoot with that image that you are sure is going to be a winner? Its one of your best shots. You add the finishing touches and hit the print command. A few minutes later, out comes the print. Everything looks fantastic -- except the banding that runs across the sky. You try several different things in the editing program, but each new print continues to show the banding. At that point, that sinking feeling begins to set in as you realize that you may have lost one of your best shots. You are the victim of posterization.
POSTERIZATION
Figure 1: Posterization
Posterization is highly undesirable. However, posterization can usually be prevented. For those cases where posterization has already occurred, it can usually be corrected if you know the proper technique. The purpose of this article is to cover the cause, prevention, and cure of posterization.
POSTERIZATION: CAUSE
The cause of posterization can be explained in one little sentence: too little information spread too far apart. The "information" refers to the tones contained in an image, and the "too far apart" refers to how far apart the tones are from each other. Figures 2 -- 5 illustrate this concept. Figure 2 (JPEG image) shows a color gradient from green to red. The gradient has a smooth transition of color across the gradient. Figure 3 shows the histogram of the image. The histogram shows no tonal gaps. In Figure 4 (JPEG image), the color gradient was subjected to some editing that caused posterization. The histogram in Figure 5 shows why the posterization occurred. This histogram shows that the tones are now spread far apart.Figure 2: Colors with No Posterization before Editing (JPEG Image)
Figure 3: Histogram with No Tonal Gaps before Editing (JPEG Image)
Figure 4: Colors with Posterization after Editing (JPEG Image)
Figure 5: Histogram with Tonal Gaps after Editing (JPEG Image)
Figure 6: Shadow Tonal Values before and after Editing
POSTERIZATION PREVENTION
It is easier to put on a coat than to cure pneumonia. Similarly, it is easier to prevent posterization that to fix it after it has occurred. There are three ways to prevent posterization.- Minimize image editing.
- Increase the number of tones through raw.
- Increase the number of tones through exposure.
POSTERIZATION PREVENTION #1: MINIMIZE IMAGE EDITING
Now, this may not seem like the most brilliant or technical solution, but posterization can, sometimes, be significantly reduced or even eliminated by reducing the amount of image editing. For example, reducing the amount of Curves or Hue/Saturation used might help reduce or eliminate a posterization problem. Another possibility is to use a tool that doesn't spread the tones apart (e.g., use Brightness/Contrast instead of Curves). By reducing the amount of editing, or using a tool that does not spread the tones apart, the distance between the tones will be less and the posterization problem may disappear.While a bit simplistic, and not possible with all images, this method may save the day for some images.
POSTERIZATION PREVENTION #2: INCREASE THE NUMBER OF TONES THROUGH RAW
One of the easiest ways to help reduce or eliminate posterization is to shoot images in raw. Raw has sixteen times more tones than JPEG. Therefore, the tones are much closer together than when the JPEG format is used. Thus, images that start as raw and are converted to high bit TIFF (or some other high bit format) for editing are much less likely to exhibit posterization.The effect of having more tones can be seen by comparing Figures 2 -- 5 to Figures 7 -- 10. Looking back at Figure 2, we see an eight bit, JPEG image of a smooth color gradient that shows no tonal gaps in the associated histogram. However, after editing, the image exhibits heavy posterization (as shown in Figure 4) with large tonal gaps (as shown in Figure 5).
Figure 7 shows the exact same color gradient as that shown in Figure 2, except that this image is a sixteen bit image. The image has a smooth transition of color across the gradient. Figure 8 shows the histogram for the image. The histogram shows no tonal gaps. In Figure 9, the color gradient was subjected to the same editing done with Figure 4. This time, there is no posterization due to the larger number of closely spaced tones in this image, and the histogram in Figure 10 shows a lack of tonal gaps.
Please keep in mind that these color gradient images were created in Photoshop for illustration purpose. They were not created in a camera (e.g., Figures 7 and 9 were not created from raw images; however, they behave similarly to raw images that were converted to sixteen bit TIFF files). Thus, these images illustrate well the impact of having more tones.
Figure 7: Colors with No Posterization before Editing (16 Bit Image)
Figure 8: Histogram with No Tonal Gaps before Editing (16 Bit Image)
Figure 9: Colors with No Posterization after Editing (16 Bit Image)
Figure 10: Histogram with No Tonal Gaps after Editing (16 Bit Image)
So, a great defense against posterization is to shoot raw and convert the raw images to sixteen bit files for image editing.
POSTERIZATION PREVENTION #3: INCREASE THE NUMBER OF TONES THROUGH EXPOSURE
Have you ever noticed that posterization tends to occur mostly in the darker parts of an image? There is a reason for that. The different tonal regions contain different numbers of tones. This is shown in Table 1. For a five stop dynamic range, JPEG image, there are 128 tones in the highlight tonal region (before application of tonal curves) but only sixteen tones in the shadow region (before application of tonal curves). Since the darker tonal regions have so few tones, posterization is a bigger problem in these regions.Light Level | Tonal Values | Notes |
|---|---|---|
| 5 Stops | 128 | Highlights |
| 4 Stops | 64 | Three quarter tones |
| 3 Stops | 32 | Mid tones |
| 2 Stops | 16 | Quarter tones |
| 1 Stop | 16 | Shadows |
However, there is a way to deal with the problem of the limited number of tones in the shadow areas. By increasing the exposure in the shadows by maximizing the exposure (see the article titled Digital Exposure) or by using multiple exposures (see the article titled Shadow/Highlight Detail -- Part IV) the number of tones is increased in the shadows. For example, as shown in Table 1, exposing in the usual way, the shadows would get one stop of exposure and would contain a mere sixteen tones. If the maximizing the exposure or multiple exposure technique was used to give the shadows one extra stop of exposure, the shadows would have two stops of exposure. The first stop of light would generate sixteen tones, and the second stop of light would generate another sixteen tones. The shadows would now contain thirty-two tones, rather than just sixteen. The addition of the extra tones would reduce the tendency for posterization in the shadows. While this may make the shadows lighter than desired, the brightness of the shadows can be adjusted in the raw converter or Photoshop to bring the shadows back to the desired brightness level.
FIXING POSTERIZATION
Figure 11: Image with Dark Tones
Figure 11 shows one of my favorite images. I was excited about this image from the time that I pressed the shutter button. However, when I made my first print, I was greatly disappointed when I saw bands running across the sky (you can not see the banding in this small, web image). Happily, the following technique saved the image.
The posterization issue is resolved by adding something that, under normal conditions, everyone tries to get rid of: noise. The noise is added in very small amounts to only the areas that show posterization (the sky in this image). The noise breaks up the edges of the bands so that the eye can no longer detect the edge (kind of like how camouflage breaks up the outline of a human figure so that it is not discernable). On the other hand, the noise is added in such small amounts that it is not readily noticeable at the normal viewing distances for prints. Furthermore, since posterization is most noticeable in dark areas that have little detail, the noise is being added to areas where there is not much detail with which the noise can interfere. This is seen in Figure 11. The noise will be added to the sky where it will not hinder any significant detail.
One important point is that the noise should be added after all editing has been completed and the image has been interpolated to its final size but before the final sharpening has been performed.
Figure 12: Layers Palette
Figure 13: New Layer Dialog Box
Figure 14: Layers Palette with the Noise Layer
Figure 15: Layers Palette with the Visible Layers Merged into the Noise Layer
Figure 16: Add Noise Dialog Box
Figure 17: Layers Palette with Mask Added to the Noise Layer
Figure 18: Layers Palette with Sharpening Layer
