How to design a curve for your needs....

In order to wrap your head around tonal scale, you are going to have to understand the tools that can be used to measure and modify the way a picture is recorded. Hopefully your monitor is close to correct calibration so you can see how the original image could possibly become a printed image by viewing it on a computer monitor. Often you see a gray strip on a web page which suggests that if you cannot see all the steps, you are not seeing the image properly. We will be using a computer generated gray strip to show how things work. Later we will use a Kodak Q-13 test target to show how well your camera captures the visible world. You should be able to see from C to W in distinct steps. If you can actually see from A to Z, so much the better. If you adjust your monitor or video driver to improve the image, do so now. The small strip in our examples has 26 boxes whose measured value starts at 255 and descends by 10 steps to 5 on the black end. It goes left to right from white to black. The histograms and curves go from left to right from black to white, so don’t be confused by the direction. The Levels histogram in Photoshop are the way most people identify the tonal scale and adjust the end points and the mid point. grayscaleThe curves adjustment window displays a histogram, but adjusts the output by using a curve that represents the values being modified on the vertical axis. The composite picture below represents how the values marked here by a dot on the midpoint of the curve is translated from 125 to 155. Curves like this were used in the manufacture of film for almost 100 years. The combination of film/paper and developers, temperature and exposure had to be predictable to use the technology. This is called an “S” curve for obvious reasons. Controlling these variables contribute to consistency of the film and printing industry.

This particular example features a much brighter mid point for faces and improved ability to see detail in the dark areas. There is a sharper slope on both the high and low end to preserve detail where it is needed. Compare the original gray strip with the adjusted gray strip.

You might want to use a curve like this to take a picture of a bride in a white wedding dress with pearls and fancy white stitching.

I want to encourage you to create a still life scene to assist you in creating the curve for your needs. I will introduce you to the one I find very usefull. It is very patient, always available and does not complain. The Ben Franklin coin bank provides the high key detail. The farmer provides flesh tones, bright saturated colors and spectral highlights. The magician provides dark detail. The gray cards and Q-13 targets provide color and tone standards. There are a variety of textures to judge focus. The scene faces a South facing window with diffuse window covers and the whole room is painted pure white for flash diffusion if needed.

The following examples show how the Picture control curves behave. All pictures are made from the same NEF file and clipped from the Picture Control Utility.

Still life using Standard Picture Control.
Using Landscape Picture Control.
Using P&S Custom based on Standard.

All the variables in Picture Controls could fill volumes and have many thousands of combinations, but you could benefit by checking to see which settings would suit your needs best and then proceed no farther.

Let's start with the basic set. Eliminate monochrome and we have five left. Close examination will reveal that there are only three curves and the others are bumped  versions of those curves.  The map at the right shows that Standard and Portrait are the same, differing only in sharpness, and Normal is very close but has -1 contrast and saturation. Landscape and Vivid are very close and differ by only one bump from each other in contrast and saturation. The column of id's on the right side show lines connecting variations with the primary Picture Control.

After a few weeks of experimentation, I discovered that some of the curves that were very usefull could be duplicated in performance by simply applying the incremental digital adjustments to the in camera settings. In a few situations, I found that I could get the same result from in camera adjustment, or a custom curve or Active D-Lighting.

What are some of the ways we can use custom Picture controls?

High Dynamic Range, HDR, is getting a lot of attention and there are macros, plugins and special applications that require multiple pictures from a tripod to accomplish. Consider the solutions to a scene which includes a sunset (or sunrise) and other objects in shadow areas.
    1 Use a graduated neutral density filter to cut the picture in half so the sky is two stops less exposure to the rest of the image.
    2 If part of the picture is close enough to be covered by flash as in the case of the room, then match the ambient outdoor light value with the flash value of the room.
    3 Lower the contrast and the exposure to gain two or three stops of tonal range. This can be done with -3 contrast and +1 brightness and -1.3 EV adjustment.

Let's explore the HDR problem to start with. My experience is that shooting into a sunset is not that different in tonal range than using a flash in a large room with a high ceiling. Foreground images get burned out by the flash and people further away get underexposed because of the geometric fall off of a single light source. Lower contrast in the camera is part of the solution. Preventing overexposed hot spots can be handled with a curve that has less contrast in the bright area and more contrast in the shadow area.

The left image is with standard Picture Control and shows blasted face in foreground and darker faces farther back. It is hard to see, but the texture is blocked up in the sweater of the center person. The result of using the curve looks like balancing the ambient light with the flash, but it is not. It is the result of lowering contrast and bumping up the mid range. A similar effect can be had by using Neutral at low contrast and bumping the brightness up and bumping the saturation up by one. Either one of those solutions would enhance the yield on wedding pictures of brides in white and grooms in black in a light sucking hall such as this one. The picture to the right was processed with the same "safe hi - low boost" curve but using the D2XMode 1 Picture Control. The lessons to be learned is that you do not have to be stuck with the "Look" of any camera when you have control of all the parameters that make that look and can apply them to any Nikon NEF file. I can't afford a D2 or D3 or even a D300, but I can make my images look like the images those cameras produce by using Picture Control.

I don't want to go into every problem area that can be solved by Picture Control, so I will just show the opposite situation to this which turns out to be what I call the classic red flower problem. Nature plays tricks on photographers and creates colors almost out of gamut and completely blindsides Nikon cameras whose metering systems have minimal concern for the colors red, orange and yellow. The failure is that the meter and simple histogram shows no clipping but the detail in the red areas often are blown when it comes time to process the image for web or print. In some cases shooting raw doesn't prevent the problem. Wary photographers sometimes bracket exposures on the underexposure end just to ensure that the problem will not present itself. The problem is tricky because sometimes when there are other flowers present, a pink or yellow flower will actually be the highest in the red channel. The D90 has an RGB histogram and should always be consulted before investing heavily in digital film and time. One of the interesting aspects of looking for the blown channel is that you can display the image and zoom in to the flowers and figure out which one is blown. It will not blink on the blinky view. People have wondered why you can zoom in greater than 100% on the D90 and I believe it is precisely so you can isolate blown channel situations. As you zoom in the histogram only reflects what is on the view screen and you can watch the peak in the red channel move to the right until it hits the edge. It is these times that you discover it is the pink blossom, not the deep red one that is the problem.

The more you can do to prevent color clipping before the picture is taken the better you will succeed. The red flower situation suggests lowering the exposure would help. I have lowered the EV value by -1.5 EV for a particular Amerylus blossom with great success. Lowering the contrast might have produced a similar result as would lowering the gamma (brightness) using a custom curve. There is not just one solution for this. One of the ways you could let the camera help you is to use one of the automatic features of Picture Controls. Instead of trying to guess about the degree of contrast to use, consider using the Auto feature of contrast within the Picture Control. My Experience has been that matrix metering can get within one stop of the overal correct exposure and that when contrast is added before the JPG process begins that the Auto feature will prevent clipping and leave a small margin for error. This is an advantage over using a custom curve with a white margin as seen in wedding dress curves. Curves created in the Picture control utility are added to the fundamental curve they are based on and so may translate maximum white value to something less than 255 in the JPG. In real practice overexposure will still show as overexposure, but to a lesser degree.

After working up a number of solutions to the red rose problem, I found that I could use standard curves with adustments to contrast and brightness and could insure success by using Auto Contrast, but one solution allowed me to absolutely nail the exposure and contrast and brightness. It turned out to be Automatic Active D-Lighting, as seen in the picture to the right. Notice that there is no clipping in the color histograms either on the light or dark end of the curves. No matter what you do with the middle values in post processing, no data was lost and maximum use of the exposure data filled the chart. The next chapter will explain how to use Active D-Lighting.

Continue to Chapter 5

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Back Home to Exposed

1  History of curves and programmable contrast and gamma

2  Picture Control and Picture Control Utility

3  Creating and installing Custom curves in the camera

4  How to design a curve for your needs

5  Active D-Lighting

6  D-Lighting applied after the shot

7  Where to go for more information

8  The DOWNLOAD page


© Leon Goodman 2009

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