To help visualize the exposure setting process, a graphical representation (often used in photography) is depicted in Figures 5.1 through 5.4 . Figure 5.1 shows the basic arrangement of the process but with no exposure being made. In the upper left-hand corner is a graph that shows the basic sensi- tometric response of a hypothetical digital camera. The input is the scene, here shown as a series of rectangles of increasing brightness. The input to the camera’s sensor is shown on the horizontal axis as Log Exposure (base 10). The output from the camera is shown on the vertical axis as Value. This is a digital value ranging from 0 to 255 for an 8-bit camera, and 0 to 4096 for a 12-bit camera. Higher levels of exposure result in higher output values.
In the upper right corner is the computer. This takes values as inputs and also outputs value. It has the ability to transform these numbers. That will be shown as a contrast shift later in this series of graphs. In all the graphs in this series, higher input values result in higher output values.
In the lower right is the printer. It takes values as inputs and outputs colo- rant deposited on paper. In the case of a printer, the lower the input value, the higher the amount of colorant on the paper.
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The fi nal graph shows the print itself. For the sake of convenience, it allows presentation of the printer output just below the original scene. It is derived from the original scene inputs and printer outputs. Note that the input to this function is on the vertical axis and the output is on the hori- zontal axis. In this rotated orientation it shows the full system output com- pared to the system input.
Figure 5.2 depicts the act of exposure. Light from the scene goes through the lens (not depicted) and strikes the sensor. This is depicted as vertical lines going from the patches to the sensor response curve. This results in a series of values, each resulting from one of the scene patches. The values, shown as horizontal lines, exit the camera and go to the computer. In the current case, the computer simply outputs the same values coming in. These are shown as vertical lines going out of the computer and into the printer. The printer responds to the input values by placing colorant on paper. The colorant levels are shown as horizontal lines going from the printer and to the print quad- rant. The points where the print output levels cross the input scene levels are connected to create a system reproduction curve. Note that the output lev- els are on the horizontal axis. These point to the reproduced patches. Note
FIGURE 5.1 Basic Tone Reproduction Framework. Four key elements—the camera, computer, printer, and print—are shown as they respond to the scene. The approach helps to visualize how the different elements infl uence the fi nal outcome. Only the individual element responses are shown in this fi gure.
that the original scene has a greater range of brightness levels than the system can reproduce. The result for the current settings is that the lightest patch is reproduced at the same level as the second lightest patch. Some informa- tion has been lost. Had the lens been set to a smaller aperture, one that is equivalent to one patch level, a set full of patches would effectively have been slid over to the left, and the output would show that both of the two lowest patches are black and the brightest two patches would have different bright- ness levels. Again, some information would be lost, but it would be shadow information instead of highlight information.
In Figure 5.3 the computer is programmed to give a proportionately larger output relative to its input. This is an increase in contrast. Again the results for each patch within the camera’s dynamic range fl ow through the series of charts to the output. The end result is that the system reproduces patches from black to white in fewer steps, which is to say that the contrast has been increased.
In Figure 5.4 , the original exposure level and the normal contrast level are restored, but the computer is programmed to add a constant to each incoming value. Graphically, this shows up as a shift of the computer ’s response line to the right. Following the tracings on through, it is clear that the patches that are reproduced are rendered with higher brightness levels.
FIGURE 5.2 Making a Print. Following the format of Figure 5.1 , the effect of the fl ow from the original scene all the way through the system is shown. The arrows indicate the direction of the fl ow.
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FIGURE 5.3 Increased Contrast. The computer’s response function is changed, resulting in more output per unit input. Ultimately this produces larger increments in the steps in the print and, in this case, a loss of highlight information.
FIGURE 5.4 Increased Brightness. The computer’s response function is shifted to the right, adding a constant to each incoming value. The ultimate result is a lightening of all the steps, with, in this case, losses in the highlight information.
This is consistent with the notion that adding a constant results in a bright- ness increase.
These fi gures are here to help you visualize what happens as exposure is changed, how the dynamic range of the sensor interacts with the brightness range of the original scene, and how the computer can be used to modify the outputs with given inputs. Note that the computer cannot bring back infor- mation that was lost because the dynamic range of the camera was less than the brightness range of the original scene.