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EXPLORING THE CHARACTERISTIC LEARNING CURVE IN FILM AND DIGITAL IMAGING
Photographers often wonder how electronic imaging works, and more importantly, how to get the best results given the wide range of unfamiliar terminology and drop-down menus. One important photographic concept is the film characteristic curve. In this article, we’ll show that the characteristic curve applies not only to photographic images, but also to digital imaging. If you understand the characteristic curve and the effect that changing contrast has on an image, you can apply this to electronic imaging. We’ll review the film characteristic curve, then demonstrate that the same concept can be used (with only a slight name change) in Photoshop and monitor calibration programs. We’ll show you how to draw a characteristic curve for a digital imaging device. While “transferable skills” may be a buzzword, it’s certainly applicable as we make the transition from silver halide to CCD.
The photographic curve
Let’s start with a quick recap of the film curve. The photographic characteristic
curve is used to show how film behaves when used in a camera. The curve
is published in technical data sheets, or you can draw it yourself if
you have a densitometer. The curve is a plot of log exposure against density,
i.e. a plot that shows how much developed density we get on the film in
response to the light falling on the film at that point. Kodak T-Max is
a general-purpose, black-and-white material capable of reproducing a wide
and continuous range of gray tones. Kodalith Ortho is a very high contrast
material used in copying line reproductions that are black or white, with
very little tonal information. The curve for Kodalith is very steep. My
point here is that the appearance of the image depends on the contrast
of the chosen film. If you choose extremely low-contrast material, you
can expect an image that may be "washed out." Likewise, a high-contrast
film may produce an equally unpleasant stark image. Only a medium-contrast
film with a medium slope will produce a pleasing range of continuous tones.
It’s difficult to describe what "low" and "high" contrast mean, so there’s a mathematical number we can use. The number is called "gamma," and is a measure of the slope of the characteristic curve. Its precise definition is a measure of the slope or gradient of the straight line portion of the characteristic curve. A general-purpose film has a gentle slope with gamma of perhaps 0.8, while a high-contrast film has a steeper curve with a gamma of about 2.0 or more. To calculate gamma, use the following equation: Gamma = DD / D log exposure, where DD refers to a difference in film density, and D log exposure refers to the corresponding difference in log exposure. When we move to digital imaging, there are two things we must remember. First, just as the choice of film affects the look of images, the choice of gamma setting for digital imaging affects the reproduction of digital images. Second, gamma is just a numerical way of specifying the slope or contrast. A higher gamma means higher contrast.
Figure 1: Characteristic curves for T-Max, a general purpose material, and Kodalith, an extremely high contrast material.
Digital characteristic curve
The concept of a response curve, or characteristic curve, is the same
for photography and digital imaging. In fact, the term "gamma"
as a measure of contrast is used in both. Let’s look at the use of the
characteristic curve in digital imaging. The characteristic curve can
be constructed for all electronic imaging devices: scanners, digital cameras,
display monitors and printers. It also can be used to predict, or alter,
the image’s appearance. For example, let’s look at the characteristic
curve for one part of the electronic imaging chain the display monitor.
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| Figure 2: The monitor gamma curve is controlled by Knoll software. | |
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| Figure 3: Measuring the light output from a gray scale in order to draw an electronic characteristic curve. |
In the same way photographic gamma tells us the contrast of a film, the computer monitor’s gamma number tells us the contrast setting for the monitor (and, therefore, the appearance of images displayed on the monitor). All monitors come from the factory with an inherent gamma of about 2.2-2.5. Monitor software can change this basic setting in a process called "gamma correction." On the Macintosh, for instance, it’s customary to set the gamma to 1.8; on the PC, the standard tends to be 2.2. A lower gamma number decreases image contrast, while a higher number increases it. If you view the same image on both platforms, it will look fine on the Mac, but darker on a PC. Why? Because of the difference in the default gamma value. To change the gamma in photography, we choose a different film stock or alter processing conditions. It’s much simpler in digital; any gamma (within reason) can be easily selected. In digital imaging, we immediately see the effect of the changed gamma on the monitor display.
The gamma for a monitor can be varied in several ways. One common way on a PC is via an on-screen calibration unit supplied with Photoshop called Adobe Gamma. On the Mac, an equivalent utility is found in Control Panels>Monitors> Color>Calibrate. (Note how both these utilities use the term "gamma," derived from the photographic characteristic curve.)
Before we can change the monitor’s gamma, we need to determine the monitor’s base or factory setting gamma. Monitor software needs our input here. The user typically is asked to adjust a slider until an inside box matches an outside one. From this, the software calculates the current gamma of the monitor. Once the software has figured out the starting gamma, it allows you to change this to any other setting. In the Adobe Gamma and Apple control panel utilities, the eye does the job of a measuring instrument. If you can afford it, it’s even better to use a measuring instrument like a colorimeter to measure and set the monitor gamma.
Figure 4: The characteristic curve for a computer monitor is a simple straight line, and the slope of the curve is the monitor gamma.
Measuring monitor gamma
How do we know if the monitor really has the stated gamma value? Simple.
We draw a characteristic curve for the monitor, and measure the gamma
just like for a film characteristic curve. It is possible to draw a characteristic
curve (or gamma response curve) for all electronic imaging devices, as
long as there is a measurable input and measurable corresponding output.
For a monitor, this requires a standard exposure meter and calculator.
First, calibrate the monitor using the calibration software (described
above) and set a specific gamma value. Next, generate a series of about
20 gray patches spread between pixel values 0-255 on the screen. The patches
should be big enough to be measured using an appropriate meter. Ideally,
this is a specialized luminance meter such as a colorimeter or spectrophotometer,
but with care, a hand-held exposure meter or camera TTL spot meter can
be used. Take meter readings for each patch so the aperture remains constant.
Convert the shutter speed reading to a relative luminance value by taking
the reciprocal of the shutter speed (i.e, a shutter speed of 1/8 becomes
a luminance reading of 8; a shutter speed of 1/125 becomes a luminance
reading of 125, etc.).
When you’ve measured each step, normalize the data so all the values lie between 0 and 1. To do this, divide by the maximum value. Also, take the log of all the values except zero, which you can ignore. Logarithm or log units have a nice analogy to the photographic characteristic curve where density is a log unit and exposure is expressed as a log exposure unit. If you’re not happy with the negative numbers, add a constant number to all the values. Finally, plot the log pixel values (the input) against the log luminance (light output). The graph obtained is the characteristic curve for the display, and its gradient or gamma value should be approximately equal to that set in the Adobe Gamma or Monitor Calibrator software.
Another way to check your monitor’s gamma is to download a special test image from ColorSynergy at www.picto. com/techinfoA.html, and display it on your screen. Cut out a long thin slit and view the image through that. Move the slit up and down until both sides of the image merge. Read off the gamma number from the right of the image. The image and simple instructions can be downloaded free.
Compare the monitor characteristic curve with the film curve. Notice how much simpler the monitor curve is, in fact it is just a straight line. Because the monitor has such a simple curve, it is adequate to just quote the gamma number and the full "characteristics" of the monitor are specified.
The gamma setting is part of setting up and calibrating a monitor. This comes under the topic of monitor profiling and color management. We’ll cover this very important issue in a future article. Although the process for a display monitor has been described, the characteristic curve for other electronic devices--scanners and digital cameras--can also be generated. If this process is replicated for all the separate devices in an electronic imaging chain, a quadrant diagram can be drawn to represent the overall tone reproduction characteristics of a digital imaging process.
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| Figure 5: Photoshop uses the characteristic curve in a dialog of the same name-‘Curves’. (See text for explanation.) | ||
Image specific curves
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| Figure 6: Determine the gamma of your monitor. Download this image and simple instructions free from ColorSynergy. |
Finally - More and more images are being digitally viewed and edited from film scanners, digital cameras, PhotoCD or Internet downloads. Thus, it’s important to know about setting the monitor gamma and using the fine tuning and control that digital imaging offers. The concept of the film characteristic curve is fully applicable to electronic imaging, often using the same names. So the next time some computer geek starts telling you about monitor gamma, tell them where it came from!
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