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     A common complaint amongst new Nikon D100 users is that their camera consistently produces slightly dark images. To see if the problem was due to underexposure, I conducted a controlled test where I compared meter readings of evenly lit scenes using the Nikon D100, FA and N70. Using all combinations of metering (matrix, center-weighted and spot) and all exposure modes (aperture priority, shutter priority, and program mode), I arrived at exactly the same exposure settings in all the cameras. I concluded that the exposure meter on the D100 does not underexpose, at least compared to any other camera.

     Still, by checking the histograms of resulting images, I saw that the tonalities were not as they should have been. In a typical 8-bit JPEG image, there are 256 possible tonal values from pure black to pure white. A histogram is a graph that displays the distrubution of these tonalities in a digital image. If I were to take a photograph of an evenly lit wall, assuming that camera meters are designed to make whatever they see medium gray, I would've expected the camera to set an exposure that would render the wall with a distribution over the center of the histogram. Instead, the peaks of the histograms I observed were to the left of center. To get the peaks of the resulting histograms in the center, I had to add between +0.33 EV to +0.5 EV to the exposures, with some variance depending upon which lens and white balance setting I used.

Figure 1: Although the D100 meters just like any other camera, to get metered objects to equal medium gray, I had to add between +0.33 EV to +0.5 EV to the exposure regardless of metering method or exposure modes.

     I later found out that my original assumption regarding camera meters was incorrect. It turns out that camera meters are not actually calibrated to make whatever they see medium gray. Instead, they are calibrated, according to ANSI standards, to make whatever they see slightly darker than medium gray by about half a stop. Ansel Adams supposedly argued to have this changed but was unsuccessful. To read more about the confusion regarding the calibration of meters, click here.

     So, it turns out that if you expect your camera to make whatever it sees medium gray, your images will be darker than you expect. A possible reason that other D-SLRs don't seem to exhibit the same characteristic may be because manufacturers compensate for the half stop discrepancy by calibrating their sensors to be half a stop more sensitive than indicated. Fortunately, Nikon's engineers designed all their D-SLR cameras to be flexible enough to be tailored for individual preferences. For those of us who have a problem with the ANSI standard, we can either use a permanent exposure compensation of +0.33 EV or +0.5 EV or we can use a custom tone curve. As we shall see later, there are several advantages to using custom tone curves over exposure compensation.

Understanding D100 Tonality

     To understand the advantages of using custom tone curves over exposure compensation, we must first look deeper into the characteristics of the D100's tonality. My initial tests with the D100 only gave me an idea of how it handled midtones. After reading "The Negative", by Ansel Adams, I was inspired with an idea to more fully understand how the D100 handles exposure. In his book, before actually using any particular film, Ansel Adams suggests taking a series of photographs of a gray card through a complete range of exposures. The brightness of the resulting test patches will allow you to plot a characteristic curve of how that film handles exposure. By doing such a test with a D-SLR--or any digital camera, for that matter--you can gain invaluable information about the true dynamic range of its sensor and the precise brightness value (the equivalent of density in the film world) to expect for any given exposure. This type of information is essential in achieving consistent, predictable results in the least amount of time possible because it allows you to visualize the resulting tonality for a specific exposure without having to spend weeks or months "getting to know" the camera.

     I conducted a similar test with my D100 by spot-metering off of a gray card and varying exposure from -5.0 EV all the way up to +3.0 EV. The results are shown below, in Figure 2.

Figure 2: D100 Characteristic Curve at ISO 200

     As you can see, without exposure compensation, the D100 rendered the gray card darker than medium gray. This verified the results of my original test. This new test also revealed problems in the D100's contrast. In a characteristic curve, the slope of the curve at any given point determines the contrast you can expect in the corresponding range of tones. The bottom portion of the D100's characteristic curve, which represents the shadows and part of the midtones, has a shallow slope, or what people who study sensitometry would call, a long toe. This long toe results in noticeably less contrast in shadows and midtones, and consequently, less detail and color saturation. This may be the cause of the "muddy" colors some people attribute the D100 with. We also see that the upper portion of the curve representing highlights, also known as the shoulder, has a relatively steep slope that ends abruptly at the maximum possible value. This is a common characteristic amongst most digital cameras and results in "burnt" highlights. In these highlights, usually one of the RGB channels reaches maximum saturation and the highlight takes on the color of that RGB channel.

     Compare Figure 2 to Figure 3, below, which shows the characteristic curve of a typical film. In this curve, there is a much shorter toe and a much longer shoulder. The short toe results in greater shadow details and midtone contrast while the long shoulder results in smooth transitions to pure white. Notice also that the "straight part" of the curve is much longer, resulting in a consistent contrast over a larger range of tonalities. All of these characteristics are much more pleasing to the eye and are what separate film from digital. However, through the use of custom tone curves, it is possible to achieve a similar tonality.

Figure 3: Characteristic curve of a typical film.

Exposure Compensation vs. Custom Tone Curves

     Now that we have a better understanding of the D100's tonality, we can begin to compare the choices we have in improving it. To correct the problems associated with the D100's midtones and shadow details, most people use an exposure compensation of anywhere from 1/3rd to 2/3rds of a stop (+0.33 EV and +0.67 EV, respectively) or they take control of their exposures manually. When exposure compensation is added, the tonality of the D100 simply shifts to the left. To see how an exposure compensation of +0.5 EV affects tonality, place your cursor over Figure 4.

Figure 4: Effect of +0.5 EV on tonality

     As you can see, adding +0.5 EV to the camera results in midtones that are just about where they should be. Shadow details are also improved, which is not surprising. However, if you look at the top right of the graph, you will notice that we lose 1/2 of a stop of highlight information. Fortunately, there is a safer way we can get the exact same results as adding +0.5 EV, but with less risk of blown highlights. This solution, of course, is the use of custom tone curves. Why is it safer? Well, with a properly designed custom tone curve, you can control which tones are affected. For example, you can lift details from the shadows and brighten midtones while holding highlights down. No other combination of camera settings can do this, not even the less contrast tone setting.

Figure 5: Comparison of tonality resulting from +0.5 EV and custom tone curves

     In Figure 5, shown above, we see a comparison of the normal tone with and without exposure compensation (gray and blue plots, respectively) and the tonality resulting from two custom tone curves, both of which are available for download from this site. As you can see, the tonality resulting from the use of custom tone curves is a near perfect match with the tonality resulting from an exposure compensation of +0.5 EV. The only difference is at the highlights, where the custom tone curves gradually decrease in contrast, much like the soft shoulder we saw in the characteristic curve of film in Figure 3. As a result, the custom tone curves are able to hold approximately 1/3rd of a stop more information than using +0.5 EV.

Figure 6:  Comparison of images using the normal tone (left), normal +0.5 EV (center), and a custom tone curve (right)

     In Figure 6, shown above, we see a comparison of the same image using the normal tone, the normal tone +0.5 EV, and finally, a custom tone curve. In this comparison, the image taken with the normal tone handled highlights relatively well, but at the expense of midtones. The other two images handled midtones well, but by looking closer at the highlight details, shown in Figure 7, we see that the highlights were blown in the image with the EV compensation. The image taken with the custom tone curve retained all the highlight information, albeit with less contrast. However, the softened highlights actually make the image appear more film-like.

Figure 7: 100% crop of highlight details of image using normal tone (left), normal +0.5 EV (center), and a custom tone curve (right).

Figure 8: Comparison of histograms resulting from the normal tone (left), normal +0.5 EV (center), and a custom tone curve (right).

     Figure 8, shown above, compares the actual histograms of the images. As you can see, the custom tone curve results in an image that has the exact same tonality as the image with the normal tone +0.5 EV from the shadows all the way up into the highlights. All the curves available on this website essentially decompress shadows and midtones while compressing the last 1/4th of highlight information. Unlike other custom tone curves available on the internet, however, these curves do not sacrifice overall linearity to accomplish this. Other curves may seem to hold highlights better but only because they begin to compress tonality at an earlier point. The unintended result of curves such as these is inconsistent contrast in midtones. This limits the camera's latitude because with different exposures of the same subject, you will get different contrasts. For example, if you slightly overexpose a photo, you may get an extremely low contrast image, whereas if you slightly underexpose your photo, you may get a high contrast image. The custom tone curves available for download here are all linear with the normal tone, resulting in a consistent contrast from the shadows all the way up to the last 1/4th of highlights. Additionally, since color is optimized for the normal tone, curves that are linear with the normal tone will result in little to no color shifts or saturation loss. Rather than trying to give the appearance of holding highlights better, my philosophy is to accept pure white but to handle it with a smoother transition than the normal tone with EV compensation. As a result, images taken using these curves appear more film-like.

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