Nikon’s palette Curves is somewhat similar to the Color Balance palette in allowing contrast, tonal distribution, and color balance correction. It differs from Color Balance because Color Balance allows only global color changes – all levels and color ranges are treated equally. The Curves function is more powerful since it allows finer corrections, but its use is more demanding. The histogram integrated with the tool helps in evaluating the corrections. Also, Curves allows selective range corrections without unwanted effects on other tones, such as often occur with Color Balance. The color channels can be corrected individually or combined in the RGB channel.
The horizontal axis of the diagram represents the input values; the vertical axis, the output values. The “0” on the left is the blackest part of the image, and “255” on the right is the maximum possible brightness. In the histogram, the vertical axis displays the frequency with which a given level occurs in the image. If a level occurs frequently, then the bar for that level is high; if the level is rare, the bar stays low. The vertical scale for the histogram is not fixed, but is adjusted flexibly to have the tallest bar always fit within the graph of the curves. The numerical values on the vertical axis define the black point and white point on the output side.
Initially, the curve is a straight line, which means that input and output values are identical. Pushing the curve increases the brightness level of specific tonal ranges. Moving the mouse over the image displays a point running along the curve to indicate the level value of the pixel currently under the cursor.
Difference from Curves in Photoshop
Nikon integrates the histogram with the Curves tool. In Photoshop, Levels and Curves are two separate tools, with no histogram display in the background of the Curves graph. A similar function is available with the live histogram of the Histogram palette; with the Curves window placed next to the histogram, the histogram levels change as the curve is pulled.
The histogram helps assess the effect curve correction has on the distribution of tones in the overall image. The histogram can show the distribution of levels not only of the full image, but also of a selected crop. The histogram displays the frequency (y-axis) of pixels for each level (x-axis) within the image. This type of display provides a very helpful tool for image processing. For example, a distribution spread evenly across the whole scale from 0 to 255 indicates a correct exposure with balanced levels. These 256 steps are based on 8-bit RGB standard. Even if your are processing 16-bit images the scale still has 256 steps. But this is just a simplified display; internally, of course, the program uses 65,536 different steps as needed for 16-bit image processing.
When adjustments are made in Curves, the histogram does not update in real time, even though the changes affect the histogram. The Histogram button in Nikon Scan is used to display changes. It also functions as a toggle to show “before-and-after” values in the histogram. In Nikon Capture, this button offers a live visual update of the histogram. Unlike Photoshop, Nikon Capture does not offer superimposed histograms for a before-and-after comparison.
There are no fixed rules for how a good histogram should look; the subjects and their level distributions differ too greatly. Gaps in the histogram after corrections are always a concern. They point to a loss of detail due to shifted levels.
Using the Before/After Histogram button displays the effects of the correction. With one mouse click, the function provides a comparison of the updated graph with the original one. Unfortunately, in Capture Editor it is hard to tell whether the “before” or “after” histogram is being displayed. The Nikon Scan interface is clearer, and it is easier to determine the proper settings. Using the Auto Contrast button lets the program make an automatic contrast adjustment. In my tests, the results of performing contrast corrections were in many cases unsatisfactory. Manual corrections seem to be better. There is no way to undo Auto Contrast except by completely resetting all curves. This can be irritating if the curves have been changed extensively; those changes will all be gone as well.
The Black Point is the darkest pixel in the image, indicated by the value at the left end of the histogram. The White Point is the brightest pixel in the image, indicated by the value at the right end of the histogram. Usually, the Black Point is at 0 and the White Point is at 255. However, in many images the darkest point is not deep black and the brightest point is not pure white. This means that a part of the dynamic range is not being used. Manually moving the two sliders to the respective ends of the histogram will use the entire dynamic range.
The fastest way to adjust levels is to use the RGB channel, although it is not the best way. It is more effective to make the effort to adjust the levels for each color channel separately. This not only makes best use of the tonal palette, but also compensates for most color casts (with the exception of high/low-key shots).
Another way to adjust levels is to use the eyedropper tools Set White Point and Set Black Point. First click on the eyedropper to activate it, and then click on the desired part of the image. For example, to set the white point, click on the pixel that is considered the brightest. Areas with even brighter pixels will be washed out. The sliders for black point and white point can be set separately for each color channel. If the eyedroppers in the RGB channel are used, then each color channel will be adjusted automatically. This is also a good way to fix color casts, provided suitable black or white points can be found in the image (which is not easy for every subject).
The gray point lies between the black point and the white point. By default, it is right in the middle. Sliding the gray point to the left makes the image brighter. Sliding to the right makes the image darker. Unlike the global brightness adjustment in Color Balance, this correction does not affect the endpoints of the curve. Black point and white point remain unchanged; only the midrange is affected.
With the gray value, the gamma changes. One can slide the gray triangle or type a number in the field below it. Alternatively, the gray eyedropper may be used to assign a certain image area the value “neutral gray”; each color channel is then corrected separately. This is a very good method for removing any color cast, but only if there is a suitable neutral gray area in the image.
There are not, however, many images that genuinely have such neutral gray values. For this reason, manufacturers such as Novoflex and Kodak produce standardized gray cards that reflect exactly 18% of the light. Such gray cards allow digital photographers to adjust white balance manually while shooting, thus virtually guaranteeing accurate color rendition. The situation is more complicated when scanning, because the film material itself can produce color casts, and because the general degeneration of film material over time produces further unavoidable color shifts. Digital photographers have an easier time of things, as the image sensor in a digital camera remains the same over time, and digital images don’t age.
Ideally, you should include a gray card in every scan you make, to enable you to adjust color balance accurately at a later stage. This is, of course, extremely impractical, and so the general use of gray points in scanning situations remains limited.
