In the Levels and Curves dialog boxes you can set the colors used by the Black and White Eyedroppers. Choose Options and specify the target colors for each eyedropper. The Shadows, Midtones, and Highlights patches control the black, gray, and white points for the eyedroppers. They also control the black/white clipping points for the three Auto commands.
For clipping points you can specify Clip percentage. This is the percentage of black or white pixels that will be clipped when Auto algorithms are applied. This avoids stray noise pixels influencing results. Small changes in clipping level can have a dramatic effect on image quality.
Installation defaults for Shadows, Midtones, and Highlights are (0,0,0), (128,128,128), and (255,255,255) respectively. Clipping percentage default for shadows and highlights is 0.5% but may be a bit high. Noise levels have declined in recent years. Experiment with values around 0.1%.
Typically you'll want to leave midtones at (128,128,128). This value is used by the gray eyedropper and, when applied correctly, removes color casts in midtones. It's not looking for an exact gray (128,128,128), but an initial value of 92 to 160 for each channel is reasonable. It then balances the three RGB components so they have the same value (a shade of gray), and proportionally applies the correction to the rest of the image.
Black and white clipping points may need adjusting depending on the intended target. Setting the black clipping point at (20,20,20), and the white clipping point at (240,240,240), and then invoking Auto Levels, has the same effect as setting output points in Levels. This is illustrated below.
It's also the same as the following setting in the Curves dialog box.
Input tonal values are compressed to a smaller range. That is, no color in the output image will be darker than (20,20,20). No color in the output image will be lighter than (240,240,240).
Targeting a monitor. Choose (0,0,0) and (255,255,255) for clipping points to exploit the full gamut of colors.
Halftone publishing. When a drop of ink hits the paper it is absorbed and spreads into the paper. The amount of spreading is known as dot gain. Cheap uncoated paper, such as newspaper, has a large dot gain. Expensive coated brochures have a small dot gain. To limit the amount of ink and avoid excessive spreading, avoid absolute blacks in the image. Pure white is also avoided so white areas on the image can be distinguished from surrounding paper stock.
It's best to consult your printer to determine the correct settings. They usually designate values in percentages. A dot gain of 92% indicates that darkest blacks should be 20 (255 - 0.92x255), and the black point should be set at (20,20,20). A white point a value of 6% indicates that whitest whites should be 240 (255 - 0.06x255), and the white clipping point should be set at (240,240,240).
Targeting a printer. If you're targeting an inkjet or photographic printer, experiment to discover the best setting for black and white points. Different printers, inks, and papers require different settings.
The following image is useful for determining black/white point settings when targeting a printer. For best results right-click on this link following link to download a TIFF file. The image is composed of vector graphics so file size is small and it resizes well. Print the image without corrections.
View your print in good light. If you can see all the circles in your print then leave the black clipping point at (0,0,0) and the white clipping point at (255,255,255). Increasing the black point, in this case, simply means that that blacks won't be black. Decreasing the white point means that whites won't be white.
If you're unable to see the all the circles on the black background, increase the black point for more shadow detail. If you're unable to see the circles on the white background, decrease the white point for more highlight detail.
As an experiment the above image was printed on both matte and glossy paper at EzPrints using a Fuji Frontier printer. The following illustration shows photographs of the prints I received. Images have been darkened/lightened so you can clearly see the circles on your monitor.
Since the circles are visible at all values, for both matte and glossy finish, clipping points should be set to their defaults: (0,0,0) and (255,255,255). Similar results have been reported by owners of ink jet printers.
For years companies such as Kodak, Agfa, and Ilford strived to make photographic printing papers that exhibited the blackest blacks and whitest whites. These papers were developed to a high DMax (black density) and employed the use of brighteners for pure whites. They were the first choice of skilled darkroom technicians for wide tonal range and high quality reproduction.
Several authors recommend increasing the black point, and decreasing the white point, without calibration. In many cases, and indeed for the Fuji Frontier, results obtained will resemble the wimpy photographic papers from the 50's.