The Darkroom

The Darkroom
Setting The Black Point; A Key Step In Getting The Right Tonal Range

Darryl C. Nicholas | Sep 1, 2004

#1
Photos © 2004, Darryl C. Nicholas, All Rights Reserved

One of the secrets to making great ink jet prints is to set the black point correctly in the image file before sending the image to the printer. The black point in an image is the group of pixels that should be printed as solid, D-max, black. If this setting is made correctly, it will establish a correct tonal range for the image, which means that there will be a correct distribution of shadows and highlights. You will have the proper density in the shadows to allow you to have a maximum of shadow detail as well as a maximum of detail in the highlights. As with many procedures in Photoshop, different experts have different opinions on just how to set the black point. So, before you all start writing in to tell me how wrong I am, let me say up-front that this is how I set the black point in my images, and this is why I do it this way. You're free, to experiment with better options.

Image #1 was taken at Baltimore's Inner Harbor on a cloudy, overcast day. The original image file was a tad low in contrast and a bit dull. After opening the image in Photoshop (I'm using Version 7.0, but the technique I'm describing will also work with Version 5.5 as well as CS), go to Layer to New Adjustment Layer to Curves. When the New Layer window opens, click on OK. That will cause the Curves adjustment window to open. Click on OK. We'll come back to this adjustment layer in a few minutes.

Next, go back to Layer to New Adjustment Layer to Threshold. When the Threshold New Layer window opens, click on OK. That will open the Threshold adjustment window and will immediately turn the image into a high contrast, black and white view as shown in #2. In the Threshold adjustment window notice the little arrowhead under the histogram (#3). Grab the arrowhead and drag it all the way to the left and the image will go to solid white. Then, slowly drag the arrowhead back to the right until you get just the beginning trace of some black pixels (#4).

Use the CTRL plus the "+" key to magnify the image. Adjust the scroll bars as you magnify the image until the group of black pixels are in the middle of the window and large enough to see them clearly. Place your cursor (an eyedropper) in the middle of the group of black pixels, hold down the Shift key, and left click. A small circle with the number one will appear (#5). Now click on Cancel.

Next, go to the Layer Palette (#6) and double click on the first square for the Curves adjustment layer that we created earlier (see the red arrow in #6). That will cause the Curves adjustment window to open (#7).

Back in #5 when you set the Threshold point, Photoshop also adjusted the red, green, and blue curves to make the Threshold point absolutely neutral in color balance. You can see the adjustments that Photoshop made by going to the Channel dialog box and selecting the color channels one at a time and looking at how they are now set. In #7, you can see that the Green channel has been offset from the corner to the right just a little (see the red arrow in #7).

At this point, some experts will tell you that you have correctly set the black point. I disagree.

If you will open the Info Palette and look at the RGB values (#8), you'll see that the black point that has been set is now reading "0" for all RGB values. Notice that before I made the black point adjustment for this image the red value had been 12, the green value had been 16, and the blue value had been 20 (#8). Since all three color channels have now been set to "0," the shadows have had their color balance set to neutral. If your printer is correctly calibrated, your shadows will now print neutral black/gray with no trace of a color tint. This can be especially valuable if you are printing pictures of men in black tuxedos (like at a wedding). How often have you seen black tuxedos with a blue or green cast in them?

Here's where I differ with some of the experts on setting the black point by this method. If you set a D-max area to RGB "0" (as we have done earlier), the ink jet printer will lay down considerably more ink than necessary to establish D-max. This means that slightly lighter shadow tonal areas will also be getting a whole lot more ink than is necessary, causing some of them to also go to D-max when they should have printed a little lighter, thus producing more visible detail in the shadows. The way to prevent this excessive ink lay down, and to get much better shadow detail, is to set the black point to something between 10 and 13 instead of "0." Here's how I do that:

In #9, you see the Curves adjustment window as it first opens when you double click on the Curves layer back in #6. If you will go down to the bottom left corner of the Curves adjustment window and grab the end of the diagonal line, then drag it up just a little (#9), you will lighten the black point. In fact, if you will keep your eye on the Info Palette window (#8) while you are dragging the diagonal line up, you will see the values of RGB changing from "0" to the desired 10-13 units that I mentioned earlier. In this example, I have set the black point to 12 units. The 12 units show up in the Curves adjustment window down at the bottom as "Output" = 12 (#9).

The last fine tuning, which you might want to do, is also done in the Curves adjustment window. Drag the window off to one side so you can see the bulk of your image and then bend the curve into a very gentle "S" shape. This will produce a slight increase in contrast in the image, and should be done according to your personal artistic taste. There is no real right or wrong. Image #10 shows my finished image. As you can appreciate, the subtle shadow details will not reproduce very accurately in the magazine, but I can assure you that the subtle shadow detail can be clearly seen in a real ink jet print. For more information visit my website at: www.colorbat.com.