In a different article I explained how to use the INPUT values of the Photoshop Levels and Curves dialogs. In that article I didn't discuss the OUTPUT values other than to say that the default values are 255 for the white slider and 0 for the black slider.
The OUTPUT values can be used to lighten dark tones or to darken light tones. This can be used to reduce the overall contrast of a photograph. In the Curves dialog, if you drag the black end point at the left of the graph upwards you will increase the value of the black output above the default of 0 (zero). Conversely, if you drag the white end point at the right of the graph downwards, you will reduce the value of the white output to less than the default of 255. This has the effect of lowering the contrast of the photograph you are editing. This is similar but opposite of using the Input values.
However, the OUTPUT values also have a more important function - they help you to calibrate your printer! When we print an image the image data is interpreted by the printer and this can lead to the all too common problem of the print NOT matching the screen image. The problem is that the printer has its own 'contrast range' and this might be, and often is, less than the contrast of the screen image.
What this means is that the printer may well print some of the more subtle tones of the image, i.e. shadows with values of 5, 6, 7 up to say 15 as solid black and the lightest tones of 250, 251, up to 254 as solid white. The result would be a print that had more contrast and less detail than the screen image.
The solution is to make some test prints using a grey scale of tones (i.e. a set of patches of various tone values) from black through to white. The tonal values of each patch should change in value by say 3 or 5 in both the darkest and lightest parts. The middle tones can change using greater differences in values, e.g. units of 10, since they are not so important for this calibration. Print this scale on your printer with your usual paper(s). You should test each paper you use because the paper will affect the visible contrast range.
Now, compare the print to the screen and find the tone value patch on the printed scale that is completely black and the one that is completely white. For example, you may have dark tones in the grey scale screen image of say 0 (black), 5, 10, 15, 20 and light tones of say 250, 252, 254, and 255 (white).
When looking at your test print, if there is no visible difference between the dark patches of the scale with values of 5, 10 and black (i.e. the tones of 5 and 10 are pure black in the print) this indicates your printer will interpret a dark tone of value 10 and lower as pure black.
Conversely, looking at the lightest tones, if the tones with values 252 and above are showing as paper white this indicates your printer will print anything above 252 as pure white. Good, now we know the limits of contrast the printer can produce. In this example any tone darker than 10 will print as pure black and any tone lighter than 252 will print as pure white.
Now we can use the OUTPUT values of the Curves dialog to compensate for this printer/paper contrast. Using our example, we now know that any tone with a value of 10 or lower will be black in the print. In other words, the BLACK POINT of the PRINTER is 10. So, to compensate we need to set the OUTPUT value of the black slider in the Curves dialog to the value 10. This will lighten the shadows in the screen image (and probably make it look dull) BUT when you print the image the printer will lower the values down again to where you want them.
The same happens for the lighter tones. The printer will produce any value above 252 as paper white (no ink), so we need to set the OUTPUT value of the white slider in the Curves dialog to 252. This will lower the values of the light tones in the screen image BUT the printer will raise them back up on the print! You now understand enough about the Levels and Curves OUTPUT values to allow you to adjust the screen image to match the contrast capabilities of your printer and paper. The visual result on screen doesn't matter!
Now you can go on to learn more skills in Photoshop by joining me, and other like minded folk, for FREE at Zone2Tone Members and after you join us you will receive a free professional quality video tutorial.
Copyright (c) Les Meehan 2010.
Les Meehan is the author of seven published digital photography books and as a qualified instructor has been teaching workshops for over 20 years.
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