The goal of this technique is to correct black point and white point values of an image, to 10-10-10, and 245-245-245, so that blacks are truly black and whites are truly white, and also to ensure the neutral midtone areas of the image are actually neutral. Based on your own experiments, you may even want to choose other values for the blacks and whites.
Why would you want to do this?
- To correct unwanted color casts introduced by light temperature, white balance issues, or off-color reflections (green light bouncing from grass, etc).
- To expand tonal range. Images can be over or underexposed, or lacking contrast. This technique will ensure a full range of tones.
- To ensure maximum detail in shadow and highlight areas. Printers may not be able to render all the details if very low or very high pixel values are present in the image.
First, you must find the darkest and lightest pixel values in the image, using Threshold. Go to the Tools palette, and select Color Sampler Tool. Make sure your Info palette is visible.
Layer>New Adjustment Layer>Threshold
Your image will turn black and white
Drag the adjustment slider to the left until only a few black pixels are remaining. Keeping an eye on your Info palette, center the Color Sampler Tool over the darkest group of pixels, and click to select Sample #1.
Drag the adjustment slider to the right until only a few white pixels are remaining, avoiding specular highlights (tiny white reflections). Again, keeping an eye on your Info palette, center the Color Sampler Tool over the brightest group of pixels, and click to select Sample #2. If you decide to change the position of the Sample icons, they can be dragged.
Now delete the Threshold layer. The Sample icons will remain on your image.
Now you will correct those values to neutral, using Levels.
Layer>New Adjustment Layer>Levels
Click on the drop-down menu in the Levels palette, and select each R-G-B channel individually, correcting your samples to 10-10-10 and 245-245-245.
To move a lower number to a higher number in the shadows, use the output slider
To move a higher number to a lower number, use the input slider
To move a lower number to a higher number in the highlights, use the input slider
To move a higher number to a lower number, use the output slider
To correct Gray Point, or midtone values in Levels, you must have a calibrated monitor. Sometimes this changes your end point values, and these must be re-corrected. I generally don’t bother with Gray Point, since correcting shadows and highlights usually does the job. When you are finished with your corrections, the Sample icons can be deleted by Alt>click on each icon. They are non-printing.
This process tends to decrease the contrast in an image. You can add contrast to your midtones in Curves. Generally only a slight adjustment is needed.
This process sounds time consuming, but once you get used to doing it, it really only takes a few minutes. It is much more precise than simply using the Levels white and black point droppers. Not all images need this process. You may decide to correct only the White point values, especially in a macro image.
This tutorial is based on Photoshop CS4, and should work for later versions as well. It is an adaptation of a Photoshop class taught at the Washington School of Photography, by John Witschey.
Submitted by Melanie Marts