AnandTech Guide to Better Photos: Post-Processing

Basic Color Correction

In this section, we are going to discuss one simple way to remove color casts from your images. Although this method cannot always correct heavy color casts, it does a very good job with subtle ones. It can also add a good amount of contrast to your image. This method assumes that your image has a point that should be pure black and a point that should be pure white. If your image is comprised of all subtle midtones, you might want to skip this method.

When most people think of color correction, they might think of the “Color Balance” option under the Image/Adjustments menu in Photoshop. Unfortunately, using the “Color Balance” option can be very imprecise. Our method takes the guesswork out of the process so that you can simply find and specify the white and black points of your image. In this method, we will be using the Curves dialog box. Although Photoshop Elements does not offer Curves out of the box, we ran across this interesting website that shows you how to get some Curves functionality in Elements. However, since this Curves manipulation works a bit differently, we will be focusing on Curves in the standard version of Photoshop.

1) Open up the image that needs some color correction and set the view to “Fit to Screen”.

Original image
Click to enlarge.

In this image, our Auto WB setting produced a strong yellowish color cast. Using our method, we will try to get the colors to be more accurate.

2) Click the “New Adjustment Layer” icon (half white/half black circle on the layers palette) and select “Threshold”.

Create a new adjustment layer (Threshold)

3) In the Threshold dialog box that pops up, drag the slider on the bottom all the way to the left. Now, move the slider very slowly to the right and stop when you first see some black patches appearing on your image. In this step, you are actually revealing the darkest areas of your image. These will most likely be shadow areas.

Find the black point
Click to enlarge.

4) Click “OK” in the Threshold dialog. Then, change the magnification of the image to “Actual Pixels”.

5) Hold down the mouse button over the Eyedropper tool on the Tool palette. Then, select the Color Sampler Tool from the flyout menu.

Select the Color Sampler Tool

6) Click inside the dark patch with the Color Sampler Tool.

Sample the black point
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7) Set the view to “Fit to Screen” and double-click the Threshold Adjustment layer again. Now, drag the slider at the bottom all the way to the right. Slowly bring it back to the left until you see some white patches appearing. These are the brightest areas in your image. Repeat steps 4 – 6 to mark the white point with the Color Sample Tool. You do not want to set your white point on specular highlights. A specular highlight is an area that is particularly bright because of a reflection. These spots may be overexposed and it is usually desirable to leave them this way. Instead, select a white patch that corresponds to a white object in the image. You could repeat this process for a midtone as well, but these tend to be harder to spot. Don’t worry, we’ll adjust the midtones in a second.

Sample the white point
Click to enlarge.

8) Now that you have marked the points for the darkest and brightest parts of the image, we are finished with the Threshold layer. So, simply drag that layer into the Trash to delete it. Our next step will include inputting our black and white points into the Curves dialog.

9) We can either work directly on our image or we can use a Curves Adjustment layer. The benefit of using Curves on an Adjustment layer is that we can always go back and edit the Curves layer or delete it entirely. If you decide to work directly on the image, you will not be able to change the Curves settings later. So, click on the New Adjustment Layer icon and select Curves.

10) The first thing that we want to do is to tell Photoshop how to define “Black” and “White”. Starting with black, double-click the black eye-dropper icon on the left (circled in red in the image below).

Curves dialog

Define RGB values

Here, we will be telling Photoshop what the RGB values are for the color black. For an image destined for the web or monitor, you can leave the RGB value at “0 ,0,0”. However, if you plan on printing your image, we have found that a value of “20 ,20,20” will produce better results. Otherwise, detail in the darkest part of the image will not be visible. Click OK and double-click the white eye-dropper icon. For the white point, specify an RGB value of “255,255,255” for the web or monitor, and “245,245,245” for print. When printing your images, the most effective values will vary depending on the output device. Therefore, we recommend making some test prints with differing values to see which is the best for you.

11) Now that you have defined the RGB values for black and white, we can tell Photoshop where these points are in our image. From the main Curves dialog, select the black eye-dropper tool. Now, move over to your image and click on the sample point for the darkest part of your image. Then, click the white eye-dropper tool and select the sample point on the brightest part of your image. You can also use the “ Set Gray Point” eye-dropper if you have a neutral midtone in your image. If everything went as planned, you should see a difference and an increase in contrast. If you make a mistake at any point and want to start over, simply hold down the Alt key (Mac: Option) and the Cancel button will turn into a Reset button.

Image after setting black and white points

12) Next, we want to boost the midtones. To do this, simply click on the middle of the diagonal line in the Curves dialog. Then, drag it up very slowly. You will notice the midtones increasing in brightness.

Brightening midtones

13) Finally, to add a bit more punch to the image, you can add a second point on the curve and create an “S” shape. You will see the image increase in contrast dramatically. You will probably need just a very subtle “S-curve” before your image has plenty of contrast.

Creating an “S-Curve”

Before curve adjustment	After curve adjustment
Click images to enlarge.

Hold mouse over image.