Curves Adjustments

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I struggled for a long while to find a good way to use curves to its full potential. I have read that curves is the most powerful tool when processing photos in Photoshop so I set out to find a good way by Googling curves, but time and time again it was still the same old way I have been doing it, using ‘S’ curves and so on. In my search I finally found a way to do it that works for me but can’t remember where now so thought I would write it down in a tutorial for everyone here to see and hopefully it will work for you as well. This is the best way yet that I have found to use curves to adjust for colour correcting photos. I use Photoshop CS, but I am sure it works in others that use the curves adjustment.

First open a photo in Photoshop. You will be using the colour sampler tool to set the grey point in the image so you will have to set the sample size to 3 by 3 sample.

Step 1

We are going to start by finding the white and black points in your photo so go ahead and open a threshold adjustment layer Layer/New Adjustment Layer/Threshold. A histogram will show up with a single slider in the middle. Slide the arrow over to the left until you only have a small amount of black showing. Place your cursor over the black and pressing on the shift key, left click (I usually zoom right in when sampling by pressing Ctrl and +). In your information window you will now have a sample of the darkest point on your photo and there will be a circle with a “1” where you clicked on the photo.

Step 2

Now slide your arrow to the right until you only have a small amount of white left and shift-left click on the white.

Step 3

Click “cancel” to cancel out of the threshold adjustment layer.

Step 4

Now we have to find a middle grey point. So using the Colour Sampler tool find a part of your photo that is close to a middle grey and left click. Just as a reminder the colour black is a RGB colour of 0,0,0; white 255, 255, 255; and middle grey is 128, 128, 128.

Now you should have three colour samples #1, #2, #3 in your information window. We will now use the curves adjustment layer now, as this is what the tutorial is all about.

Step 5

Open a Curves adjustment layer by clicking on the half black half white circle on the bottom of the layers window. Make sure the white in on the outer most side (top and far right) and black is on the inner most (bottom left) part of the grid. This is where we will use the numbers from the samples we took earlier. Click on the red channel and click near the middle of the diagonal line for that channel. In the input put the number for red in the sample #3 (in this case 126), now take the numbers for red in the three samples add them together and divide by 3 (15+250+126/3=130) and put that in the output. Do this for all three channels and click OK.

Step 6

Open up another curves adj layer by clicking on the half white and half black circle again. Click on the red channel and click on the white square on the diagonal line this is on the top end of the line. In the Input box put the number you got from your white sample (#2) and in the output put 240. Now click on the bottom part of that line to set your black. In the Input enter the number you got for your black sample (#1) and in the output enter 0. If you are going to print your photo enter 10 for your output, use 0 only for posting on the web.

Click OK and you are done. Sounds difficult don’t it. Do it a couple of times and it won’t take you long at all.