A potent tool needs soft hands to match

PUBLISHED : Thursday, 15 May, 1997, 12:00am
UPDATED : Thursday, 15 May, 1997, 12:00am

Say the word curves, and what do you think of? A fast car tearing down a country lane? Not me. You say curves, I say PhotoShop.

Curves are PhotoShop's most powerful tool, letting you correct colour, contrast and brightness from a single window.

But the most important thing is: Be gentle. Open the Curves window; you will see something that looks like a graph. The graph shows a straight line from one corner to the other. At the bottom is a grey scale running from white to black. The black end may be on the left or the right, and can be swapped by clicking on it. For the purposes of this article, I will assume the black is at the left.

Look for a small check box labelled Preview. Make sure that it is marked. This will allow PhotoShop to change the values of the image alone rather than change the lightness, darkness, colour, etc of the whole monitor.

Hold down the Option key on the Mac, or the Alt key on the PC, and click anywhere on the graph. The graph will become finer, with nine lines vertically and nine horizontally rather than the usual three.

Down the side of the window are six buttons - OK, Cancel, Load, Save, Smooth and Auto. The use of the first two is obvious. The second two can be used to load and save changes, should you want to apply the same correction repeatedly. This can be useful if you have several pictures shot under the same conditions which are to appear next to each other.

Scan the first image, open the Curves window, make your corrections, save them with the Save button, then click OK.

After that, provided the images were exposed correctly and you scanned them with the same settings, you can simply open the Curves window for each image, load the Correction curve you just saved and click OK.

The next button, Smooth, is used with the button that looks like a pencil at the bottom of the graph. I will not be discussing either.

Pressing the Auto button causes the computer to find the darkest part of the image and make it a neutral black, while making the lightest highlight a neutral white.

The Auto function presents some problems. It may make neutral colours where they are not appropriate. It may also make blacks too dark or highlights too light.

Grab the endpoint on the graph line in the lower left hand corner of the graph. This should be directly above the black end of the grey scale below the graph. Move the endpoint along the bottom of the graph to the first vertical hash line.

The shadows in your image should have darkened, as you have told the computer to make everything that was 90 per cent black 100 per cent black.

Take the endpoint and put it back where you started. The shadows will lighten. Move the endpoint up the side of the graph one hash mark. The shadows should get lighter. This time, you told the computer to make everything that was 100 per cent black, 90 per cent black.

By grabbing the endpoint in the upper right hand corner, we can repeat the process with the highlights. Move the endpoint left one hash mark and the highlights will brighten up, move it down one and they will get muddy.

Remember that highlights in a digital image are not like highlights in an enlarger.

If you load a negative in an enlarger and darken the highlights by burning them in, they will get darker and more detailed. In a digital image they will just get darker.

The important thing here is to be careful with your scans. If you haven't already scanned in the highlight details, you can't make them appear in PhotoShop.

Move the endpoint on the highlight end one notch to the left, then move the shadow endpoint one hash mark to the right. This makes the highlights brighter and the shadows darker. You have increased the contrast.

Now return the graph to its original position. Hold down the Option key. The Cancel button changes to Reset. By clicking this, you can return the image to the state it was in when you opened Curves, without having to exit the window.

Grab the line in the middle of the graph and bend it up and to the left. Now the whole image will lighten. Bend it down and to the right and the image will darken.

You are lightening or darkening the whole image by changing the mid-tones. The white point and the black point in the image have been left in place. This lets you lighten or darken the image without losing too much highlight or shadow detail.

Move the cursor on to any part of the image. Hold down the mouse button. An open circle will appear on the graph showing the value of the image at the point of the cursor. This can be very useful if you want to lighten or darken a certain range of tones.

You can see exactly where you should grab the graph in order to affect that part of the image. By placing points on other parts of the line, you can pin the line down so that the curve moves only at the point you want to affect.

Above the curve is a pull-down menu labelled RGB. The menu will allow you to change the colour balance. Changing from RGB to Red allows you to affect the red/cyan content, Green affects green/magenta and blue affects blue/yellow.

Overall colour balance is changed by grabbing the midpoint on the curve line. To correct blue highlights caused by a flash, grab the highlight endpoint in the blue channel and move it down the side of the graph.

Another common problem with colour negatives is slightly red shadows. This can be fixed by going to the Red channel, grabbing the shadow endpoint and moving it right along the bottom of the graph. This will add some cyan to the shadows to make them more neutral.

What was the reason for gentle treatment? Basically, you should never make huge corrections to an image all at one time. If you have a picture that is too dark, don't just grab the curve and give it a mighty pull. Lighten it just a little bit and then close the window. Open the Curves and lighten it a bit more.

Repeat the process until you get the image you want. Try not move a point more than about 5 per cent at a time. This means never move a point more than half the distance between two hash marks.