Image Correction with Curves

Digital Imaging

by Hans Weichselbaum

Image Correction with Curves

Image Correction with Curves

Image Correction with Curves

Image Correction with Curves

Looking at the histogram should be the first thing to do when you evaluate your digital images. Many of today’s cameras show you the histogram of the image you have just taken, some even display a ‘life’ histogram, before you press the button.

We discussed the histogram in the last issue. If you want to make any changes to your image, the Level command with its three sliders is usually the best starting point.

There is another tool in your image editing programme which allows you to do all the adjustments you normally do in Levels – and more. I am talking about the ‘Curves’ interface. I know of people who are real experts in Photoshop, but have never used Curves. As always in image editing, there are usually different ways of getting you to your optimum image and you can get away without ever using Curves. But it is a very powerful tool and you will love it once you have tried it.

Looking at Photoshop, the Curves interface has been around since the beginnings of digital imaging and hasn’t changed until we got to CS3, about 18 months ago. Not everybody has upgraded to CS3, so I will show you the old and the newer version.

Many people use Photoshop Elements – it is so much cheaper and gives you 90% of what its bigger brother can offer you. But one of the big drawbacks was that it didn’t have Curves. This has changed: since version 5, Curves has been included in Elements.


The Older Curves Interface

If you work with Photoshop CS2 or earlier then your Curves interface will look like this (see Fig.1). When you call it up (Image > Adjustments > Curves) you will see a box with a diagonal straight line. The graph plots the relationship between input levels (along the bottom) and output values (along the side).

Anything you can do in the Levels dialog box, you can also do with Curves. Grab the curve at about the middle of the line and pull it up or down and your image will lighten or darken. This is equivalent to moving the middle slider in Levels. Moving the two endpoints in Curves towards the middle has exactly the same effect as moving the two end points (black- and white-point) in Levels: it increases the contrast of your image. Unfortunately you don’t get the clipping preview as in Levels when you hold down the Alt button (unless you move up to CS3 – more about that later).

But there is more you can do in Curves. Remember, when you try to increase the contrast of an image you need to push either the black-point or the white-points towards the middle. Often you can’t do that without losing shadow- or highlight details. In those cases grab the curves at two points and make an S-curve, as shown in Fig.1. Wherever the curve slopes at an angle of more than 45 degrees, the contrast is going to be increased. This means that in our example the contrast is increased in the midtones and reduced in the shadows and highlights.

With the Curves menu open your mouse pointer will automatically change into the Eyedropper tool. Move it across your image and you will see a point running up and down the diagonal line. If you Control-click somewhere on your image, say in the shadow region, Photoshop will make a fixed point on your curve. You can then move the point up or down and only affect those particular tonal regions. To be more selective you can fix the curve around your point, just by clicking on it. For finer adjustments you can select a point with the mouse and then fine-tune it with the arrow keys.

This sounds all quite complicated, but will make sense once you open an image and play around with the Curves interface. The dropdown box on top lets you select the individual colour channels. If you want to make your image warmer looking, simply select the red channel and pull the curve up. In fact, I do all my colour corrections in Curves: it even allows me to correct a magenta cast in the shadows and a green cast in the highlights.


The New Curves Interface in CS3



What could one possibly add to the old Curves dialogue to make it more powerful? A histogram and a clipping preview would be on my wish list. And that’s exactly what happened in CS3 and I can’t see any reason anymore for using Levels. Certainly a most welcome addition to streamline your workflow!

There is a special tick box which lets you see shadow- and highlight clipping and, yes, it also allows you to do the Alt-key trick to show you the clipping while you move the endpoints.

Don’t forget to have a look at the built-in presets to which you can add your own settings.


Curves in Adobe Camera Raw

If you work with camera Raw files and use the latest Adobe Raw converter, then have a look at the second tab (behind the ‘Basic’ tab).

This will be a new experience if you have used the Photoshop Curves dialogue for years. Here you get four sliders, each controlling an exposure range in your image: highlights, lights, darks and shadows. In addition you can change the width of each of the four ranges with three sliders below the curve. You can also switch from the Parametric curves to the more familiar Point display.

Lightroom offers you an additional tool on the top left. You grab it, go back to into your image and you can directly lighten or darken any area you click on by moving your mouse up and down. This is really cool – hopefully we will see this also in one of the next ACR upgrades.