Photoshop and image correction.

When you go out and take photo’s or scan in artwork, you will probably have to do some work on the image to correct it.

The most common correction you will need to make on most images will be tonal and colour correction.

Before you correct an image it is important that you have cropped the image down to the composition you want to use. This will get rid of any excess visual data that you don’t need and will also mean that any correction only takes into account the tonality and colour range of your exact image.

Most of the tools to adjust your images come under image>adjustments.

There are quite a few tools for correction your image but we will focus on a couple that are the best for correcting image tonality and colour correction. These are:

• Levels
• Curves
• Hue and Saturation

But before we use these tools it is important to understand how to use them so you don’t spoil your original image.

Adjustment Layers

Without doubt the best way of "minimising" image degradation is to use Photoshop "Adjustment Layers". Almost every adjustment tool found under the "Image > Adjustments" menu can be found in the "Adjustment Layer". We can create an adjustment layer by simply selecting "Layer > New Adjustment Layer" from the Photoshop "Layer" menu.

One advantage of applying the adjustments using the layers approach is that it avoids working on the actual image. By working on a clear film "overlay" we can iteratively fine-tune the adjustment; turn it off; or even delete it at any time without impacting upon the original image. Once you are then happy with all the adjustments you can flatten the image to completely adjust the image.

Levels

This is likely the most useful tool in Photoshop because it allows you to alter not only brightness and contrast but colour balance as well, all in one step. Use Layer / New / Adjustment Layer and select Levels. The histogram of the image will be displayed along with several tools. The histogram shows the distribution of light values in your image with a range of 256 values — from darkest to lightest, with the vertical axis showing the amount.

The three tools that you'll be using are the left, right and centre input level triangular sliders. You see that the one on the left makes the image darker and the one on the right makes it lighter. Move them both inwards and you're increasing the contrast. The centre slider allows you to make overall changes to the general brightness of the image.

If you now look at Window / Show Layers you'll see that a new layer has been created called Levels. This layer contains the changes that you just made. You can turn the layer (and therefore the changes) off at any time by clicking the eye-icon located next to it. You can delete the layer by dragging it to the garbage can icon or right clicking and then selecting Delete Layer.

Most importantly you can recall the histogram and tools at any time by double clicking on the Levels layer in the palette selector. With the Levels Histogram displayed again you can make further changes. You can also create another new Levels layer and make additional changes to it. This can be convenient when you want to see what such changes will do without disturbing what you’ve already done in the previous Levels adjustment layer.

The image below of a stitch drawing was taken using a regular digital camera and even there is a lot of visual information there it is far to dull. Changing the levels of this image will bring out the image to it’s full potential

Move the right hand white slider to the edge of the black graph, and move the black slider on the left in towards the graph also, this should be the result:

To have a go at this yourself download the image click here.

Levels are very handy but they are not the only correction tool and sometimes using other tools will be more suitable.

Curves

With the curves tool you can get a very high level of control over your images, you can control tonality very accurately in both highlights and darkest tones. It takes a bit of time to get an understanding of curves and again that is why it is important to use adjustment layers, so that you can make small adjustments and then change them at a later date.

The best way to get to grips with curves is to just play around a bit with it, but here is a couple of simple lessons to get you started.

So what’s the first, most obvious thing you notice? There's NO CURVE!! What's up with this? The idea behind Curves is all about re–mapping brightness values. Does that make sense? A pixel starts out at a certain brightness, and you change it to be brighter or darker. Have a look at these simplified versions:

Think of the gray ramp at the bottom as the image’s tonal values before any changes; think of the vertical gray ramp as representing what the image’s tonal values will be changed to. I hope it’s clear that the Curves dialog opens as a straight line because you haven’t made any changes yet. That means that the brightness values before and after are the same. You will effect a change by changing the shape of the curve! You accomplish this by clicking once somewhere on the line. This will establish a “point”; this point can now be dragged to a different place within the grid, which causes that tonal value to change, either lighter or darker depending on whether you drag it up or down. The reason it’s a curve is so that the change blends smoothly throught the image. An abrupt change in value can be very noticeable. The increasingly gradual change of the brightness values on either side of the change permit a very smooth and believable adjustment. These next illustrations demonstrate how a certain value is affected by a curve, and how you can start to visualize how the shape of the curve affects tonality. (Note: I have drawn pretty exaggerated curves for illustration purposes. Sometimes you need curves this strong, but for many images the curves will be much more gentle. A little bit goes a long way!)

Here’s a before and after example of what I’m talking about, with the real life curve below:

Curves can get pretty in depth especially when dealing with colour correction.

Let me introduce you to our second most helpful tool, the Info Palette. Windows>Info brings this little puppy up, and it should be given a permanent place of honor on your desktop. Run the cursor around your image (doesn’t matter which tool you have selected) and notice that the info palette displays the RGB values for whatever pixel lies under the cursor! Color correction “by the numbers” is the ultimate hallmark of the master color technician, and is way beyond the scope of this introductory piece. But just knowing one fact will make an enormous difference in how you approach color correcting: for a pixel to be neutral, just remember that the values for Red, Green and Blue should be the same. It doesn’t matter what they are, just that they are the same. A light gray will have different values from a dark gray, but if R=G=B you know they will be neutral. (As a sidebar: our eyes and brain are tremendously adaptable; they work together to pull things almost neutral, or almost black, or almost white to what the brain thinks they should be. Colors are okay to evaluate from the monitor, but black, white and especially neutrals should be confirmed with the info palette.)

I can’t tell you what color skin should be, or what values to use for green grass or a navy blue suit. But neutrals are your point of reference. Of course not every image has a neutral in it, but if you start looking, it’s amazing how many photos have something in them that can be used as a neutral reference point. A white shirt; a car tire; a paper on a desk; asphalt paving; a white fence; granite foundations… you get my point. If you need a neutral, chances are better than 50/50 you’ll find something. Now, what can you gather about the color of the pixel being described by the info palette in the previous paragraph? It sure isn’t a neutral! The value of Blue is much lower than Red or Green. What's the opposite of Blue? That’s one pretty yellow pixel. If that was supposed to be neutral, we’ve got to bring blue into line with the others.

In fact, that reading came from this image, shot with a digital camera in a flourescent lit room. You might look at it and have the distinct impression of “something’s wrong”, but what? We tend to think of flourescent lighting as being deficient in red, leading to green images, but in this case the combination of light and camera is less green than it is yellow. How do we know that? Well, I’ve done you a favor by picking an image with loads of potential neutral reference points. We run the cursor around any one of the things that our eyes say probably ought to be neutral, and over and over again the Info Palette announces that blue is seriously low.

Here’s the reference point I chose, and the Info display, followed by the correction applied, followed by the net result:

To get the image and try for yourself click here

Locate the source of the problem with the Info Palette. Go straight to the problem channel in the Curves dialog. Make the problem go away. As you move the curser out of the image window into the Curves dialog, the Info paletted display goes away; but if you have made a change to one or more curves, when the cursor goes back into the image window the read–out now displays a “before and after” reading. Wow! And, as we saw in grayscale, a command/control click in the image window plants a point on the curve. In color it gets better: if you have, for instance, the blue channel displayed, that's where the point gets planted. Wow Wow! Simple! Of course not all images are this simple, and even this one could have used other help too, but the point is just look at what a difference pulling one point in one channel made! I could have spent ages having at it area by area, but pulling a global curve based on a known neutral worked wonders. Are you shocked? Are you stunned? Are you ready to go?

Here’s a super helpful tip Adobe introduced with Photoshop 5. It’s annoying to have to keep making trips back into the image window to check on whether you've brought the color value to where you want it. And it’s hard to get the curser back to the exact same place each time. Hiding under the Eyedropper tool is a locking eyedropper. Click in the image on the point you want as a reference; the little cross hair icon stays there and a new level appears in the Info Palette. Adjusting curves is simple now: the before and after readout is always there to refer to until you dismiss or accept the changes.

The eyedroppers still work, just as they did in grayscale, and now the middle, “neutral” dropper has some meaning. As we discussed before, double clicking on each of the eyedropper icons will open the color picker. This allows you to set exactly how that dropper will function. If you have some specific parameters, this can be very useful. If you need to keep whites no lighter than 250 – 250 – 250 and blacks no darker than 5 – 5 – 5, set up the droppers and that’s what you will get. I still prefer direct manipulation of the curves most times, but the droppers can be handy. I still have same guarded opinion of the Auto button however. A click on this will set the darkest point in each channel to 0 and the lightest to 255. Sometimes it’s great, but often it does violence to both color and contrast. However, if you are stumped by an image, give it a try. The preview will tell you the story pretty quickly; if you hate it, try this trick: hold down the option/alt key and notice that the button which had been “Cancel” changes to “Reset”. Now wasn’t that thoughtful?

Two final points before concluding with some Power User tips. Point One: We have looked so far at doing everything globally (to the whole image at once). But you might see that there are times when it is in fact warranted to use curves on a local selection. There are times when the foreground doesn’t match the background, or the background is too dark but the subject is okay… any number of cases can come to mind. After having done everything possible to the image overall, it’s certainly legal to have at it with selection tools and work locally with Curves! They are just as powerful and capable inside a selection boundary.

Point Two: I have invested many many words describing how to discern and remove a color cast from an image, bringing it to neutral. But not all images want or need to be neutral! There will be times when you will want to enhance a tan, exaggerate a sunset or brighten up a lawn. There will be times when an image needs to be warmed or cooled to work better with other images in a montage. You should have no fear now! Your eyes will have to be your guide since this is interpretation and there are no numerical reference points, but if you had to put a little more red into a face or take some yellow out of moody twilight shot, you know how to approach it now. If at the same time you need to make it darker and less contrasty - one trip to Curves and you can take care of it all!

There are other images for you to play with, go to the HOMES server and the workshops folder and a folder called colour.