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Using the Curves Tool
Presented by Pixalo - Submitted by Rob Barron
I asked for suggestions as to what you would like tutorials on and one that has been mentioned to me a few times is the Curves tool. After all, the Curves tool is one of the most powerful tools in Photoshop but a lot of people are baffled by it. My aim in this tutorial is therefore to demystify this tool and encourage you to give it a go. It is one of my favourite tools in PS and I can't remember the last time I processed a photo without using it at least once!
I am going to keep this simple and I am including screenshots to make things a bit clearer but this is not the ultimate guide to Curves as that would take a whole book. I am going to show you various uses for tools and that will hopefully get you started so that you can go on and experiment with it.
Thanks for reading it and if you find it helpful, please let me know. If you find it has confused you even more, please let me know that to! All feedback is helpful for future tutorials that I make.
Ok, let's get started.
What is the Curves Tool?
Well, it is a fair question and one that will probably be clearer after the tutorial part than after this short explanation but let's try:
Let's first look at the Levels tool as these two are related. The Levels tool can be found by pressing Ctrl+L when you have a picture open. What it shows you is something like this:
It shows the range of shades that you have in your picture from fully black to the left through to fully white on the right. In the middle is a grey triangle that can be moved to adjust what we call the 'mid-point' which is basically a mid-grey. Move it to the left and things get darker, move it to the right and things get lighter. You can do the same with the black point and white point in order to make sure your picture contains a full range. Move the black triangle to the right until it points to the first bit of the historgam that is showing any content. Move the white triangle to the left until you again meet the first bits of histogram content. Click OK and you'll now have what is called a 'Full dynamic range.'
Ok, the reason I wanted you to understand the layers tool first, basic as it is, is that this has a direct relationship to the Curves tool.
When you open the Curves tool (Press Ctrl+M) it looks like this:
Ok, let's take a quick look around:
- At the top there is a window that says 'RGB' by default. This means your adjustments will be affecting all the colours in your picture or selection. You can select Red, Green or Blue individually from this window to change one channel. more about that later.
- Main window looks like a box with a diagonal line from bottom left to top right.
Along the bottom and up the left hand side of this is two strips showing graduated from black to white. I like to have black at bottom and left but you can switch them to black at top and right by clicking on the two way arrow head in the middle of the bottom window. Again, more on the diagonal line in a moment.
- Below this is Input and Output which basically shows numerically the shades from black (0) to white (255) but you will usually ignore this to be honest.
- To the right of that are two buttons: the default is an S-curved line with adjustment nodes on it, this will be selected by default and you will use this 99% of the time, esepcially at first. The second button has a pencil on it. We'll return to that later too!
- Down the right are the usual buttons which do what they say on the tin and at the bottom of those are the three eyedroppers which are identical to those on the Levels tool. You can select any part of the picture to be true black, true white or mid-grey and all other shades will be adjusted relative to those.
- Then there is the usual preview check box, if checked it shows you live in the picture what effect you are having. Under that is a button for making the Tool window a bit larger to allow for more accuracy if needed.... I never bother!
Imagine that histogram line you saw in the levels tool, lots of ups and downs on it. Ok, now lift that line and stand it up on the line in the Curves tool (the one that goes from bottom left to top right). It would sort of look something like this:
In other words, when we start flexing the curves line, you will be adjusting the whole of the histogram which means you have far tighter control over the results. If you don't understand that, don't worry. Hopefully the rest of this tutorial will show you the practical effects anyway :o)
Ok, the most common adjustment to use Curves for is to increase the contrast. So how do we do that? Ok, first make sure you have a picture open and then open the Curves tool. Now, you remember we have those graduated lines showing dark is bottom and left, light is top and right.
- So, when we pull the line down, things will get darker, when we push the line up above the centre, things will get lighter.
- Where you adjust on the line determines the area of shade in the picture you are adjusting most. So if you pull it down (Click and drag) from about a quarter way up from the left, you are adjusting the darker shades and making them even darker.
- If we then click and drag on the line about a quarter down from the right and push it upwards, we are now adjusting mostly the lighter shades and making them even lighter.
- If we make dark darker and light lighter, we will be increasing the contrast. The effect will be like this:
I have made it an extreme S-curve to show the difference, normally you would use a shallower curve than this but it's up to you. If you just click in the middle of the line first and then make the adjustment to the dark areas, you will see the line automatically goes upwards in the lighter areas. This is a flexible curve that adjusts around the points you put on it so you can make it do extremely complex curves. But let's keep it simple for now shall we?
Ok, so what would happen if we did the S-curve the other way? Push the darker areas up and pull the lighter areas down? Yup, you'll reduce contrast and make the image 'flatter'. If something is overly contrasty, this might be useful but would generally only be a very small amount.
Here is what it looks like if you do it (again extreme for effect!):
You'll see that the light areas have gone down to light grey tones and the darker areas have pushed up to lighter tones. Not what we would usually want but again a shallow S-curve can help if there is too much contrast in the picture.
Of course, you might not want to adjust the contrast, you might just want to lift the lighting across the whole image. In that case all you need to do is click and drag from the middle. Push up to lighten, pull down to darken. Here is an example of lightening:
So far we have adjusted the entire picture by having 'RGB' selected in the Curves window but we can do all the same things with just one of the three channels if we wish. In this example I have selected Red and then increased it throughout. In other words, I have done exactly the same as in the last example but instead of lightening the whole picture, I have simply lightened all the red tones:
Of course you can do the same to the Gren and Blue channels and you can go down as well as up.
With the Curves tool open, if you click anywhere on your picture, (Marked A on my example below) you will see the point is marked on the Curves line (Marked B on the example). You can then push or pull from that area specifically to adjust those tones. If you click on either side of that area and then adjust from the point you found, you will adjust a much finer range of shades.
Ok, the blacks are too strong and the white areas are burnt out spots. Can I do anything? Well yes you can. If we would rather have a gentle range of shades, we can do the following:
- You can drag the bottom end of the line up the left-hand side to any point you wish.
This has the effect of making all areas of the picture that are darker than that point, restricted to the level of that point. This means the darkest areas of your picture will be dark grey instead of black.
- At the top of the line, if you drag it down the right-hand side, all areas of the picture that are lighter than that point will be restricted to that shade. So all whites will become light greys. This is useful if you have a couple of bunrt out highlights that are distracting in your picture. By pulling them down a bit, they will not be so intense.
Conversely to the above, if you want to give a strong contrast or high key look to the image, you can do the following:
- Slide the bottom of the line aliong the bottom to the right.
This has the effect of saying every shade from this point and darker will all appear black in the picture.
- Slide the top of the line along the top to the left.
This will mean that all shades lighter than this point will appear white in the picture.
Ok, we have done some normal stuff, albeit rather exageratedly to make my points but now we are going to seriously wild! On the Curves window, we saw that there is a button with a pencil on it. This allows you to draw your own curve exactly how you want it. For the example I have drawn over the line as when you get radical, the line will break up in the window but the effect will still happen. So, get wild, draw a crazy line and this is the kind of thing that can happen:
Colours and shades get pushed up and down all over the place and can give some freaky effects. So that makes it pointless right? No, wrong!
Remember that this tool does not have to be used for the entire picture. You can make selections and apply it in just those areas as much as you want so if you want to get radical with just a bit of the picture, you go right ahead :o)
So if you want to just give my friend some radically funky hair and leave the rest of her looking normal, you can do just that:
Ok, like I said before, this is not everything you can do, you need to have a play with it and see for yourself but hopefully this tutorial has got you enthused about giving Curves a go and releasing its power at work into your post-processing.
There are two key principles I want to leave you with:
1. Make small adjustments: Better to make a small adjustment, close the tool, reopen and do
another step rather than make big adjustments. This also helps reduce the introduction of
graininess which can happen.
2. Instead of making the adjustment direct onto the layer, use a Curves Layer Mask instead. This
allows you to go back and adjust it as much and as often as you want. Very useful if you print
something out and find it isn't quite how you saw it on screen. You can go back and adjust it to
get it right.
Thanks for reading and I hope you find it helpful.