tutorial - how to use Curves in Photoshop

#1

OT because it’s mainly for photographers, but I thought some of you would like to read it. It’s part of a small series of articles I’m writing for a photo site. A bit techy, but not that hard to understand :slight_smile:

A quick guide to using Curves in Photoshop

At first glance, the Curves dialog can look intimidating, it’s hard to visualise what is going on. In actual fact, it’s quite simple to understand, and more importantly it’s very powerful, and in many cases much more versatile than using Levels. Using Curves allows you to control exactly where in your image you can fine tune contrast, and it also allows you to increase contrast without losing detail in the shadows or highlights. What’s more, this can all be done visually, intuitively, and easily!

To start off, here is a fairly typical image that could benefit with some tweaking in curves. I shot it on my compact digital camera – which I deliberately set to record images as flat as possible: turning all the settings for contrast, saturation and sharpness to their minimum settings. This means that images out of the camera look dull, but I prefer to control these things myself later as the camera defaults are indeed vivid but not always to my liking.

A quick look at the histogram reveals that this image is fairly high contrast, with values at both ends of the range:

This means that any contrast increase using Levels will clip some of the detail in the shadow and highlight regions.

So here is the result of a Levels operation, the contrast is higher, but it hasn’t really improved the image. Some of the detail in the leaf has got lost, as has some shadow detail:

In this case, what we really need to do is to increase the local contrast in the leaf, without losing any of the detail in the shadows or highlights. This is where Curves come in, Levels are too crude a tool for this job.

If we revert back to the original and call up the Curves dialog, we can easily perform the corrections that are necessary to bring out the best in this image.

The first thing you should realise when opening the Curves dialog is that you shouldn’t be looking at the graph, you should be looking at your image instead – this is where the work gets done. I think that once this idea is grasped, much of the fear of using Curves vanishes.

What the Curves command does is to take pixels of one value, and re-map them to another (either lighter or darker), the reason for the curve is to make the transitions gentle. So by re-mapping one value the curve will also affect neighbouring values in order to make the transition subtle. The steeper the curve is, the higher the contrast between values. Conversely, the shallower the curve, the less contrast between those values.

How does this impact on our image? Well, it allows us to target the specific values we want to work with. With the Curves dialog open, if you left-click with your mouse anywhere in the image the value underneath your cursor with be plotted on the curve (which is a straight line at the moment). If you control-click (command on the Mac) on any part of your image, the value represented underneath your cursor will be placed on your curve as a point – this is the key to working with Curves, and it’s very simple.

Looking at the image, the area which needs more contrast is the leaf itself, increasing the contrast in this area will help reveal the detail in the leaf rather than destroy it like the Levels command does. So I simply control-click on the lightest and darkest areas of the leaf to plot those respective values on the curve. Illustrated below are the two points I clicked on:

And the resulting curve:

#2

What we need to do is to make the line between our two points steeper, by moving the points either up or down. Moving a point up will make it lighter, moving it down will make it darker. I also personally like to move the points slightly to the left and right to widen the range of values being altered, as I generally find this more effective.

The curve below is the result of steepening the area of interest (the leaf) by moving the bottom point down and to the left, and the top point up and to the right. Remember that Curve adjustments should generally be small and subtle.

And here is the resulting image:

Two important things have happened: we have increased the contrast in the leaf itself and therefore emphasised the detail within it, and we have done so without clipping the highlights – the curve tapers to the top right hand side in a gentle shoulder meaning that highlight detail is preserved all the way to white.

The grass area is still a bit too dark though, so before clicking on OK I want to lighten it somewhat. By running the cursor around the image with the left button clicked down I can see that all the grass values lie beneath my existing points. Instead of control-clicking, this time I simply click roughly in the middle of the area beneath my existing points (pretty much the bottom half of the curve) and simply drag the new point upwards to lighten all those values:

The image now looks like this:

Finally I decide to make the very darkest shades a little blacker and contrastier, so I move the point in the bottom left corner slightly to the right to clip some of those dark values to black (this is exactly what the sliders in the Levels dialog do):

After clicking OK this is what the image looks like:

By using Curves instead of Levels I have been able to increase contrast selectively in the leaf area, lighten the whole of the grass area while also increasing shadow contrast there and preserve the dynamic range of the highlights. All this was done by referring directly to the image in a very visual and intuitive way. As you can see this is a very powerful way of controlling local contrast, and a much finer and subtler tool than Levels ever can be.

Some tips for using Curves:

If you alt-click on the grid you can toggle the divisions from quarters to tenths, giving you either coarse or fine grid squares.

You can if you wish use the eyedropper tools to set black, white and grey points. The grey eyedropper can be used for fixing colour casts, by clicking it on a neutral point in the image. For instance if you have an image with a strong orange cast because it has been taken under tungsten lights, click the grey eyedropper on a white wall to correct the cast. The more neutral your source object (a proper grey card is ideal), the more accurate the result.

You can use the shortcut control-M to call up the Curves dialog. Even better, using control-alt-M calls up the dialog with the previous used settings already dialled in, this is very useful when working with batches of images that need the same or similar settings.

Curves are also available as an Adjustment Layer – use this option if possible as it is non-destructive.

#3

Awsome this should come in handy :slight_smile: Thanks for posting :thumbsup:

#4

hey very nice tutorial, thanks man !

Didn’t know you are on cgtalk aswell, have seen you on dA, your vector art is great !

Edit: ok I’m clearly mistaken and your not the frog guy from dA (well i think you aren’t) lol, well still thanks for the tutorial.

Cough this never happend :shrug:

I should learn to check peoples profiles out before posting =/

#5

Awesome. These are the kind of quick tutorials that one day save you :slight_smile:

#6

Thank you for a very easy to follow tutorial. Clear, and informative. I encourage you to write more. (Not being greedy, just like your style very much).

Thank you!

#7

Curves DEMYSTIFIED!!:beer:

THANKS FROG!

#8

Very helpful, indeed! Thank you for posting. :slight_smile:

#9

your info combine with the dylan cole dvd’s on matte painting are priceless thanks

#10

This looks like a useful tutorial. I never did get much out of Curves before - after reading this I suspect I just wasn’t using it right. Cheers! :slight_smile:

#11

thanky u very much for the explanation, it´s very useful…

#12

I always knew curves were a powerful tool, but I never really got much out of them without a lot of putzing around. Naturally, I had no idea you could ctrl-click on the image! :argh: Thank you, sir, you rock. :buttrock:

#13

Ditto. Years of using Photoshop, and I still never quite understood how to take advantage of curves… until now. This is immensely helpful. Thanks!

-mike

#14

This was very helpful. Thank you for helping taking “sharper turns” without cutting corners.

#15

and the phrase “sharp turns without cutting corners”, i had never heard it before but its so damn right. good stuff. that really sums up the philosophy that i feel a lot of tech directors miss… on the one hand you dont want to cut corners but on the other hand you want to turn as sharp as you can, keep it simple as you can. the point of techniques and mastery should be to speed workflow for artists, many good techniques will take longer to write the tutorial than to actually implement :slight_smile:

#16

This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.