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Adobe Photoshop Tutorial




Getting Started with RAW

The Curves correction Tools

acr lens correction toolsUp until recently I looked at this tabbed window and simply blanked it out as being too fiddly to use. 'Life's too short' was my main feeling with this set of tools but I have managed to convert myself. I had some images that were really not giving me what I wanted light wise so I had a play with them in Curves and suddenly a whole new set of possibilities became apparent. It's still not something I would use all the time but for some of those tricky shots, lighting wise, this could be the answer.

Just to recap, we have already seen how we can change the shadow and highlight detail in the Basic tabbed window by using the Exposure slider, Recovery slider, Fill Light slider plus the Blacks, Brightness and Contrast sliders but they basically either work on the light or dark areas of the image. what if you wanted to just change the semi dark areas without effecting the rest or maybe just increase the smallest of highlights? The curves tools allow you to select different ranges of shade and light to change individually and you can even set the parameters of how wide the range of these areas should be.

There are two sub tabs to this window. The first is the Parametric curve, I find these the most useful. The other is called Point and here you can add points to the graph and position the curve how you want it by dragging them about.

Parametric Curve Window

Lets quickly look at the Parametric curve first. You will first notice a graph made up of 16 squares 4x4 with a black line running evenly from the bottom left to the top right. Behind that in grey there is a histogram of the image displayed. This graph represents lightness values just like the histogram. The top right corner is the pure white value while the bottom left is the black value. If you now look at the 16 squares the 4 vertically stacked to the right show the range of the Shadows and are linked to the slider by the same name below. Similarly the next stack of 4 squares immediately to the right of them represent the 'Darks' or areas of darkness but not full shadow. This also has a corresponding slider. Next it the 'Lights' which are areas of well lit image that is lighter than mid grey but not highlight and the last vertically stacked set of 4 squares full right are the 'Highlights'. The vertical lines that divide these columns of squares are moveable. Click on the little triangle tabs below each line and drag it either left or right to either increase the range of values contained within it or decrease it. Initially doing that won't effect the image at all because the sliders below are all set to '0' by default. We will come back to this to see how it works on a image shortly or you can click here.

acr curves point windowPoints curve window

Now let me explain the Points window. You will see much the same graph but without the sliders at the bottom or the triangle tabs to change the columns. Instead you will see some points fixed to your diagonal line or 'curve' as it will soon become. By default you will have at least 5 points already on the curve, there are 6 on mine but that me be due to my camera settings. The bottom left point sits at the black end and the top right point at the white end. You can see a histogram of the image in pale grey behind the curve. There is a centre point that represents mid tones and two more half way between the black and mid tone and the mid tone and white. You can click on these and drag then where you like or you can click on another part of the curve and add a new point to position the curve or to anchor it to a fixed point.

You will also notice you now have a drop down box which provides 4 options. 'Linear' where no correction is set and the curve will simply be a straight line between the bottom left and top right. 'Medium contrast' shown here where some contrast enhancement has been applied and the line has curved gently. 'Strong contrast' which darkens the shadows and brightens the highlights more causing the curve to almost snake and lastly, 'Custom' where you can create your own settings by dragging the points around as you wish.

Although the little tabs at the bottom of the vertical lines have gone that does not mean you cannot move the position of them. The points that are placed by default on the graph will each sit on one of these lines and by clicking on that point you can drag it left or right to reposition the line. Moving them up or down will lighten or darken their value. This is something you will need to practice with but I will sown you some of their effects.

Using the Points curve

For this tutorial I will be working on this still life shot of some pomegranates, below.

acr curves

Some of the changes are quite subtle so may not be easily apparent seen on the web. You really will need to experiment with this for yourself to see the full effect. First I shall show you the changes you can make in the Point tabbed window. I shall make these edits quite extreme.

acr curves

Using the drop down box at the top of the window I've selected Linear. This has flattened out any contrast the camera set as a default and flattened the image slightly. It has less 'punch'.

acr curves

Now I have re-selected the Medium Contrast which has increased the contrast in the image slightly. The colors are deeper and the image appears more 3D almost. I say 'reselect' because with my camera settings the Medium contrast option is shown as the default when i click on this window so this image should look the same as the top one.

acr curves

Now I've selected the 'Strong Contrast' option. Notice how much more curved the line is and how dark the shadows have become. The vibrancy of the whole image has increased also. If I attempt to move any of the points on the graph while any of these options are displayed the drop down box instantly shows the 4th setting of Custom. This is because Custom refers to any settings that you select manually.

acr curves

By dragging the points further to increase the curve still more I can really darken those shadows and saturate the colour in the light areas. Take note of which way the curve is going. Moving a point higher lightens that range of shades while moving it lower darkens them. Generally any edit will involve lightening the highlights or deepening the shadows so the curve will form a lose 'S' shape. However, what would happen if I was to reverse this?

acr curves

You can see that now the shadows have been lightened and you can actually see detail where previously there was only black. The highlights have become very dull and flat, almost drained of colour. However, compared to the previous, super saturated and contrasty image, there is at least a ton more detail visible in this image.

Using the Parametric curve

Now let me show you how you can make similar changes using the Parametric curve window. This is the one that is presented first by default and a little more simple to operate. I will use the same image as before so to recap, here is the image as it came off the camera.

acr curves

I want to give the image more bite but like the quality of the hard shadows and dim lighting.

acr curves

To achieve what I require I shall deepen the Shadows but lighten the other three sliders by varying amounts. I don't have to worry about points on a curve as the program sorts that all out for me. This example is maybe a bit too subtle so let me show you what happens if I take the shadows slider all the way down to -100.

acr curves

The shadows have completely turned to black. Now I can show you the effect of moving the vertical lines in the graph to shrink the darkness values that are effected by this change. Remember the first set of 4 vertically stacked boxes on the far left of the graph show the range of shades included and so effected by the shadows slider. By default it takes the darkest 25% of the values available. The smallest you can make this range is 10% and to do that simply click on the first triangular tab and drag it all the way left. It won't go right to the edge but you will see a numerical value appear by the tab as you move it sowing you the percentage.

acr curves

Now if you look closely and compare this image with the one above it you will see that the areas of deepest shadow have reduced.

I hope this has shed some light on the mystery of this particular set of tools and that you, like me, might find it useful occasionally. If you have any questions do please e-mail me.

Next I shall discuss ACR's sharpening and noise reduction tools.


RAW introduction,

RAW - Camera Settings

Importing your RAW files

ACR Tools and Interface Explained

Creating and Applying Presets

ACR Basics & Exposure Tools

ACR Sharpening & Noise Reduction

HSL/Grayscale tools (Hue, Saturation and Luminance)

Split Toning tools

Lens Correction tools

ACR Top Menu Bar (Crop, rotate and blemis removal etc)

Saving your settings and converting your RAW files

Bridge and Converted RAW Files



Recommended Further Reading for Photoshop

Return to introduction & contents page

"A-Z of Digital Editing"


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