Photoshop Curves

Photoshop curves were a real mystery to me when I was a rookie digital editor.  I would occasionally open the Photoshop Curves dialogue box and mess about with it.  The results were always a complete and total disaster.

People told me that Curves were great - ha - it was a complete mystery and did not make any sense at all.  The darn thing seemed to defy any logic whatsoever!

At this point in my digital editing career, Photoshop Curves are less of a mystery and the time has come to offer you this tutorial on Photoshop Curves.  There's a lot to learn and it'll be worth it.

In the old style wet darkroom curves would be analogous to the black and white mult-grade papers and the little filters that were place below the light source.

I have to admit that I never really understood how to use the multi-grade papers with the filters and that is probably why my black and white prints tended to lack contrast.

The curves dialogue, at first, was also a mystery to me until I understood the importance of good contrast in an image and how easy it is to modify the contrast with the curves dialogue.

Photoshop Curves Quick Links

Auto Setting

Let Photoshop do all of the work.

Modifying the Curve

Moving the endpoints

Changing the Shape of the Curve

The well known 'S' Curve

Using the Eyedroppers

By the numbers

Photoshop Curves
The Panel

You can see from the following three screen shots of the Photoshop Curves dialogue that they have different looks from CS2 to CC but the important parts are pretty much the same.

What's In The Panel

This is the Photoshop curves dialogue box for CS5.  When Curves is first opened the line is straight from the bottom left (Shadow area) to the top right (Highlights Area).  It covers all of the tones from 0 on the left to 255 on the right and the Histogram for the open image is visible behind the line.

The panel opens with a 4X4 grid arrangement.  If you want to have a 10X10 grid, simply Alt-Click on the grid.

The line roughly represents the three tonal areas in an image ...

  • The Shadows (from 0 to 85).
  • The Mid-Tones (from 85 to 171).
  • The Highlights (from 171 to 256).

If you grab the bottom left point and pull it to the right and/or click on the top right point and pull it to the left the adjustment is just like a Levels adjustment.  

When you click anywhere on the line a point will be added to the line.  Once you've got a point on the line you can click on the point and push the line up (to make things lighter) or pull the line down (to make things darker).

Input and Output

The horizontal axis of the graph (the bottom one) represents the input levels (original image values) and the vertical axis (on the left) represents the output levels (new adjusted values).  When Photoshop Curves is first opened there are no values for Input or Output.

As soon as the curve is changed then the Input and Output values will change.

In this example a point was added at 128 (middle grey) and it was dragged down to 102, making the image darker.

It's not really necessary to work with these numbers but they are interesting.

Dialogue - Left Side

Down the left side of the CS5 Photoshop curves dialogue are some additional icons.

The top one allows for on image curves adjustments - quite a neat idea.  Activate the icon, go over to the image, left click or put the pen on the tablet and move around.  The curve and the image will change.

The eyedroppers make it easy to do a quick adjustment based on the darkest and brightest part of the image  and you can do much more exact adjustments in conjunction with the Info Panel.

When the curvy line is activated it means you can place points on the line and move them up, down, left or right to affect the contrast of the image.

The pencil is not used very often - it allows you to draw a freehand curve.

The bottom icon will smooth out the curve.

Bottom of the Palette

At the bottom of the CS5 and CC Photoshop Curves panels are 7 more buttons (this is a busy panel, isn't it).  From left to right they are ...

  1. A big arrow to return to the Adjustment list.
  2. Switch the panel between expanded and standard view (that means big or small).
  3. A button to clip the Curves layer to the layer immediately below or and if it's not clicked than the Curves layer affects all layers below it.
  4. The eyeball to toggle the visibility of the adjustment layer.
  5. The next icon, when clicked, shows the previous state so you can toggle back and forth between two adjustments.
  6. The backward arrow resets the panel to the default adjustment.
  7. The last icon (Trash Can) will delete the layer.

All of the Photoshop versions have an Auto Button - it does what the name implies - clicking on it creates an adjustment based on the the image evaluation that is done by Photoshop.

Working With Curves

The Auto Setting

This is easy and is really only useful if time is of the essence.  

Click the AUTO button and you're done.  

When Photoshop Curves performed an evaluation of this image it must have seen that the Histogram did not extend from black to white.

What Photoshop did was pull the black point to the right, to the bottom of the histogram and it pulled the white point to the left to the other end of the histogram.  

This adjustment, moving the Shadow point up and the highlight point down, is very much like the Levels adjustment.

Here's an example ...

There are, in fact, options available for the Auto setting. When you Alt/Option click on the Auto button then this dialogue box comes up.

Enhance Monochromatic Contrast

Clips all three channels identically. This preserves the overall colour relationship while making highlights appear lighter and shadows appear darker.

Enhance per Channel Contrast

Maximizes the tonal range in each channel to produce a more dramatic correction. Because each channel is adjusted individually, this selection may remove or introduce colour casts.

Find Dark and Light Colours

Finds the average lightest and darkest pixels in an image and uses them to maximize contrast while minimizing clipping.

Modifying The Curve

Moving the Endpoints

If you want to make your own Photoshop Curves adjustments rather than use the Auto Adjustment (and who doesn't) then modifying the curve is the next step.

The most basic adjustment is to mimic the Auto Adjustment and that is to ...

Move the end points in to the Histogram so back to the sailboat.

The method is simple - just grab the little triangle at the left (shadow end) and move it to the right until the vertical line that appears just touches the bottom of the Histogram.  The image will get darker.

Now go to the other end, grab the little white triangle and move it to the left until the vertical line touches the top of the Histogram.  The image will get lighter.

Overall, as a result of the two adjustments, the image contrast has improved.  That's because the curve is much steeper and a steep Curve always creates more contrast.

Here's how it looks ...

So what's happened?

Here are the before and the after Histograms for the sailboat image and it's easy to see there are substantial differences between the two.

The Histogram for the original image on the left stretches from 20 to 226 so it's not covering the full range of brightness (it represents 206 of the 256 brightness levels).

The Histogram on the right is the result after the adjustment.  It covers the full range of brightness from 0 to 255.  The white areas within the Histogram are the areas with no information - Photoshop eliminates some brightness levels as it stretches the Histogram to cover the full range.  The missing information is not noticeable.

Changing The Shape

So far the Photoshop 'curves' have been straight lines - no curves to be seen so it's time to get to the next level.

What you can do is click anywhere on the diagonal line and notice a point is set there.  Make sure Preview is checked, open your Histogram window and then select the point and move it around - up/down/left/right and watch what happens to your picture.

It changes - getting brighter or darker and if you watch the histogram you will see it changing as well.  If you make big changes then the result with your image will be dramatic and generally not particularly pleasing.  

Most of the time with Photoshop Curves - less is better

Back to the Sailboat

We know a couple of things now ...

  1. Moving the points in to the edge of the Histogram produces a very positive change.
  2. The steeper the line, the greater the contrast.

So the question is this - now that we've got a nice improvement in the image, can we improve it even more?

Of course we can and it's a simple matter of adding some points and moving them around so let's look at how to do that.

Just to refresh your memory, here's the original sailboat picture.  It lacks contrast.

Once again, here's the Photoshop Curves Dialogue for the sailboat with a couple of differences.

Points 1 and 4 on the curve are the same as the curve that was created when the AUTO button was selected.  

It's also the same as the curve that was created when the shadow (left) and highlight (right) points were moved to the edges of the Histogram.

The new points are 2 and 3.

Point 2 was placed about 1 1/2 squares to the left of  the Highlight point.  It made the overall picture just a bit brighter.

Point 3 was placed about 1 1/2 squares to the right of the Shadow point.  It made to overall image just a bit darker.

The overall effect is an improvement in the contrast of the image.  If you recall a steeper line increases the contrast and the line certainly is steeper between Points 2 and 3.

The change created an "S" Curve which is the go-to curve to improve contrast in an image.  Sometimes the S will be dramatic and other times it will be subtle - in every case the steeper the line the greater the contrast.

This image was done entirely using the eyeball test which is quick and generally accurate.


With Photoshop Curves you can make modifications to an image in a variety of different ways.  So far we've looked at the Auto setting, moving the black and white points in to meet the Histogram and increasing contrast.

Using the eyedroppers on the Photoshop Curves dialogue may well provide the same result as the other methods so what's the point?  For you folks who love numbers then this is the method for you.  It really is working the image by the numbers - so let's take a look ...

The Photoshop Curves dialogue has three eyedroppers on the left side of my CS5 dialogue and they can be found on every version of Photoshop.  

Curves Eyedroppers

They are, from top to bottom ...

  • Shadow Eyedropper
  • Mid-Point Eyedropper
  • Highlight Eyedropper

When the Shadow Eyedropper is clicked on a black or almost black pixel then that pixel is set as black.  By default this sets the black point to 0 on the Histogram.

When the mid-point Eyedropper is clicked on a middle grey area then that pixel is set to middle grey.  By default this sets middle grey to position 128 on the histogram (half way between black (0) and white (255).

When the Highlight Eyedropper is clicked on a white or almost white pixel then that pixel is set as white.  By default this sets the white point to 255 on the Histogram.

The black and the white points can be changed and this is generally recommended while it's best to leave the middle grey point alone.  Changing the Shadow and Highlight points means that the blackest part of the image will have a bit of texture as will the lighest part of the image.

Reset the Shadow Target

  1. Double click on the Shadow Eyedropper to bring up the following dialogue.  
  2. Enter new Target Shadow Color number into the RGB data points (no larger than 10).  You can see I am using 5 as my Black Target.

Reset the Highlight Target

  1. Double click on the Highlight Eyedropper to bring up the following dialogue.  
  2. Enter new Target Highlight Color number into the RGB data points (no lower than 245).  You can see I am using 250 as my Highlight Target.

Now that we've found the eyedroppers and set the black and white points, it's time to move to the next set and that's locating the darkest and lightest area in the image.  There are two different ways to do that - either with an eyeball and the Info Palette or with a Threshold Adjustment layer.

We're going to do that with the seldom used yet very useful Threshold Adjustment Layer.

This is the image we'll be working with ...

As you can see it's somewhat washed out and lacking contrast.  The Photoshop curves eyedroppers can fix that ...

Find the Black Point

With the Background layer selected add a Threshold Adjustment Layer.

When the layer opens up the triangle adjuster will be sitting at position 128 (middle grey).  More the slider all the way to the left and the screen will become all white.  

Now slowly move the slider to the right, or better yet, highlight the Threshold Level box and press the up arrow key to move up one number per key press.

Whichever method you choose, stop when you see a black pixel appear - that is the darkest area in the picture.  Zoom in so you can easily see the black pixel.

Get the Color Sampler Tool (it lives with the Eyedropper in the tool bar), move over the black pixel and click once.  A small circle with the number 1 next to it will appear - the darkest area has been located and marked!

In this image I added a couple of additional screenshots so you can see ...

  1. I lowered the Opacity of the Threshold layer so you can faintly see the underlying layer. 
  2. The Threshold Adjustment - for this image the first black area appeared at position 27.
  3. In the bottom right corner is where the Color Sampler tool was clicked.  This is the Black Point.

Find the White Point

After finding the Black Point more the slider all the way to the rightand the screen will become all black.  

Now slowly move the slider to the left, or better yet, highlight the Threshold Level box and press the down arrow key to move down one number per key press.

Whichever method you choose, stop when you see a white pixel appear - that is the lightest area in the picture.  Zoom in so you can see the picture easily.  

At this point you need to insure that the first light pixel to appear is not a Specular Highlight (like the glare off of glasses or other hotspots).  To do that make sure you're zoomed in on the first light pixel, turn off the visibility of the Threshold layer and see what's under there.  With my picture there's something on the side of the boat with a bright glare - I didn't want to use that because it will throw off the end adjustment.

I zoomed out and kept moving the slider to the right until the next light area appeared and checked that one as well.  It was the top of the boat cabin so that it was fine.

Get the Color Sampler Tool (it lives with the Eyedropper in the tool bar), move over the white pixel and click once.  A small circle with the number 2 will be placed where you click - the lightest area has been located and marked!

In this image I added a couple of additional screenshots so you can see ...

  1. I lowered the Opacity of the Threshold layer so you can faintly see the underlying layer. 
  2. The Threshold Adjustment - for this image the first usable white area appeared at position 225.
  3. To the right of the adjustment panel is where the Color Sampler tool was clicked.  This is the White Point.  The vertical white line is a specular highlight

Find Middle Grey

Middle grey is important in photography and in Photoshop color correction.  In most cases finding the middle grey on your image and clicking with the Middle Grey eyedropper will correct a color cast.

Finding middle grey is a bit different from finding the black and white point.  Here's how to do it ...

First, add a new layer above the Background Layer and fill it with 50% grey using the Fill Menu
(Edit > Fill or Shift-F5).  Drop down the menu and select 50% Gray and OK.

The next step is to change the Blend Mode of this new layer to Difference - the image will look weird.

If your Threshold Layer is still in the layer stack (at the top) you'll see that the histogram only appears on the left side of the adjustment panel.

Move the slider all the way to the left and start moving it back to the right until you see a pixel appear - this is middle grey in your image.

Get your Color Sampler (as before) and click on the Middle Grey pixel to add a point.  It will be labelled number 3.

Now that all three points have been found (Black Point, White Point and Middle Grey)you can delete the new 50% grey  layer and the Threshold layer and then it's time to move the next step.

One hint - when all of the layers have been eliminated it can be difficult to find the three points to make the adjustments.  

To eliminate the agony of searching for the three points go up to the top ruler and drag guides onto each of the points and then do the same from the left ruler.  This makes it much easier to complete the next step and that's what all those pretty colored lines are the next image.

With the Middle Grey and Threshold layers deleted, add a Curves Adjustment Layer and select the Black Eyedropper.

Zoom in to find the black point that was marked with the Color Selector and click in the middle of the little circle which will re-map the black.

Find the White point, select the White Eyedropper and click in the middle of the little white circle which will re-map white.

Now locate the Grey point, select the Middle Grey Eyedropper and click in the middle of the little middle grey circle.  This often eliminates any color cast in the image.

Now you can use the Move Tool to eliminate the guides.

Here's how things look for each step with one extra at the end ...

After clicking the black point with the Black Eyedropper.

After clicking the white point with the White Eyedropper.

After clicking the middle grey point with the Middle Grey Eyedropper.

Extra Step - Filter > Sharpen > UnSharp Mask and set ...

Amount - 50
Radius - 20

This is a nice De-Fog setting.

The Photoshop Curves eyedroppers takes a bit of effort to complete but it does give great results.


Photoshop Curves are fantastic.  They do so many different things and they do them very, very well.  Take some time to become familiar with all aspects of the Photoshop Curves adjustment.

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