Laura Shoe's Lightroom (and Occasionally Photoshop) Blog for Digital Photographers

An Introduction to Curves

In Uncategorized on January 22, 2010 at 8:41 pm

Curves is certainly not one of those intuitive features of Photoshop (or Lightroom), or at least that was my perspective when I tried to figure it out on my own before I got formal training. Curves is used to brighten or darken tones in your image, and at a more advanced level, to do color correction work. The best way to use it in PS is with an adjustment layer (Layer>New Adjustment Layer>Curves.) In Lightroom simply go to the Tone Curve panel in the Develop module. The curve will look like this (with different buttons and features around it depending on your software and version).  Photoshop gives you more point-by-point control, so I will begin there.

The Curve Dialog Box Before Any Changes are Made
The Curve Dialog Box Before Any Changes are Made

So how do you read the curve? It is a two-dimensional chart showing tones before and after. Along the bottom (X) axis, you have tones ranging from pure black at the left to pure white at the right. These are the tones  of your image before you make any changes. Along the left side  (Y axis) are the tones, again from pure black to pure white, AFTER your changes to the curve. The diagonal line running bottom left to top right says what change you are making. If you visually run a line straight up from a tone on the x-axis Before to the Curve and then straight over to the left to After, you will see what that tone will be after, as shown below:

Before We Make a Change
Reading the Curve: Before We Make a Change

Of course in this case we haven’t moved the Curve, so there is no change from before to after, all tones are the same.

Let’s put a point at the center of the curve by clicking on it, then hold  and drag  straight up. Now if you follow medium grey up from the Before axis to the Curve and over, you see that it is now a significantly lighter grey, so we have lightened that tone. All other tones in your image have been lightened as well, other than pure black and pure white —  but not by as much as the midtones.  (Note that the histogram has changed in the screen print — it is simply that when I started writing this post I was using one image, than a few months later when I got back to it, I had to choose a different one — so ignore it.)

Brightening an Image with One Point to Raise the Curve

The opposite would happen if you put a point in the middle and you dragged downward — you would darken all tones in the image except pure black and pure white, but the most  darkening would be applied to the midtones.

Darken, Focused on the Midtones

This technique to brighten or darken your image protects you from blowing out your highlights, or blocking up your shadows.  A point in the center moved up or down achieves the same effect as using the Brightness slider in LR or ACR.

You can target the tones you want to affect and protect others — for example, in the following screen print, I brighten the lighter tones, and put two more points on the curve to prevent the midtones and darker tones from brightening as well.  Be careful that you don’t make the curve too abrupt.

Brighten bright tones, Anchor mid tones and dark tones so that they don't changeBrighten lighter tones, Protect midtones and dark tones from brightening

Hint:  If you put a point on the curve that you want to delete, click on it and drag it off the chart.

Adding contrast to your image involves brightening the bright tones and darkening the dark tones.  This is done with a classic S-curve:

A Typical Contrast "S" Curve

Of course you can also reduce contrast in an image, by darkening the bright tones and lightening the dark tones:

Reduce Contrast:  An inverted S Curve

For contrast, points exactly on 1/4 and 3/4 tones, as I demonstrated above, give you the same result as you would get with the Contrast slider in Lightroom.  With the curve though you can make a more conscious choice of which tones to focus on.

Lightroom also allows you to click and drag on the curve to bend it, though visually points aren’t laid down.    However, you don’t get quite the same degree of fine control as you do in Photoshop.  In a way, this is a good thing — Lightroom protects you from yourself by not allowing extreme (and therefore potentially disastrous) curves with many, many points or shifts in direction.  On the other hand, if you know what you’re doing with curves, and want that fine control, you will want to take your images from Lightroom into Photoshop to achieve it.

Lightroom also offers sliders below the tone curve.  This is particularly handy for those who don’t understand the curve itself, but is also a convenience for all of us.   Highlights, Lights, Darks and Shadows by default each represent 25% of tones from black to white.

Lightroom Curve, with Tones Represented by Each Slider Shown

If you want to brighten highlights in your image you can click and drag on the curve, or you can drag the Highlights slider to the right.  If you want to target just the brightest 10% of tones, for example, rather than the brightest 25%, you can narrow the range that the Highlights slider affects by clicking on the triangle slider at the base of the curve diagram (i.e. on the x-axis) and dragging it to the right to redefine “Highlights”, and then drag the Highlights slider to brighten:

Highlights Defined as only the 10% Brightest Tones, Then Brightened with the Highlights Slider

In the same way you can redefine what tones Shadows, Lights and Darks cover.

In Lightroom, I tend to use Brightness and Contrast in the Basics Panel to achieve overall midtone Brightness and Contrast, and then I move to the Tone Curve to finesse any specific tones that need attention — often I use it to dampen down my highlights, or to brighten up deep shadows.

Give it a try!

UPDATE MARCH 2010:  The point curve, previously only available in ACR and Photoshop, has now been introduced in Lightroom 3 Beta 2.  Click HERE for my post on how to access it.

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  1. […] This post was mentioned on Twitter by Tina Grant and Gregg Lowrimore, hoda. hoda said: #Photoshop: Digital Daily Dose: An Introduction to Curves http://ow.ly/ZAK3 via @thetinagrant […]

  2. […] Adobe lately has been introducing tools to make it more accessible.  As I mentioned in my “Introduction to Curves” post, the sliders available underneath the curve in Lightroom (and Camera Raw) to adjust […]

  3. Best explanation of curves in the world!

  4. in lightroom, is there a difference between brightening the shadows with the”fill light” slider vs. brightening them with the “shadows” slider under the tone curve? (and similarly for the highlights–the “recovery” slider vs. the “highlights” slider)

    • Excellent question, George, thank you. Yes, there are siginificant differences. Recovery in the Basics panel is used to recover blown-out highlights. The tone curve does not have a way to allow you to bring down and restore detail to these blown out highlights. If I want to dampen down highlights in my image, I will first slide the Recovery slider just enough to recover any of the highlights that are blown out, and no further (too much Recovery can give you unattractive halos). Then I will go to the Tone Curve, and darken the highlights further with the Highlights slider, limiting the Highlight range as needed with the Highlight point at the base of the curve, as explained in my post.

      The fill light slider in the Basics panel essentially slides the whole shadow and dark section of the histogram to the right, brightening all these tones, including anything that is pure black (blocked up), bringing light into the dark areas of your photograph. The shadow and dark sliders in the Tone Curve almost do the same thing — the difference is that the endpoints are protected — if you brighten with these, you’ll see in the histogram that the darkest of the dark tones stay dark. Fill light can also add odd halos where your shadows run up against brighter tones, so use in moderation and after applying, zoom in to 1:1 to make sure you don’t see halos.

      Watch for a post soon on this — I’ll repeat and elaborate on the explanation and include examples with screen prints. Thank you for the idea.

  5. […] precise control over the tones in your image than the default parametric curve does.    See my Introduction to Curves post for an explanation of how to read and use the curve. (Note that this post was written before […]

  6. […] precise control over the tones in your image than the default parametric curve does.    See my Introduction to Curves post for an explanation of how to read and use the […]

  7. […] Adobe lately has been introducing tools to make it more accessible.  As I mentioned in my “Introduction to Curves” post, the sliders available underneath the curve in Lightroom (and Camera Raw) to adjust […]

  8. […] precise control over the tones in your image than the default parametric curve does.    See my Introduction to Curves post for an explanation of how to read and use the curve. Posted by lshoe at 2:43 pm Tagged […]

  9. […] Adobe lately has been introducing tools to make it more accessible.  As I mentioned in my “Introduction to Curves” post, the sliders available underneath the curve in Lightroom (and Camera Raw) to adjust […]

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