Photoshop Curves

When I was a Photoshop rookie 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 seemd to defy any logic whatsoever!

At this point in my Photoshop career Curves are less of a mystery and the time has come to offer you this tutorial on 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.

The Basics of Curves
The Dialogue

This is the photoshop curves dialogue box.  The first thing you notice is that there is no curve - just a nice straight line.    

curves palette

This nice straight line goes from the bottom left to the top right and is a graphical representation of the full range of brightness from 0 (black with no detail on the bottom left) to 255 (pure white with no detail on the top right).

Roughly speaking, the bottom third of the line is the shadow detail, the middle third the midtones and the top right third the highlights.  In addition - everything below the line is darker than at the line and everything above the line is lighter.

Confusing?  Just play with the curve and it will all become clear.

So - if you move a spot on the line up then that area of brightness will get lighter and if you move it down it will get darker.

When you open a photoshop curves adjustment layer (by clicking on the little black and white circle at the bottom of the Layers palette and selecting Curves) and then take your cursor and run it around your picture then you will see a dot move up and down the line corresponding to the brightness.

Changing the Curve


What you can do is click somewhere 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.

Changing Brightness


A simple and small change to the curve can have a really dramatic effect on the brightness of the image.

Take this image of the Tattoo & Barbershop.  It's an interesting (and very unusual) subject but not particularly pleasing photographically.

curves tattoo

Anyway ...

The picture is too light and too flat - not at all what I remember when I pressed the shutter on my trusty Olympus E-300 DSLR.

It was annoying to see this lmage after I rushed home full of excited anticipation and loaded the RAW file into Bridge.

Your keen eye can tell this is not my best work and the Histogram confirmed your opinion - most of the Histogram was kinda clumped off to the right - the light area.  There is very little information in the shadow area.  In my defense - it was a brilliant overcast day with the light falling on the front of the store.

curves histogram

The histogram tells all you need to know about this image ...

To get the brightness where it is required the administration of a Photoshop Curves Adjustment Layer and that's what I did.

The histogram is saying that there is not enough shadow area in the image and this is confirmed visually - it really is too light and quite unappealing!

There are so many ways to modify the curve to improve the image - here are a couple...

curves mid point adjust curves mid adjust
The Mid-Point of the
Curve Pulled Down

 The Adjusted Image
curves adjust curves after adjustment
The Mid-Point Pulled Down
And The Bottom Left Corner
Moved to the Right		 The Adjusted Image

This image suffered from being far too light so the Curve was dragged down into the dark area.  If the image was too dark then the curve would go the other way - up into the light area.  

In the situation where the bottom left point was moved to the right it increased the slope of the line and this not only modified the brightness, it also changed the contrast ...

Improving Contrast

An image that just looks "flat" is generally lacking contrast and this is where Photoshop curves really shines!  

An image that covers the full range of brightness levels (0 to 255) will generally have good contrast.  The contrast will suffer to the degree that the full 255 brightness levels are missing.

Here is an image of some wall art.  It is not a bad image but can probably be better with a contrast adjustment with Photoshop Curves ...

curves wall art

and here is the histogram ...

curves histogram

The majority of the pixels are in the mid-tones and shadows which creates the overall flat appearance - gotta have the full range of brightness levels (from 0 to 255) to have a pleasing image, right?  This image has brightness values ranging from about 0 to 225 meaning it will improve with a bump up in contrast.

A Photoshop Curves adjustment can fix that ...

This is the curve I eventually ended up with after some messing about to get the image looking like I wanted it to look ...

curves dialogue

The Points ...

  1. The top right point was moved to the right one square which increased the slope of the line creating more contrast,
  2. Point 2 was nudged up just a bit to make the highlights a bit brighter (the points can be moved about with your pen or you can highlight one and use the arrow keys to move the points).
  3. The third point, the mid-tone area, was also moved up a tad which brightened the mid-tones.
  4. The fourth point was moved down a bit to add some richness to the shadow areas.
  5. The fifth point was left alone.
The overall appearance of the curve resembles an "S" which is the classic curve to improve contrast with Photoshop.

Here is the image after the change in contrast ...

curves contrast adjusted

The reds are redder - the little blue coaster now has some color, the yellow menu has some attitude and the whites are brighter - overall it is a better image - and you know what - I think the guy on the left is in love with the gal at the table!

Who knew?

Useful Stuff


To move forward from point to point on the curve - Ctrl-Tab (Windows) and Cmd-Tab (Mac).

To move backward from point to point on the curve Ctrl-Shift-Tab (Windows) and Cmd-Shift-Tab (Mac).

To reset the curve to normal press the Alt key and the Cancel button turns into a Reset button. Click it!  This is same in all of the Adjustment Layer dialogues.

The little eyedroppers in the dialogue can be used to set a black, white and neutral (18% gray) point.  The black and white are easy - the neutral can be a bit of a problem.  The neutral gray point is really important and deserves its own tutorial page.  When you are using a light meter it is simply looking for the 18% gray point and that is why you can find the 18% neutral gray cards for sale in the higher end camera stores.

The Auto button will set the curve automatically although the appearance of the curve will not change.

You can select the pencil tool and draw your own curve (ya, right).  

The curves for the Red, Green and Blue channels can be individually modified.  This is more of a high end, experienced user thing and somewhat beyond the present scope of this site.  A detailed tutorial can be found in the manual Photoshop In A Day.

Combining Photoshop Curves and Photoshop Levels can have a really, really, really dramatic effect (sheez - I read that and thought of Zoolander).

For instance - this image improved with a Photoshop Levels Adjustment and with a Photoshop Curves Adjustment but when they were both done the improvement was dramatic ...

curves dog park curves dog park


Changing the brightness and the contrast of an image with Photoshop Curves will make some amazing changes.

Give it a try and play with the curve to see what happens.

Digital Darkroom


In the old style "wet" darkroom curves are analogous to the black and white mult-grade papers and the little filters that were placed 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 dialgogue, 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.

Why Use An
Adjustment Layer
Photoshop Curves?


A Curves Adjustment layer is more versatile that doing the adjustment on the original image.

Why?

The Opacity of the adjustment layer can be lowered to decrease the effect if it is a bit too storng, and

The adjustment layer comes complete with its own Layer Mask whicl allows for slectively removing part of the adjustment, and finally

The adjustment layer preserves the original image and that is a good thing!

And finally - if you do a bunch of adjustments and realize the curves need to be re-adjusted it is a simple matter to open them again and make further adjustments.  

If you do it on the original image then it's back to square one on the original image.


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