Levels vs Curves

Both Levels and Curves tools in Photoshop are very useful in matching lighting and color of an image or a part of an image.  Brightness and Contrast are generalyl tools for beginners, but serious users use Levels or Curves, and rarely use Brightness/Contrast [this is why that tool does not have a hotkey, but Levels and Curves do.]

First, we'll have a little Levels-anatomy, then compare it to the Brightness/Contrast tools.  Then, we'll show how Levels compares to Curves, and follow up with how Brightness/Contrast compares with Curves.  If you feel confident with Levels, you can skip that part of hte lesson by clicking here.  If you feel confident with both Levels and Curves, then you can skip this whole thing in entirity, or read for acedemic interest, or to fill in holes in your knowledge of each tool.

Hope this helps!

First off, here's an image:

Bird Warrior

We will now try some adjustments on this image.

In Levels, you will see a histogram.  This is a graph of the *amount* of pixels for each brightness value in the source image.  The higher the black line goes, the more pixels that are that particular brightness.  The histogram graphs the image *before* it has been changed by the Levels tool -- this is why this part of the Levels tool is called the "Input".

Input sliders -- Whites
The slider on the right side sets the cap fro the whites in an image.  All of the brightness levels to the right of this slider are pushed to white, and the rest are made brighter, but the blackest black remains the same.  As you can see below, the image has gotten brighter.

If you push this slider too much, you will "blow out" the image -- causing so much of the highlights to become pure white that the image looks over-exposed.
Levels input white

Input sliders -- Blacks
Similarly, the slider on the left side sets the cap for the darks in an image.  All of the brightness levels to the left of this slider are pushed to black, and the rest are made darker, but the whitest white remains the same.  As you can see below, the image has gotten darker.

If you push this slider too much, you will seriosuly darken the image -- causing so much of the shadow areas  to become pure black that the image looks under-exposed.
Levels input black

Input sliders -- Gamma
The slider in the middle is one of the most useful.  It sets the "gamma" for the image.  Basically, it can make the image brighter or darker without changing what is the whitest white or darkest dark  in the two images below, you can see one gets brighter, and one gets darker.

The middle slider indicates which value will become 50% gray -- so as the slider moves into the lighter areas, the image gets darkers, because what was once bright gets moved darker towards gray.  Similarly, as the slider is moved into the darker areas, the image gets brighter, because those darker values are getting pushed brighter.  But for both, the lightest lights and darkest darks remain the same.
Levels gamma up

Levels gamma down

In general, you don't need to know what this is doing, but if you're a bit of a geek, this might be interesting to you!

Output devices each have their own dynamic range as well as gamma.  Dynamic range means how bright the whitest white and darkest dark can be.  For instance, for a printer, the whitest white is the white of the paper, the darkest dark is the color of the black ink when printed on the paper.  The gamma is the way the grays are spread out inbetween -- it is rarely uniform, and the way those grays are spread is called the gamma.  For instance, if a printer prints 600dpi [dots per inch], but a single black dot shows up as a little larger than 1/600th of an inch, then 50% gray will actually be a little darker than 1/2 black.  If you can determine exactly how much the printer is off by, you can adjust the gamma to correct for it, so 50% gray prints correctly at 50% black.  Because monitors each have their own gammas, and which can change based on the temp of the room, as well as be different at startup and after the monitor has wartmed up, etc, there was an old Mac utility to allow you to adjust the gamma for your monitor.  It displayed a square filled with 50% gray pixels, and next to it another square that was a checkerboard of pure white and pure black pixels.  You adjusted the gamma until both looked the same.

More geeky info: the quick answer is gamma is a "power curve" over the normalized pixel values.  In more plain speach, this means that if you converted the pixel brightnesses from 0-255 down to 0-1, you would then take each pixel value and raise it to the *power* of the gamma value, then convert that number back to 0-255.  eg 128 goes to 0.5 raised to the power 1.31 is 0.403 -- so the input of 0.403 goes to 0.50 and the image gets brighter.  You can see this on the histogram: the gamma slider signifies what brightness value will be middle-gray -- if it is moved to the darker areas, those vaules will be pushed more to middle-gray.  Similarly, 128 goes to 0.5 raised to the power of 0.59 above is 0.693 and the image gets darker -- the gray slider is moved into the lighter areas, which are pushed darker.


A combination of adjusting all three of these sliders can do drastic wonders for adjusting an image!  Levels is far more powerful than Brightness/Contrast.


Now let's talk about the slider at the bottom.

Output sliders -- Whites

The slider at the bottom describes what brightness values are *output* in the image.  If you slide the slider on the right, it designates what is the brightest white value -- pure white gets pushed to gray.  The rest of the iamge gets darker too.

Levels output white

Output sliders -- Darks

Similarly, if you slide the slider on the left, it designates what is the darkest black value -- pure black gets pushed to gray.  The rest of the iamge gets brighter too.

Levels output black


Now that we have a better understanding of Levels, let's compare it directly to Brightness/Contrast.

Turn Contrast UP

Compare these two images:

Contrast up
Brightness/Contrast

Levels contrast up
Levels

We can accomplish the same results as adjusting Contrast up by pushing the Levels Input sliders closer together.

Turn Contrast DOWN

Compare these two images:

Contrast down
Brightness/Contrast

Levels contrast down
Levels

This time, we get the same results as adjusting Contrast down by pushing the Levels Output sliders closer together.

Compare these four iamges:

Turn Brightness UP

Brightness up
Brightness/Contrast

Levels brightness up
Levels
Turn Brightness DOWN

Brightness down
Brightness/Contrast

Levels brightness down
Levels
As you can see, brightness is conrolled by the Output sliders.

Levels control per channel

Yet, levels has the advantage that it can manipulate each channel separately!

See the Levels sliders acting on the red channel:

Levels red gamma


Now, let's take a look at the Curves tool and compare it with the Levels tool.  We'll just walk through it, without comment.

Curves vs Levels Input White slider

Levels input white
Levels
Curves input white
Curves
Curves vs Levels Input Black slider

Levels input black
Levels
Curves input black
Curves
Curves vs Levels Output White slider

Levels output white
Levels
Curves output white
Curves

Curves vs Levels Output Black slider

Levels output black
Levels
Curves output black
Curves

Curves vs Levels Gamma up -- note that the middle control point moves *horizontally*

Levels gamma up
Levels
Curves gamma up
Curves
Curves vs Levels Gamma down

Levels gamma down
Levels
Curves gamma down
Curves
Curves vs Levels Red sliders

Like Levels, Curves can act upon each channel separately:

Levels red gamma
Levels
Curves red gamma
Curves

Now compare Curves to Brightness/Contrast

Brightness upLevels bright down
Brightness up
Curves brightness up
Curves




Brightness down
Brightness down

Curves brightness down
Curves




Contrast up
Contrast up

Curves contrast up
Curves

Contrast down
Brightness down

Curves contrast down
Curves