Using Unsharp Mask Filter to Increase Contrast

photoshop, usm and contrastThis article will demonstrate three techniques for increasing contrast in a picture. The tool we are going to use for this is Photoshop.

Ok, so you took a picture and now it look dull and flat, the colors are dead, or it has a milky look to it. You know what it is. The picture lacks contrast.
Let’s take a look at this picture I took – “role model”.

photoshop - using unsharp mask (usm) for contrast - original

As you can see, this picture definitely needs some contrast boost. The first thing you can try is the auto level tool. It is like “contrast for lazy-man”. Sometimes it ill give you good results, and sometimes not so good. Plus you have no control over what Photoshop’s auto levels do for you. Let’s look at the result of Photoshop’s auto levels command (CTRL+SHIFT+L) for the “role model” picture.

photoshop - using unsharp mask (usm) for contrast - auto levels

Well, the result is slightly better, but still the colors are not popping. And another thing, here is the original picture, with the picture’s histogram on the bottom right corner, see how the histogram shows values in each of the shades.

photoshop - using unsharp mask (usm) for contrast - original + histogram

Now, let’s look again at the histogram of the picture after we applied auto levels. See how some shades have dropped. This is bad, the histogram is now unbalanced.

Let’s review another method to increase contrast – Photoshop’s curves tool. I applied a new “S” curve to the picture using Photoshop’s curves tool (CTRL+M). The result is much more pleasing to the eye. Here is the curve I used.

photoshop - using unsharp mask (usm) for contrast - curves

And here is the “role model” picture after I applied a new curve.

photoshop - using unsharp mask (usm) for contrast - curves

Better, right? But look at the histogram, now it is really has some really bad holes now. This why I recommend a third way of increasing contrast. Hopefully we can both get better contrast and leave the histogram ok. Try this: bring up Photoshop’s unsharp mask dialog (filter->sharpen->unsharp mask). Now select a very high radius and a small to medium amount. Selecting such values will only “sharpen” your picture as a side affect. The main effect will be increased contrast. For the “role model” picture I used 50% and 161 pixels.

photoshop - using unsharp mask (usm) for contrast - usm values

Now let’s look at the picture. (move the nouse over the image to see before and after)

photoshop - using unsharp mask (usm) for contrast - after usm

The colors are quite contrasty. The histogram is ok. Looks like we have a winner. This method is great for removing the milky curtain under exposed pictures sometime have. This is why this method is also knows as unveiling the curtain.

To sum up:
If an image lacks contrast you can use several ways to regain it using Photoshop:

  1. Use auto levels – you get no control and a comb-like histogram
  2. Use the curves tool – you get more control, but the histogram is really missing out and really looks like a comb.
  3. Use unsharp mask creatively and gave both contrast and good histogram.