Posted by: David Vernon | May 26, 2009

Photoshop Friend #1 – Levels

Wind Turbines 03A lot of the tidbits we pass on to the Intro to Digital Photography class at the Peoria Art Guild come in lists. You know – “Seven Tips for Doing This” and “Ten Ideas to Make That Better.”

The level of post-production we do in the Intro class varies. A lot of folks are new to this level of photography – they’re shooting JPEG and they’re not ready for a ton of post-production (or they’re doing it in camera). So we keep our Photoshop list pretty simple – we’re just going to call it “Three Important Things to do in Photoshop.”

So this week, I’m gonna dig deep and come up with three blog entries on what I think are the three most important things you can do in Photoshop if you want to improve a photo. These are basic techniques that fix tonal range, color and sharpness. There is no magic here – these are some of the most elementary things one should master in any version of Photoshop – and any of them can do a lot of good work to fix an image.

Let’s cover some basics first…

EIGHT-BIT SCALE

Each pixel in an image is essentially represented by four different numbers. All four of these numbers exist on what is called an “eight-bit” scale. An eight-bit scale has exactly 256 possible values. How do we get to 256? Because the eight-bit scale is derived from taking 2^^8 = 256. We actually start at zero (meaning an absence of that value) and stop at 255.

The first three numbers are the values of the RED (R), GREEN (G), and BLUE (B) – the three primary colors of light that make up ALL other colors. So R=255, G=0, B=0 is pure RED. If you mix them up, you can have 256 shades of red, 256 shades of green, and 256 shades of blue – for a grand total of (256x256x256) 16,777,216 colors in an eight-bit system. There are a couple special colors: R=0, G=0, B=0 is PURE BLACK. R=255, G=255, B=255 is PURE WHITE. But mix these three values and you can create just about any color you’ll ever need.

Now let’s get to the fourth number. If the first three numbers represent the final color of the pixel you can use the fourth number to represent the luminosity of the pixel on a greyscale. Again, zero represents pure black and 255 represents pure white. Now however, 127 – right in the middle – represents middle grey. We’re going to let this number scale represent something we call TONAL RANGE.

TONAL RANGE

One superb way to understand tonal range is too look at an image that has limited tonal range.

Levels01

We’re going to start on the amazingly beautiful island of Inisheer, the smallest and easternmost of the Aran Islands, off the southwest coast of Ireland. I spent a few hours on the island on a stunningly overcast day a few years ago and a lot of my images that day were just that – limited – in tonal range. The overall image is almost muddy. That’s because there is NOTHING in the image that is pure white and there is NOTHING in the image that is pure black. That’s how you define limited tonal range. Instead of running from zero to 255, the darkest pixel in this image is actually 15, and the brightest pixel is in reality 206. We can see that in a histogram representing the tonal range of the image:

Levels02

Luminosity Histogram

Sure enough, there are gaps – a small one on the left side and a larger one on the right side. The histogram again represents a count of the number of pixels in the image that have a certain luminosity value. Since there are no values between zero and 15, there is no data there. The big spike on the left side – those values are in the mid-50s. This just means there are A LOT of pixels in the image that have those luminosity values. And again, once we get past 206, there is no data again.

LEVELS ADJUSMENT

So let’s get to our goal today. One of the easiest ways to improve your image is to do a Levels Adjustment. And the purpose of the Levels adjustment? It’s simply a way to “stretch” your tonal range – so that the brightest thing in your image gets stretched out to pure white (but probably no more) and the darkest thing in your image gets stretched out to pure black (but again probably no more). When you stretch the ends, every other pixel in the image gets stretched along with those extremes and the resulting image is much more contrasty and punchy. And it’s certainly less muddy.

Let’s look at the Levels Adjustment Panel for this image:

Levels03

Levels Adjustment Panel

At a high level, this is simply a mapping function – you are mapping the Input Levels (the actual luminosity in the image) to the Output Levels (the desired luminosity in the image). The basic flow is very easy – and depends on the three sliders / numbers beneath the Input Levels (for this lesson we’ll totally leave the Output Levels and Eyedroppers alone). The leftmost slider controls the shadows (the pixels with low luminosity). The rightmost slider controls the highlights (the pixels with the high luminosity). We’ll get to the middle slider – which controls the midtones – in a moment.

Let’s do it. We’ll grab the left slider and simply slide it right – to the point where the histogram first has shadow values. We’ll then grab the right slider and slide it left – to the point where the histogram first has highlight values. It now looks like this:

Levels04

Levels Mapping

The left (black point) slider is now mapped to zero (so what was 15 now becomes zero) and the right (white point) slider is now mapped to 255 (so what was 206 now becomes 255). And just that easily we’ve expanded the tonal range of the image. Take a look at the before and after:

Levels01

Before

Levels05

After

This has clearly punched the picture up. But we can take it one step further. We still have the middle (midtone point) slider. The midtone slider compresses or stretches the tones in the direction it’s moved (also referred to as adjusting the gamma). Move it left and the histogram is compressed beneath that point and stretched above it. This compresses the shadows and stretches the highlights, making the picture brighter. If you go the other way, and move it right, the picture goes a little darker. The end result is the ability to brighten or darken the midtones. Let’s brighten this up a little more:

Additional Midtone Adjustment

Additional Midtone Adjustment

Total time spent – about 15 seconds – and we’ve taken a muddy picture and got the ball rolling. Setting your end points is relatively easy – you just want to meet the histogram. If you go much beyond the ends of the original histogram, you will clip pixels. Go beyond the shadow end and you’ll lose detail in the shadows. Go beyond the highlight end and the same result will occur – details in the highlights will disappear. It’s usually enough just to get close to the ends. Adjusting the midtones – to me – is more by feel – and what you like.

And if you open that histogram, and you already have pixels at the ends, you may not need a levels adjustment – but it’s still an option if you’re willing to part with some detail. Good luck!

Tomorrow: Removing Color Cast with Curves – do not fear the Curves!

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Responses

  1. It never hurts to read a post even if you think you already know about it. Case in point… this article! I knew about levels and use/adjust them all the time but I was unaware of the relationship between the input and output levels.

    While you don’t cover it is detail I now know why adjusting the output levels modifies the contrast. Cool!!

  2. […] Last week I was all set to write two more Photoshop articles – to complement our lessons on levels, color correction and sharpening from last year. And then I come to find out that our own Bill […]

  3. […] Photoshop Friend #1 – Levels […]


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