Contrast Part 6: Levels and Curves, 101

Part 6 of a continuing new blog series

“All About Image Contrast for Photographers”

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Levels and Curves – The Basics

No other contrast adjustment tool is its equal

We have lots of post-processing programs to choose from

and they all offer different contrast adjustment features

.

HOWEVER

Level and Curves is the common denominator

It is the one contrast adjustment tool that

All good post-processing programs share in common

A perfect place to begin our journey

(and it doesn’t hurt that it’s the most powerful, also)

D800E_130514_094430__DSC8652 acr cep-1

Calla Lily

(iPad camera & Snapseed)

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Today’s post is only about L&C basic principles

Apologies to those for whom it’s way too basic

I’ve found over & over that

Assuming everyone knows something or another

Is a bad assumption when explaining anything

Go grab a cup of coffee & stop back tomorrow 😉

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I’ll use the Levels & Curves from Color Efex Pro

Other programs’ L&C are similar

All operate on the same basic principles

In a later post, I’ll examine a few program specific implementations

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If you were to  open L&C in Color Efex Pro you’d see this

We’re interested in the contents of the red rectangle

6-27-2013 8-28-00 AM

Fig. 1

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Begin by ignoring everything except the numbers in red

It’s essential to understand what they tell us

In the simplest sense when you see

0 – think black

255 – think white

A number in between – think shades of gray

6-27-2013 8-46-31 01

Fig. 2

Those numbers are showing you tonal levels

In comparing their meaning to that of the more familiar histogram

The axis labeled INPUT is identical in meaning to the histogram’s

It’s the tonal levels of the input image’s pixels

The axis labeled OUTPUT is totally different

A histogram Y-axis measures pixel count

How many input pixels have a tonal value of 0,

How many 2, 3, 4, …., 255?

In the L&C display the Y-axis shows us the

Output tonal level for

Pixels with a given input tonal level

As an example, in the default L&C, shown in Fig 2.

Output tonal levels are identical to input levels

That is – the image is unchanged

128 in gives us 128 out (middle gray)

0 in, 0 out; 255 in, 255 out

———–

Base on the above, let’s see how things change if we

Change L&C’s default settings

6-27-2013 10-17-15 AM

Fig. 3

By the numbers –

1. Change the dark-side from

0 In & 0 Out to

0 In & approximately 50 Out

This means a black tonal input level (0) is now

Gray with a tonal level of 50 when it’s output

Further, the new “curve”, the diagonal white line,

Differs  from the default (red)

Everywhere (except at 255, 255)

This says that every input tonal level

Is now lighter when it is output

2. The above changes are reflected in the histogram

Note how far the darkest area has shifted

Further, the entire histogram is compressed

RECALL

The wider the histogram, the greater the contrast

This simple change greatly reduced the image’s contrast

3. Shows the visual effect of 1 & 2

—–

The above tells how I made this before|after shown yesterday

Raised the 0 end as shown AND dropped the 255 end

6-26-2013 9-10-43 AM

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500 word cut off – see you tomorrow 😉

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