Contrast Part 8: Levels & Curves – Refining Adjustments

The same post-processing adjustment is rarely needed in equal-measure over 100% of an image. Ways to apply adjustments selectively are needed. This post looks at ways to apply the levels portion of the Levels & Curves tool in a selective manner (curves in the next post).

 Part 8 of a series

“All About Image Contrast for Photographers”

THE SAME IMAGE ADJUSTMENTS OFTEN ARE NOT NEEDED IN

Every part of an image

This post examines

Ways to selectively apply tonal adjustments

Using the Levels portion of Levels & Curves

(In general, this will also apply to Curves – a future post)

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Levels Tonal Adjustment

<<Before | After>>

Adjustments made at the channel level; read on


The What – Application of Levels’ adjustments to

All three color channels, R/G/B, at once or

Individual color channels alone or

Just the Luminosity channel or

Any & all combinations of the above

________________________________________________________

The Why – Global  tonal adjustments aren’t always desirable

Color casts or shifts may be an issue

A pre-existing cast you wish to remove

Saturation shifts may occur

When tonal adjustments are applied across the board


The How

Use the L&C Channel selection menu to choose among

RGB (global)

R or G or B individually

Luminosity (hue & saturation remain unchanged)

—–

Black point shift applied to RGB

Click any of the following images to enlarge

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Black point shift applied to red channel

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Black point shift applied to green channel

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Black point shift applied to blue channel

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Black point shift applied to RGB (left)

and to luminosity channel (right)

Note the color change with RGB

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Mid-tone shift applied to RGB

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Mid-tone shift applied to luminosity channel

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Mid-tone shift applied to red channel

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Mid-tone shift applied to green channel

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Mid-tone shift applied to blue channel

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Shift of all three sliders applied to luminosity channel

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Shift of all three sliders applied to RGB channel

There’s that color shift in the daisy’s center

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The Luminosity channel is better than RGB in many cases

This is because applying tonal adjustments to

All three channels (RGB) simultaneously

Can cause unwanted color changes/shifts

Changing only Luminosity

Leaves hue and saturation unchanged


A practical example – removing a color cast

There are many ways to remove a color cast

Levels may not be the best (depends)

Here’s a Before image with a green cast (left) and

After – using red & green channel adjustments

Increase red & decrease green via mid-tone slider

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The same can be done without

Using the green channel (see next image)

This red/blue adjust (ignoring green altogether)

Is in line with how

Many “built-in” White Balance tools work

They set WB by adjusting ONLY Red & Blue

Green is reduced by raising both

vice versa to increase

Red cast removal = less red & more blue

Blue cast removal = you can guess….

Changing (adjusting) colors on a per channel basis

Requires familiarity with how colors mix

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