Contrast Part 3: ………………… Measuring & Describing Color

In order to work with and control color contrast we should know how color is measured and described.

Part 3 of an 7-part series

“All About Image Contrast for Photographers”.


Measuring & Describing Color

“That rose is red” is a subjective statement

To actually use & work with color, we need to do better


Many physiological factors influence color perception

These factors keep us from

Making objective verbal descriptions

Further, everyone

Verbally describes a color differently

What is needed is an objective color standard

Fortunately, such standards exist

(as well as devices to perform the measurements)

 

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There’s red – and another red, and yet another….

Saying that this rose is red is correct – as far as it goes

(which isn’t very far)


Color Models  (Standards)

A means of representing colors by a set of numbers

Typically as three or four values or color components

In the illustration, figure 1 below, the RGB model

Represents the circled color by the three numbers shown

Every color (within a practical range)

Can be represented by a similar set of three numbers

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Figure 1

This series on Image Contrast for Photographers

Will only consider the two models shown above

(with emphasis on HSL)

If you’re interested in a broader discussion

Go here

The HSL & RGB models are the most familiar & useful to photographers

Most post-processing software has one or both available

For purposes of tonal and color contrast measurements

HSL is the more useful of the two

Lightness is one of its parameters

Thus tonal contrast is more straight forward

(but not perfect, see L*a*b below)


I opened Figure 1 in Photoshop Elements

and – Opened the PSE Color Picker (see figure 2)

I entered Figure 1’s RGB values (must convert to a 0-255 scale)

and – You can see the color picker’s rendition of the color

Stating that the color is R=51, G=153, B=205

Is more precise than saying it’s “sort of blue-green”

(but not as descriptive 😉 )

You can see that PSE provided the corresponding HSL values

H=200, S=75, B=80 (done automatically)

Click for full size

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Figure 2

Open PS, LR, or whatever you use

Find its color measuring|setting tool (color picker in PSE)

Convince yourself that these are the values for B, W, R, G, B

…….RGB………………..HSL

B…..0/0/0……………*/*/0

W…..255/255/255…..*/0/100

R…..255/0/0…..360/100/100

G…..0/255/0…..120/100/100

B…..0/0/255…..240/100/100

NOTE – an asterisk (*) above means any number will work

– The HSL hue for red is 360 (or equally valid, zero)

G is 120 & B is 240


You can see from the above that even the HSL model (as shown)

Does not give us the entire “tonality” story

i.e., the tone you’d see on a histogram’s luminosity channel

That level of tonality is embedded in two HSL values

Both S & L

Tones get brighter with decreasing S & increasing L

Pure white (histogram’s right side) is when

S = 0 & L =100 (regardless of H’s value)

Important note!!

The model shown as HSL is more correctly known as HSV

(V for value; an art term denoting lightness)

In PS it’s labeled HSL which is unfortunate

Since there actually is a HSL model which is

Very similar but different enough to confuse you

I’ve chosen to go with the PS nomenclature shown in HSL above since that’s what most users will encounter

I’m sorry for the confusion this causes

Blame Adobe

There’s yet another model, L*a*b, where

L is the same tonality as shown in a luminosity channel

Unfortunately, apart from full PS, it’s not a common feature of most post-processing programs

More here


Enough on color

Tomorrow –  tonal contrast


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