Color Temperature And Color Correction In Photography.pdf

Color Temperature and Color Correction in Photography

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Color Temperature and Color

Correction in Photography

CYBAEA : photo : Color Temperature and Color Correction

Note: this page is still under development. I have put it up here

early in the hope that (1) it may be useful even in it's current state

and (2) I may receive some early feedback that will improve the final version.

The

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Colour

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In this document we discuss color temperature and color correction as it applies to photography.

We are concerned with taking pictures - typically transparencies - rather than the development and

printing of photographs.

If you have taken photographs under a variety of lighting conditions you will have noticed how the

resulting images show a distinct color cast depending on the light source. In the image at the top

of this document you will notice the blue-white color from the fluorescent tubes and the warm

yellow-orange color from the tungsten lights over the fish.

Color temperature is the main way in which we measure the different colors and color correction is

about filtration and other techniques that we as photographers use to achieve a desired color

effect. (The desired effect may be a neutral "daylight" color, or any other effect, e.g. a slight

warm-up effect for portraits.)

This document should be considered an introduction to the subject. I have included some links to

additional material, but there is no substitution for going out there and trying for yourself. So read

on and then pack your camera bag and try it out!

The Theory

Learn everything, and you will see afterward that nothing is useless.

— Hugo of Saint-Victor (12th century): On the Sacraments of the Christian

Faith

Digital

Photographer's

Handbook

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Color Temperature and Color Correction in Photography

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This section may seem boring or unnecessary to some. After all, all the filters are labeled with their

effect and the manufacturers publish long lists of which filter to use when. Surely you can just give

us the techniques - after all, we do not need to know the theory behind the combustion engine in

order to drive a car?

Call me old-fashioned if you like, but I think it is useful to have at least some understanding of the

basic concepts. There are several reasons for this.

Firstly, I firmly believe that you remember things you understand better than tables and rules you

have just learned by heart. Secondly, no table can cover all situations. An understanding of the

principles will help you in new situations. And thirdly: it's interesting! So let's get on with it.

Color Balance

Film sensitivity: one, two,

three, and four layers

This side bar contains a short

history on the spectral sensitivity

of films.

In the beginning there was black

and white film. The early films

were sensitive to only to blue

light (and to some extent to

invisible ultra-violet light). With

this type of film you obtain white

skies and black lips. Apparently

people got used to this and

considered these effects quite

natural!

The next development was

orthochromatic films. These

extended the sensitivity of the

earlier films into the green

spectral area by adding another

set of dyes. The benefits included

a more natural tonal rendering of

subjects and faster films.

Most black and white film that

you can buy in the shops these

days is panchromatic which

means it is sensitive to all colors.

If you ever wondered why so

many black and white films have

the word "pan" in their name

(Kodak Tech Pan, Ilford Pan F,

Fuji Neopan, ...) then you now

know the reason: it was originally

to distinguish these films from

the earlier orthochromatic films

and the name stuck. Infrared film

is also available, but we will not

discuss it here.

Then color films arrived. Most

color films have three layers with

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Color Temperature and Color Correction in Photography

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dyes to record each of the three

primary colors: blue, green, and

red. Since all photographic dyes

are sensitive to blue light to

some extent, the blue-sensitive

layer is usually on top followed

by a yellow filter layer to prevent

the blue light affecting the lower,

green - and red -sensitive, layers.

This layered structure is found in

all modern color films, with some

variations.

Three-layer color film work well

in many situations and, with

suitable filtration as discussed in

the main article, can give

excellent results under most

lighting conditions. However,

fluorescent light is becoming

increasingly widespread and this

particular light gives a

characteristic green color cast to

un-filtered photographs. For this

reason, and for other difficult

situations, Fuji has developed

print (color negative) films with

an additional fourth layer. This is

a magenta layer and it does an

excellent job of correcting this

color cast without affecting the

overall color balance. Originally

introduced in the Reala films it is

sometimes known as "Reala

Technology" but it is now being

made available in a much wider

range of negative films, including

the consumer Superia range and

the professional portrait films

(NPS).

This concludes this side bar

discussion.

With a gross over-simplification - but one that is useful for this discussion - we can assume that

film is sensitive to three colors. These roughly correspond to red, green, and blue - the traditional

primary colors.

In this model, which is similar to what is used in computer graphics if that helps some of my

readers with the following discussion, each point in the image can be described with three values.

These could be chosen to be the percentage intensity of the colors red, green, and blue, relative to

their maximum values for the particular film. This is completely analogous to the RGB color space

in computer graphics.

Three values describe the image at any given point, but only two values are required to describe

the color balance. Think of it this way: the overall intensity doesn't matter; if it is dark blue or light

blue it is still blue. If you mix 25% of each of red, green, and blue you get a neutral gray color. If

you mix 50% intensity you still get neutral gray, albeit a slightly lighter gray.

In the table below cells in the same row have the same color balance, only the intensity changes.

All the colors in the first row are red, and red only with no trace of blue or green. (This may not be

exactly true, depending on your monitor.)

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Color Temperature and Color Correction in Photography

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We are of course free to choose any two (different) values to measure the color balance.

In photography it is traditional to choose as the two variables the ratio of blue to red and the ration

of green to the overall intensity. These correspond to the traditional light-balancing filters (80, 81,

82, and 85 series filters), and green and magenta filters (CC-G and CC-M). More about this below.

Color Balance Variables

Light Balance (LB), or

Color Temperature

The ratio of the intensities of blue to red

Green-Magenta Balance The relative amount of green

Color Temperature

Optics

Eugene Hecht

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Color temperature is a term that is borrowed from physics. In physics we learn that a so called

"black body" will radiate light when it is heated. The spectrum of this light, and therefore its color,

depends on the temperature of the body. You probably know this effect from everyday life: if you

heat an iron bar, say, it will eventually start to glow dark red ( "red hot" ). Continue to heat it and it

turns yellow (like the filament in a light-bulb) and eventually blue-white.

Be careful with the terminology here! The hotter the body gets (measured as the temperature in

degrees Kelvin) the more the color moves from red to wards blue. But we say that red is a

"warmer" color than blue! So a warm body radiates a cold color and a (comparatively) cold body

radiates warm colors . I know, it's confusing...

The photographic color temperature is not the same as the color temperature defined in physics or

colorimetry. As mentioned above, the photographic color temperature is measured only on the

relative intensity of blue to red. However, we borrow the basic measurement scale from physics

and we will measure the photographic color temperature in degrees Kelvin (K).

The following table should give you some feeling for the scale. These are of course not exact

numbers but rather typical values. The list is not a substitute for a color meter .

Temperature Typical Sources

1000K Candles; oil lamps

2000K Very early sunrise; low effect tungsten lamps

2500K Household light bulbs

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Color Temperature and Color Correction in Photography

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3000K Studio lights, photo floods

4000K Clear flashbulbs

5000K Typical daylight; electronic flash

5500K The sun at noon near Kodak's offices :-)

6000K Bright sunshine with clear sky

7000K Slightly overcast sky

8000K Hazy sky

9000K Open shade on clear day

10,000K Heavily overcast sky

11,000K Sunless blue skies

20,000+K Open shade in mountains on a really clear day

This means that you will find photographers talking about "daylight balanced" film (nominally

5500K) and type A and B tungsten balanced films (3400K and 3200K). This gives the color of the

light: below we will define a measure of how much a filter moves the color temperature (the mired

shift).

For now remember that the color temperature is only half the story, albeit often the most

important part of the story.

Filters

Now that we understand the two variables that define color balance, we need to tie this in to filters

and define a way to express the effect and strength of a filter.

Light Balancing Filters

Filter guide for color and

black & w...

Used $19.00!

Light balancing filters are used to change the color

temperature of light. If you place a light balancing filter

in front of your lens, the overall temperature of the

scene will be changed. These filters are sometimes

called conversion filters because they may be used to

"convert" daylight balanced film to use in tungsten light

or tungsten films to use in daylight.

The filters are traditionally labeled in the Kodak Wratten

system (I have no idea why they choose these names),

and a typical selection is listed in the table below.

Blue filters

Amber filters

Filter Exposure

increase

Conversion Mired Filter Exposure

increase

Conversion Mired

80A

3200K to 5500K -131 81

1/3 3300K to 3200K +9

80B 1 1/3 3400K to 5500K -112 81A

1/3 3400K to 3200K +18

80C

3800K to 5500K -81 81B

1/3 3500K to 3200K +27

80D

2/3 4200K to 5500K -56 81C

1/3 3600K to 3200K +35

82C

2/3 2800K to 3200K -45 81D

1/3 3700K to 3200K +42

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