PHOTOSHOP TIPS FOR AVIATION PHOTOGRAPHERS - UNDERSTANDING COLOUR

Primary and secondary colours

I would like to begin this section on image manipulation with a few details about colour. In photography the three primary colours are Red, Green and Blue which when mixed together in equal quantities produce white. Various combinations of these primary colours produces the three secondary colours Cyan, Yellow and Magenta which when mixed together in equal quantities produce black.

 

 

 

 

 

 

 

 

The diagram above illustrates the following:

  • Red = Magenta + Yellow

  • Green = Yellow + Cyan

  • Blue = Magenta + Cyan

  • Cyan = Blue + Green

  • Yellow = Red + Green

  • Magenta = Red + Blue

 

The diagram above also tells us which colours are opposites:

  • Red and Cyan

  • Green and Magenta

  • Blue and yellow

 

All of this information is important when it comes to removing a colour cast from your image.

 

Colour casts

To remove a colour cast that is composed of a primary colour from a digital (positive) image we simply subtract the offending primary colour, which in effect is the same as adding equal amounts of the colours that make up its opposite:

 

PRIMARY COLOUR CAST

COLOUR TO SUBTRACT

EQUIVALENT TO ADDING

Red

Red

Cyan (Blue + Green)

Green

Green

Magenta (Red + Blue)

Blue

Blue

Yellow (Red + Green)

 

To remove a colour cast that is composed of a secondary colour we simply add the opposite colour, which is effectively the same as subtracting equal amounts of the colours that make up the cast:

 

SECONDARY COLOUR CAST

COLOUR TO ADD

EQUIVALENT TO SUBTRACTING

Cyan

Red

Cyan (Blue + Green)

Yellow

Blue

Yellow (Red + Green)

Magenta

Green

Magenta (Red + Blue)

 

Colour temperature

Colour has a temperature, measured in Kelvin (K), with red being the coldest and blue being the hottest. The table below lists some of the colour temperatures of various light sources and weather conditions/times of day:

 

LIGHT SOURCE

COLOUR TEMPERATURE

Candle

2,000K

Sunrise/sunset

3,000K

Early a.m/late p.m

3,500K

Noon daylight

5,500K

Electronic flash

5,500K

Overcast

6,000 - 7,000K

Open shade under blue sky

7,500K

Clear blue sky

10,000 - 15,000K

 

 

Colour temperature is important because it determines which colour cast will be present for any given White Balance (WB) setting. Take a WB setting of 5,500K for example, the temperature to which standard photographic film is balanced, and refer to the colour chart above. If you shoot under noon sun (5,500K) then no colour cast should be present. If however you then shoot indoors in a room lit by domestic 100W tungsten bulbs (2,500-3,000K) then an orange colour cast will be present. If you then shoot outside again, this time under cloudy conditions (6,000-7,000K), then a blue colour cast will be present. In simple terms, if you shoot under conditions that have a colour temperature lower than your WB setting than you will get a red/orange colour cast. Conversely, if you shoot under conditions of a higher colour temperature than your WB setting then you will get a blue colour cast. Modern digital cameras allow you to manually set the WB to a specific temperature, a feature that is particularly useful if you have a separate colour temperature meter.

 

 

 

 

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