Photoshop tips for aviation photographers

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.