Understanding Colour 3 – Colour Correction

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Posted by mike on Tuesday, August 31, 2010 · Leave a Comment 

In the first post in this series we looked briefly at colour balance and colour temperature and how varying ambient light conditions can affect the way our camera records the colours of the scene.  This post is going to go into a little bit (ok, actually quite a lot..) more detail about these colour shifts and how to correct them.

At high altitudes the proportion of skylight to sunlight is much higher giving the image on the left a blue colour cast. This can be corrected by either adjusting the white balance or by using a warming correction filter.

The difference between daylight and skylight

Let’s start with outdoor light.  The sun emits a huge spectrum of electromagnetic radiation, most of which is absorbed by the Earth’s atmosphere.  The human eye is receptive to a band of the radiation that reaches earth, that with a wavelength of 400-750 nm, and our brains process it as light and colour.  Shortly outside the range of human vision are the ultra-violet and infra-red wavelengths and beyond those are x-rays, gamma rays,  micro waves and radio waves.  Camera film and digital sensors are generally designed to record the visible spectrum but can also be adapted to capture information beyond the limits of human vision – e.g. infra-red, x-ray.

As we already know, the light from the sun is pure white and contains every colour in the spectrum – i.e. it is made up of a combination of short blue and green wavelengths; longer red, orange and yellow wavelengths and everything in between.  As the light passes through the sky it hits tiny molecules of air which absorb and then redistribute the light in all directions.  The shorter blue wavelengths are most susceptible to this and so the sky appears blue and the remaining light from the sun appears somewhat yellow.  The blue light emitted from the air molecules is what we call skylight.  It is skylight that casts light into shadows on a clear sunny day and hence why shaded scenes have a blue colour cast – without this effect the shaded areas would be much darker as they are on the moon.

With the sun directly over-head the light from the sun has the least amount of air to pass through and is still white – this is the colour temperature at which all objects appear in their natural colour i.e. daylight.  The lower the sun sits in the sky the more blue light is absorbed by the air and so the redder the remaining light appears.  This is most exaggerated at sunrise and sunset when the light is mostly orange and red and the scattered skylight isn’t strong enough to reach us.

Blue light from the sun is scattered by air molecules in a process known as Rayleigh scattering. The further through the atmosphere the light travels the more blue light is lost along the way so the warmer the remaining light appears. Scattered blue light from the sky illuminates areas shaded from the sun.

Indoor lighting

Every light we use indoors can also be placed on the colour temperature scale.  Generally indoor lights are low colour temperature as traditionally they are powered by an incandescent source and heating them to a high enough temperature to produce blue light isn’t practical.  Modern fluorescent lighting and LED lighting can be made to produce higher colour temperatures but generally people prefer light that mimics sunlight and hence feels more natural.  One complication of artificial lighting is that the distribution of colour in the spectrum of light it emits can vary – the most common example being cheap fluorescent tubes which to daylight balanced film will appear to cast a sickly green light.  The scale on how full a spectrum a light source emits is known as the Colour Rendering Index, ranging from 0-100, the lower on the scale the more inconsistent the light and the more difficult it will be to obtain a correctly balanced image.

White Balance

Colour film is generally balanced to record around 5500K as neutral white.  Any colour temperature outside of this will require correction filters.  Tungsten film is balanced to 3200K and works better with indoor scenes but is fairly rare in these days of the digital darkroom.

Digital photographers have an easier time of it as the sensor in the camera is flexible and can be adjusted to the conditions you are working in.  Depending on how advanced your camera is there may be a range of automatic settings or a manual control to adjust the image yourself.  This only applies if you are shooting JPEGs though as shooting to RAW means you can adjust the white balance later in your editing software once you have the image up on your big colour-balanced monitor.  (I’m assuming you have colour balanced your monitor – if you haven’t then make it a priority before you work on any more images or you’ll forever have disappointing prints).

A colour meter like the Sekonic C-500 is the most accurate, but most expensive way to measure the balance of light hitting the subject.

White balance means the exact same thing as colour balance -  it refers to the neutrality of highlights, mid-tones and shadows.  Generally it accounts for colour temperature along with the balance of magenta and green, which can be used to correct most colour casts.  In camera colour meters aren’t always completely accurate – particularly if they are only able to choose from a list of default settings.  Where colour is vital to the work – perhaps for high ticket architectural shoots or video, there is no substitute for a dedicated colour meter.  These tend to be expensive though so generally we make do without.  What every photographer ought to carry though is an 18% grey card.  This is what it says on the tin – a small, neutral coloured card that reflects 18% of light.  For every location shoot, whether shooting RAW or JPEG, the first frame should include an 18% grey card somewhere near the main subject.  This can then be used as a neutral point of reference for ‘white’ balance and the settings adjusted accordingly for the rest of the images.  If the light changes during the shoot be sure to get another shot of the grey card.

Corrective filters for film

When working with film it’s even more important to obtain an accurate colour reading.  After scanning of course it can be edited digitally but due to the unique characteristics of film this is playing to it’s weakness.  If you are intending to bypass digital workflow and print straight from the transparency then there isn’t much room for mistakes.  A colour meter will produce a reading and specify which colour correction filters to use – there are a large number to choose from, depending on how accurate you want to be.  Of course a colour meter and a large range of filters are still an inconvenience to the pocket, both financially and physically, so most photographers make do with a few that they use most from the following range.

In addition to these a magenta filter – FL-D, can be used to adjust for fluorescent green tints.

Previous: Converting colour to black and white

Related posts:

1. Understanding Colour – RGB, CMYK and colour temperature explained
2. Understanding Colour 2 – Converting Colour to Black and White Tutorial
3. Top tips for Studio Photography – Using flash gels to colour the background
4. Top tips for studio photography – Using snoots, grids and barndoors
5. Getting Started in Studio Photography