Wednesday, 5 June 2013

Exercise: Judging colour temperature 1

This section looks at colour temperature and being able to recognise colour changes throughout the day.  It would be very easy to get caught up in the science, but for practical purposes, ie to help me appreciate the colour of different light sources and their impact on the image, all one really needs to know are some of the basics.

First of all, I would like to illustrate this with a single image.  This I took in the City of London in late afternoon a couple of weeks ago and I felt it really illustrates the difference between colour temperature in the shade and in bright sunlight.
ISO 400, f/6.7, 1/45 sec
The differences between shade and sunlight are very marked, considering that the stonework is a consistent colour throughout.  I used auto WB for this shot as I wanted both the sunlight and shade to be treated equally.  Here the stonework which is in the shade has a blue colour and this is very much attributed to the almost cloudless blue sky, which I can see in my images taken of other buildings adjacent to this at the same time.  The warm colour of the sunlight is really shown up in this image.

One of the benefits of shooting in RAW is that all information available at the time of recording the image is preserved and not altered in any way, so that it is then possible to alter the white balance of a photograph if the "as shot" does not look right.
The original was shown in Lightroom as being shot at 4900 Kelvin (K).  I then altered this in Lightroom, creating 3 further images, and these are shown above with the following settings, left to right:

  • Daylight, 5500 K;
  • Cloudy, 6500 K; and
  • Shade, 7500 K
To me, the most pleasing colour balance is that described as shade, where the "cold" blueness of the stonework is "warmed" and there is less of a contrast between that and the sunlight flooding in at the back.

There are very many charts and other diagrams available in books and on the Internet, in particular from light bulb manufacturers, so I am not reproducing any of these here.  A summary of the science is that the temperature, expressed in degrees Kelvin (K), is recorded for the colour that an object would glow at if heated to that temperature.  An objected heated to give a red to orange glow would be at approximately 1,000 to 3,000 K.  Heating the object further until it emits a white glow, will record a temperature of 5,500 K.  Applying further heat, the object will take on an increasing blue colour, in the range of 6,000 K to 10,000 K.

In photographic terms, therefore I would expect the following:
  • Midday sun is generally white with a temperature of 5,500 K as all the wavelengths making up light can be seen;
  • Blue sky goes up to around 10,000 K
  • Shade under a clear sky will reflect a large amount of blue and therefore will be between 7,000 and 8,000 K;
  • Cloudy sky will be around 6,500 K;
  • A fine morning or evening sun will be around 3,500 K; and
  • Sunrise / sunset will be around 2,500K
Particularly in the morning and evening, at sunrise and sunset, atmospheric conditions will affect the colour and give a tint, due to diffraction of particles causing the short blue wavelengths to scatter, leaving the longer wavelengths.

Moving on to the exercise itself, the objective was to photograph a neutral coloured object in full sun in the middle of the day, in shadow at the same time and then towards the end of the day.  All need to be shot using the daylight WB setting.  

This is where I noted differences between the interpretation of settings in camera and those generally accepted and seen in Lightroom.  For the Canon EOS 5D II the following settings are given:
  • Auto - 3000-7000;
  • Daylight - 5200;
  • Cloudy - 6000; and
  • Shade - 7000
Left to Right:  Sun, Shade, early Evening.
My garden does not see much sun in the late evening, so I chose to take the last image in early evening, where the light was already tending towards yellow / orange.  Not the best of sets, but it does illustrate the point, with the shade being a bluer colour than sunlight.  

One of the learning points which I have noted during this and the following exercises, is that the eye sees colour differently to that which is recorded on the sensor, so the blueness in shade, is not always apparent.  Knowing what to expect in different situations then enables me to "see" these colours and make use of them compositionally.

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