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The colour of the light source measured in Kelvin (K). Most colour films are balanced for 5500K which is the colour temperature for average daylight conditions. Lower values produce a yellow/orange cast, higher colour temperatures produce a blue cast.
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Artificial light appears in a variety of forms - tungsten and fluorescent being two of the most widely used. Each type of lighting produces a different colour temperature that our brain compensates for to make everything appear as though it's neutral light. Digital cameras and film are not so forgiving and record the colour as it really is, so in tungsten light the picture comes out orange/yellow and fluorescent goes green. These colour casts can be corrected using filters on a film based camera, and digital cameras have a white balance setting to make the pictures look like the view our eyes see. Some models have manual white balance control where you select the type of lighting from a list, but most take care of the colour automatically.
Colour film balanced for subjects lit with a colour temperature source of 5500K.
A measure of the colour of light whos values are found by taking one million and dividing it by the colour temperature in kelvins. A 5500k light source, for example, has a mired value of 182 (1,000,000/5500). When you know the mired values it's easier to calculate which filter is needed to balance one type of film with different lighting.
Measurement of colour temperature named after the scientist Lord Kelvin.
A bright tungsten bulb with a colour temperature of around 3400K that is sometimes used in portrait studios. Most studio photographers prefer the convenience of electronic flash.
A filter which is used in front of the lens in order to filter out UV light that can cause a blueish haze. UV filters have much the same purpose, but a skylight filter is also coloured slightly pink (or yellow) in order to give pictures a warmer appearance. In the digital age these filters are used less than before, since colour temperature can be influenced in-camera and in processing, especially when using the RAW file format.
A development by Dr. Mike Ware of Kallitype and Van Dyke Brown, Argyrotype produces brown silver prints of subtle graduation on plain artist's paper. The sensitiser, which can last a year or more if correctly stored, should be spread evenly on the print using a glass rod. After a few minutes to allow it to soak in, the print should then be dried for ten minutes at 40C or for half an hour at least at room temperature. A contact printing time of 5-10 minutes under the sun or a good source of UV light is normal. After exposure, develop and clear the print in gently running water for around 5 minutes, and then use a fixing solution for 3 minutes. At this point the colour will intensify, changing from red to a deep brown, which can be turned more blackish-brown by ironing dry at this stage.