What is Unevenness?
Lighting unevenness most often happens when you use hard lighting from an undiffused spotlight or flashgun, too close to the subject, so that elements of the picture nearest the light source are much brighter than elements farther away. When you double the distance of what is almost a point source of light, the illumination at your subject drops to one-quarter the brightness.
This means that if you have a still life setup 1 metre wide and then side-light it from a position 1 metre away the illumination across your subject will be four times brighter at one side than the other. For an example of totally even lighting see the section on copying where two lights of identical intensity are placed at equal distance on opposite sides of the subject.
If you want to avoid unevenness (with minimal effect on lighting quality) just pull the
light source back farther, in a direct line from the set. At 2 metres the variation across the set becomes one-and-a-quarter stops, and at 3 metres only two thirds of a stop.
Alternatively, add an additional light source, diffuse the light, narrow the set, or help yourself by positioning the darkest, least reflective objects nearest the light source.
Most light sources used for photography produce so-called ‘white’ light, a mixture of all colours. They are said to have a continuous spectrum, although its precise mixture may vary considerably from an ordinary domestic light bulb, which is rich in red and yellow but weak in blue, to electronic flash containing relatively more energy in blue wavelengths than red.
shows, most sources can be given a ‘colour temperature’; the higher the kelvin (K) value, the bluer the light.
When shooting colour, especially colour slides, you must be careful to match the colour temperature of your lighting to your film. Normally this is 5500 K (for ‘daylight’ film) or 3200 K (‘tungsten light’ film). Alternatively you can use a tinted correction filter to bring light source and film into alignment, such as an 85 B or 80 A.
If all your subject lighting is the same colour temperature, the adjusting filter can be used over your camera lens. If it is mixed – daylight and studio lamps, for example – and you want even,
consistent colour you can place a filter over one of these sources to make it match the other, as well as the film. Some light sources, such as sodium street lights and lasers for
example, do not produce a full range of wavelengths and so cannot be filtered to give a white light result.