Editing Techniques

Please see techniques from the content list on the left.

About the Light

In digital imaging it is mandatory to have high quality lighting. Viewing prints and slides under the light from typical luminaries that are found at home and at the office completely strips accuracy from the workflow. Photographic lighting has to have smooth, continuous spectral content that simulates that of the blackbody radiator and the color-temperature has to simulate the daylight in the range of 5000K to 6500K. The light bulbs and tubes have two important specifications the CRI (Color Rendering Index) and the color-temperature. These are interrelated.

The CRI is a figure of quality in respect to the spectral content. CRI=100 means that the spectra of the lamp is fully continuous and completely follows that of the blackbody radiator. CRI from 98...100 is generally considered as being a good value. The quality of the color drops very rapidly with CRI values less than 98. E.g. a lamp with CRI=92 can have only three very narrow spikes in it's spectrum. Remember that CRI it self does not tell anything about the quality of the light.

The color-temperature in Kelvin is the hue (or color) of the light. It too by it self does not tell anything about the quality of the light.

The light quality can only be assessed from the combination of the CRI and the color-temperature.

Normal incandescent light bulbs have CRI=100 but their color-temperature is only 2700K or below, the low color-temperature means that they are effective only in the red end of the spectrum. So hues in the cyan, magenta and blue range suffer seriously. A low voltage halogen bulb is not much better than the normal incandescent, they can only reach color-temperatures in the range of 3000K to 3200K. Normal incandescent as well as the tungsten lamps can have a "daylight" filter (either directly on the bulb or in the light path) this just attenuates the red end of the spectrum, reduces the overall efficiency of the lamp considerably and still does not make the light daylight, it makes it blue.

The above image shows a photographic quality fluorescent tube, the Dulux /12. It has CRI at 98 and the color-temperature at about 5500K. The type number "/12" is important here, Osram as well as other manufacturers have a wide range of tubes that are sold as being "daylight" but they either have a low color-temperature (4100K is common) or they have spiky, non-continuous spectra. E.g. the color "/11" is a "daylight" tube, it has color-temperature at 6500K but the CRI is only 92 due to it's three spike spectrum.