Although the source is permanent and basically invariable, when it reaches a studio set, daylight is not constant and cannot always be predicted. Its intensity varies with the time of day and the season of the year, an also with the weather. Its color varies with the same factors, from orange to blue. Its softness depends on how much cloud, or must, covers the sun. For studio photography, these local variables always need watching, and can be inconvenient where a hot has to be planned in advance or is likely to take more than an hour to do.

In any particular instance, the size and position of the window is important; because it accepts light from only a small area of the sky, it modifies the variables. The color of the light, for example, may be strongly affected on a clear, sunny day if the studio skylight faces only blue sky.

Intensity

To be efficient, a studio window should not face direct sunlight at any time of the day; this is because it is difficult to control direct sunlight when it has been filtered through a frame. For most typical daylight studio skylights, the brightest lighting conditions are at midday in summer, with a thin cloud cover. This is not likely to allow exposure settings better than about 1/60 of a second at f2.8 with ISO 100 films. White walls and ceiling may improve this setting by about a half to a full f stop.

Diffusion

More than anything, the window shape controls the diffusion of the light, particularly if there is no direct sun. The diffusion can be increased by covering the window with white translucent material, particularly if this is hung or mounted a little in front of the window frame, or the lighting can be made directional by covering up part of the window to reduce its area.

Color

For the purposes of calculating filtration, the color of daylight, as of tungsten lighting, is measured as color temperature. This is the temperature at which some notional material would glow if it were heated, from red-hot to white-hot to blue. Daylight film is balanced to 5500K, and for any color temperature lower or higher than this, color balancing filters can compensate, so that the light reaching the film is ‘white’. You can find various charts online that show how this is calculated, but a color temperature meter is the only precise way of judging it. Diffusing material over the window may affect it.