With the sun below the horizon, it is the afterglow that lights the scene, often with a surprising delicacy not found under any other conditions.
Like the normal daytime skylight, twilight is a reflected light source, but is rather more complex in its effects. It is the light that appears a little before sunrise (“first light”) and that remains for a short time once the sun has set. In a clear sky, the intensity shades smoothly upwards from the horizon, where it’s brightest, and outwards from the direction of the sun. (The studio equivalent is a light placed on the floor, aimed up towards a white wall, and used as back lighting.) The sky, in fact, acts partly as a diffuser and partly as reflector. The actual light levels vary considerably from a just-discernible glow to actual sunset or sunrise.
These conditions allow a fairly wide choice of exposure. If you are shooting directly towards the twilight, you can try a short exposure in order to make a silhouette of the horizon and subject. In this kind of back-lit shot, the shading of the sky from bright to dark gives some choice of exposure, particularly if you use a wide-angle lens. Less exposure intensifies the color and concentrates the view close to the horizon. More exposure dilutes the color in the lowee part of the sky, but shows more of the higher, bluer parts. In other words, increasing the exposure extends the area of the subject within the frame. A range of exposures is acceptable, depending on what kind of effect you are trying to achieve from the photograph.
Not only does the brightness shade upwards from the horizon, but the color does also. The exact colors depend on local atmospheric conditions, and different light-scattering effects are combined in a twilight sky. At a distance form brightest area-opposite and above-the color temperature is high, as it would be during the day. Close to the horizon in the direction of the light, however, the scattering creates the warmer colors at the lower end of the color temperature range: yellow, orange and red. These merge in a graded scale of color.
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