NOW THAT we've picked up an extra hour of sunlight in the evening, there's time enough to grab a quick dinner and your camera (in that order) and get in some twilight photography.

The hour after sunset is filled with unexpected photographic delights. Light changes quickly in quantity and hue, from sunset's red-orange to cool purple-blue. City lights flicker on, adding their own palette of colors to the natural light. Long-time exposures can record the "afterglow" of the darkening sky that isn't apparent to the eye. And those long exposures can work magic on color film emulsions, shifting colors toward purple- blue in dramatic, sometimes unpredictable ways.

The key to twilight photography is striking a balance between sky and foreground, or between sky light and artificial light -- be it the gleam of offices in a skyscraper, a stream of auto headlights on the highway or the glow of streetlamps on Pennsylvania Avenue.

Assume for the moment that you're shooting a scene lit only by the setting sun. Remember that film can't record the range of brightness your eye can; you're bound to lose detail in highlights or shadows. (Squint at the scene and you'll get a visual approximation.) Begin by metering a bright area close to, but not including, a portion of sun, and make an exposure at the meter's recommended setting. The result should be a good exposure for sky, with little or no foreground detail. In each successive exposure, open up a half-stop (or set your automatic camera's override to do the same). A half- dozen exposures should reap at least one that pleases.

One way to solve the high-contrast problem is with a graduated neutral- density filter. Half of the filter is clear glass; the other half blocks some of the light. By rotating it so that the light-blocking part's aligned with the sky, you can bring the relative brightness of sky into better balance with the foreground.

When you're shooting a scene with important artificial illumination as well as natural light, the best solution is to wait a few minutes. After sunset, sky light dims rapidly, but the artificial light remains constant.

If you can meter artificially lit portions of your scene, go to it; if not, Kodak's Master Photoguide can provide trial exposures for you. It suggests the following for Kodachrome 64 and an f/5.6 lens opening (you'll need to adjust your exposure accordingly for other film speeds or choices of f/stop): 1/8 second for store windows and bright office interiors; 1/4 second for brightly lit downtown street scenes; two seconds for average streets and most floodlit buildings or monuments.

Set your camera to the appropriate reading, then meter the sky light illumination periodically. When it drops to within three f/stops or so of the artificially lit scene, begin shooting. As twilight advances, the sky will grow darker and soon match the artificial light in intensity. Keep on shooting, and you'll discover the wide range of changing-light effects obtainable from a single scene.

To record automobile headlights as long streaks of light on the highway, you'll need a lengthy time exposure. Count the seconds as an auto moves through your viewfinder, use that as your trial shutter speed, and adjust aperture to provide proper exposure. (Kodak suggests a K64 exposure of four seconds at f/8.) If only a few cars zip past during a single exposure -- not enough to make an interesting effect -- multiple exposures should do the trick. The easiest method (one that will eliminate fumbling in the dark for your camera's multiple-exposure control) is to calculate the necessary exposure as above, then set the required f/stop. Put your shutter speed on "B" (bulb), and trip the shutter as traffic goes by. When it has moved through, leave the shutter open but cover the lens with a cap. Remove it when you see the next stream of cars.

Remember to keep track of the elapsed time, and don't forget that other light sources may overexpose and "burn up" surrounding areas if you overdo it.

When making exposures of 1/10 second and longer, you must also account for a strange beast called "reciprocity failure." The technical explanation for this makes for dull reading unless you're a photochemist; suffice it to say that film isn't as efficient at recording light during long exposures as it is when shooting at hand-holdable shutter speeds. You'll need to open up from a half stop (for a 1/8 second exposure) to three stops or more (for a four-minute pop). The recommended compensation can be found on the film data sheet that you'd probably toss out if I didn't warn you not to.

With color film, reciprocity failure causes a reddish-purple bias that becomes more pronounced as exposures lengthen. You can use green filters to correct this, if you want. But the shift can be strikingly beautiful.

What film should you use? Color negative film has more exposure latitude than slide films, which can prove handy if keeping down contrast is your goal. But your color lab's attempt to interpret proper color balance during printing is necessarily subjective, and may not be what you'd prefer. It's a tossup.

Daylight film, its name notwithstanding, is fine for most nighttime and twilight shooting. Reddish skies will be emphasized; streetlamp-lit scenes will register in warm but pleasing hues. Tungsten film will render street scenes more accurately, but will deemphasize reddish "afterglow" effects.

Finally, leave the high-speed stuff in the camera bag. You'll be shooting from a tripod anyhow, so you may as well get the best possible sharpness and resolution.

While you're shooting, use a few frames in quest of some special-effects magic. At small apertures, for example, diffraction will make bright light sources appear star-shaped. If you want, you can get the same effect by using a cross-star filter or fine- mesh screen over the lens. Rack your zoom through its range of focal lengths during all or part of a time exposure to create a bright-light pattern. Or do a double-exposure, using different focal lengths on the same frame. Intentionally jiggling the camera during an exposure will create random, unpredictable light streaks.

You'll waste some film on "losers," sure. But the ones that work may include some show-stoppers. And you can always tell your friends you planned it that way all along.