Q -- In looking over the specifications of the Vivitar 90-180 zoom lens, I came across some terminology that I don't understand. What is the difference between elements and groups? Does it make any difference as to how many? Also, what is a flat field?

A -- The Vivitar series I 90-180 mm f4.5 zoom lens has an optical construction of 18 elements in 12 groups. The 18 elements are individual "optical units," or lenses that are organized in groups for maximum effect.

The flat field refers to the ability of the lens to produce edge-to-edge sharpness. When using it as a macro-close-up mode it automatically makes the f-stop adjustment so that you don't have to change the shutter speed.

Q -- Why can't the labs print my 35mm full frame instead of cropping it?

A -- The proportions don't match standard print sizes. Many labs have a special service of printing 35mm to a 7-by-10 and a 10-by-14 size that is closer to the right proportion. Prices for this service fall between the mass production prints and the more expensive custom finishing.

Q -- Sunsets are are often spectacular but I can't seem to capture them in color. Any suggestions?

A -- The trick to susets is underexposure to retain the brilliant sky colors. Take a meter reading of the sky itself, not including the dark landscape, and underexpose a half to a full stop from that.

Many photographers in taking sunset pictures mistakenly include large sections of dark foreground in their metered area.

Q -- I shot some party pictures with bounce flash and all the faces turned out red.

A -- The room probably had red-colored walls or ceiling that reflected your flash, tinting the light reddish. This can happen with other colored wealls as well. You can end up with green faces if the walls are green or a jaundiced yellow if they're that color.

Bounce your light off a white umbrella or a piece of white cardboard.

Mount the umbrella or cardboard on a light stand and place it behind you, high and pointed in the direction of your subject. When you shoot, point the light backward at the white surface instead of a colored wall.

There are gadgets on sale you can attach to the flash itself so you don't need a lightstand.

Q -- What's a Kelvin rating and how is it used in photography?

A -- A Kelvin rating is the yardstick for measuring the relative color quality of light. The lower the number, the warmer the light source. Sunsets and candlelight are rated lowest, at about 2,000 degrees Kelvin; daylight and electronic flash in the high numbers at 5,500 to 6,000 degrees Kelvin. The importance of this is that the warmth of the light affects your picture. Warm light, like candlelight or a setting sun, will color your picture red; bluish light, as on overcast days, will cast a cold blue tint.

The eye doesn't see this, by the way, because it adjusts to the light and can't tell the difference. This is why you may be surprised and say, "That's not the color I saw," when you look at your slides or prints.

The Kelvin rating lets you match up your light source and the film or make the needed change with filters.

A film rated at 3,200 degrees Kelvin should be used with a lamp rated the same for the most accurate color result. A daylight color film rated at 5,500 degrees Kelvin should be used in sunshine at about midday. Yarying the film and the light can give you some unusual results: Indoor tungsten film, for example, rated at 3,200 Kelvin, will turn an overall blue in daylight; daylight film exposed by candlelight will look reddish. Using the Kelvin rating, you can match up your film and light for the truest colors or mismatch them for special (or weird) effects.