HERE'S HOW to take pictures of fireworks: First, of course, pick a vantage point that will give you a dramatic background. In our area, the classic ones are the Capitol dome, the Marine Corps Memorial and the Washington Monument.

You'll need a tripod, and a camera that has a "Time" or "Bulb" setting, which allows you to hold the shutter open. Aim so that your background object is to the side of your viewfinder, leaving the center for the fireworks to fill.

Don't feel that you have to use superfast film. Try the ISO 200 or, at fastest, the 400. Remember that fireworks are very bright and you're going to use long exposures.

Watch the first couple of bursts through your viewfinder to allow you to line things up. Set your camera on "Time" and hold a piece of cardboard up against the front of your lens. Hold it there and open your lens. Then when the next burst of fireworks begins, move the cardboard aside. The results will be time exposure of a fireworks burst.

Try several single bursts, but try some multiple bursts, too. Just leave the shutter open and remove the cardboard for each exposure. Be sure you close the shutter when finished with each frame.

Recently I reported on photographic security at Dulles Airport. I commented with some suspicion on the lead-lined bags made to protect film. I now have some new facts that have eased those suspicions.

I spoke with Irwin Diamond, president of Sima Products Corporation, makers of the bags:

Q.

When, and under what circumstances, did you first manufacture these bags?

A.

We first made them in 1973 at the time the first airport x-ray machines were put into service.

At the time the consensus was that the machines were fine, and safe, but Ralph Nader took issue. He instituted a suit saying that the machines were unsafe for women of childbearing years who attended them.

It was proved in court that many of the machines did exceed government standards for x-ray emissions.

Sixty of the machines were immediately taken out of service and kept turned off until they could meet these standards.

Even in compliance, however, there was a small cumulative effect that had to be a problem for photographic material. A test was set up and, as a result, the protective bag was born.

Q.

How does it work? Why was I able to see through it on the x-ray machine screen?

A.

The bags are not designed to stop all of the x-rays. Their purpose is to produce an attenuation of the x-rays -- a reduction in power -- rather than total blockage.

Q.

This reduction makes it safe to pass film through the machines?

A.

Yes, but with this caution. We make two bags. One for films of ISO 400 and lower, and the "super bag" for film rated between 400 and 1600. The super bag has more than double the protection, and obviously has to cost more.

Q.

If this is the case, why worry about x-rays at all? Just stuff your film in a super bag and away you go.

A.

It's not that simple. Remember that on one vacation flight a person can change planes six or eight times. The machines may be out of adjustment and certainly can vary in output. Sometimes this variance can cause problems.

The best thing is to ask for hand inspection of your photographic gear.

In this country, there is an FAA regulation that says you can have your photographic material inspected by hand. In most of the nation's airports, there is not a problem with this. But I have boxes of foreign letters from people who have been harassed in places like New York and, of course, overseas.

Q.

What about overseas? Do things work the same way?

A.

Not really. In England and Ireland some feeling is shown, but there are six countries that will not do hand inspections. In some cases, even in the ones that will hand inspect if pressed, there is danger. Frequently security people will say that, if you insist on this kind of inspection, they will insist that you pull the film out of all the cartridges for inspection.

We've packed an information sheet in each of our bags that tells you which airports still won't provide hand inspection.

So plan your travel with a little common sense. Keep your film together in the proper bag, and try for hand inspection.

Sima is now providing a plastic baggage tag that not only carries an identification card but also bears the FAA ruling giving the right to hand inspection in the United States.

It is available from the Sima Products Corp., 4001 West Devon Ave., Chicago, Illinois 60646. The tag's free, but you must send $1 for postage and handling.

Carl Kramer answers questions of general interest, but cannot respond individually. Address mail c/o Weekend, 1150 15th St. NW, Washington DC 20071.