With the 4th of July falling on a Wednesday this year, there are going to be an unusual number of opportunities to practice long exposure photography. Official firework displays are definitely more spread out than usual, plus there will be opportunities to play with sparklers in your own backyard. Time for some fun!
The steps below apply to any sort of long exposure photography that is used to capture a light source. It could be fireworks, sparklers, or even a glow stick. Real talk: glow sticks are easier to work with. They don't burn out, so you test and retest and shoot and shoot again until you get the effect you want.
Step 1: Forget auto mode. It won't work. Instead, switch to manual mode.
Step 2: That goes for your focus as well. Auto focus just won't work when you're trying to do long exposure photography. Sparklers and fireworks move too much and it's too dark for auto focus to do a good job of tracking. Set your focus manually and don't stress about it. It won't have to be precise in order to get the job done.
Step 3: Drag out that tripod or find a level surface at the right height to put your camera on. You're going to need to leave the shutter open for longer than you can possibly stand perfectly still. On the topic of equipment, a remote is helpful in this instance. It's not required, but it is helpful.
Step 4: Set your ISO relatively low. I know that normally if you're shooting at night, you want a higher ISO. Not this time. You are going to use ISO to control how much of your background is visible. A higher ISO will make it possible to see the person behind the sparkler or glow stick, the entire city, or whatever. A lower ISO will help you get a solid black background. Important note: you can prevent overexposing the image with the ISO.
(If I had used a lower ISO and a higher aperture, you wouldn't be able to see the trees in the background. Neither way is right or wrong ... it's a personal preference.)
Step 5: Set your aperture somewhere between f/4.0 and f/7.0. You are going to have to fire some test shots to see where you need to be based on your conditions. Aperture is also responsible for helping you make the background disappear an appropriate amount.
Step 6: Set your shutter speed to a slow setting. How low depends on what you're trying to capture. One second is plenty long for some fireworks, while I had to bump all the way to 5 seconds to do some sparkler writing. I like to time out what I'm planning to draw with the sparklers and glow sticks before I actually take the shot. It's never an exact science, but it saves me some time to draw an "F" in the air without the sparkler before I set the shutter speed. That way I know how fast it needs to be before I have fire shooting all over the place.
I'd like to tell you exact numbers for steps 4-6, but it will depend on what you have going on around you. Fire off some test shots, thank the lords of digital photography that you can delete the duds, and adjust as needed.
There's more about writing with sparklers and fireworks photography over here.