You won't be able to take the type of firework photos that DSLRs are renowned for, but with newer smartphones, you can still adjust exposure, and mimic long exposures. Here are some tips to get the best firework pictures.
Disable the Flash
Your flash will do nothing when taking photos of fireworks in the night sky. It may even make your photos worse and annoy people around you. The fireworks themselves are all the lights you need in your pictures. Tap the flash icon and switch it to "Off," not "Auto."
High Dynamic Range blends three different photos taken very quickly together at different exposures. There's the original photo at normal exposure, then there's one that focuses on the darkest areas and one on the brightest. The combined image is more vivid and detailed than the normal exposure alone.
Fireworks move quickly so taking three different photos in rapid succession will help to capture light trails, duplication effects, and blurs which you normally wouldn't be able to capture.
In the Camera app, tap on the "HDR" button, then select "On," not "Auto." If you don't see the "HDR" button, have a look in the camera app settings.
If you're not entirely sold on the idea of trying out HDR for your fireworks pictures, then you should make sure that you keep the original photo as well, not just the HDR version. Most camera apps have the option to store originals as well as the HDR processed versions.
Try Out Live Photos
If you have an iPhone 6 or above, another thing you might want to try is Live Photos, which records a tiny video. To enable Live Photos, tap on the circular Live Photos icon in the Camera.
Live Photos has a few benefits - you can capture many frames with one tap allowing you to select the best one, you record a video with sound, and you can convert the Live Photo into a long exposure as long as you're running iOS 11 and higher.
When viewing a Live Photo in the Photos app, swipe up on the screen to access the Live Photo effects, which include Loop, Bounce, and the one we want, Long Exposure.
The "Long Exposure" effect uses software to simulate a long exposure, which will blur things out and create "bright streaks across the night sky," as Apple puts it.
Use the Exposure Lock
As the display gets started, take a few photos to test settings. Whenever you get a picture with decent exposure and focus, you'll want to try to recreate what you just did but don't take the photo yet. The exact method for locking exposure varies but on an iPhone, simply tap-and-hold the screen when the next set of fireworks explodes, until you see "AE/AF Lock" symbol.
Avoid Using Digital Zoom
Most camera apps will let you zoom in on a subject in the Camera app, but they all use digital zoom unless you have an iPhone 7 Plus, 8 Plus or iPhone X. These have a built-in 2x optical zoom.
Use Burst Mode and Take Lots of Photos
When it comes to fast-moving subjects, taking more photos is always better than fewer photos. The more photos you take the greater the probability of capturing that perfect moment.
To help you take more photos faster, most cameras have a burst mode. On the iPhone camera simply hold down on the shutter button or one of the volume buttons, and it will automatically begin taking photo after photo in rapid succession. Let go when you want to stop.
Shoot a Few Videos
If you're not getting the results you want with photos, try changing over to video form. This way, you can steal a frame from one of them as a regular photo.
Keep Your iPhone Steady
To get really clear fireworks photos, keep your iPhone as steady as possible, especially when using the 2x optical zoom. Find something to place your iPhone so your shaky hands aren't the only support. You could also try propping your body up against a tree or similar object to steady yourself and your arms.
If you really want to keep things steady, there's no better way than with a tripod, or put your phone in the selfie stick and rest the stick in a jar or vase.
If you're trying to keep things steady without a tripod, or even if you have a tripod, you can use most wired headphones with volume buttons to trigger the shutter. Simply plug them into the jack plug and press the volume up or down to take a photo.