If you're using iOS 11...

With iOS 11, Apple introduced a new HEIF format for photos, which stands for 'High Efficiency Image Format'. The handy little feature allows photos taken on your iPhone to be squashed down to the smallest possible size without losing any quality. Naturally, this means that you can take a lot more photos before you run out of storage space on your device. It's really easy to turn on and could be the cure to your storage problems. Go to the Settings menu of your phone followed by Camera, then Formats, and select “High Efficiency” as your format. One small problem with HEIF is that many computers are simply set up to manage JPEG images rather than HEIF, which could mean some software issues when transferring your photos. But there's also a simple fix for this. Go to Settings and then Photos and then select Transfer to Mac or PC followed by Automatic. This means that the photos will be stored on your device in HEIF format but when they're exported, they will automatically get converted to a format usable on other devices, most likely JPEG.