(used iMore.com for reference)

The images above show differences between taking photos that are too dark vs. brighter

TIPS:

1. Experiment with 3rd party apps: There are ones like a manual-custom exposure camera for $1.99 which let you work in a DSLR mode by allowing manual adjustments such as shutter speed, ISO, white balance, and exposure.
   
2. Turn on the grid function: Settings > Photos & Camera and enabling the Grid switch. You can switch it off by going back to the Photos & Camera preference screen.
   
3. Turn off the flash mode: The flash is not that powerful and can create a strange or dark hue. Try shooting with strong, natural light sources. Sunlight is preferable, but a good overhead light source can work as well. When shooting in dark settings or at night, use the camera app’s exposure slider to boost light in the photo.
4. Turn on HDR Auto: This lets you snap photos that may have high contrast light sources and still capture a nice image without distorting either the light or the dark area of the picture. You will have several shots at different exposures. There is an auto HDR function on 5s and up. A little yellow “HDR” box will appear at the bottom of the screen.
   
5. Hold down a spot on your viewfinder to lock focus: This focuses specifically on your subject matter. Tap and hold on the focus point until you see the yellow AE/AF Lock. To remove the lock, just tap anywhere else on the frame.
   
6. Slide the exposure meter to brighten (or darken) images: Adjust the yellow exposure slider next to the focus square. Tap once on the focus square and exposure slider, then use the sun icon to increase your exposure by sliding upward or decrease exposure by sliding downward.
   
7. Snap photos with volume button: If you want to avoid blur or shaking this is your best bet.