While being able to make exposure adjustments on your iPhone camera has been around since iOS 5, this option was limited to relying on the iPhone’s automated sensors for judging the optimal exposure; you could tap on a location to make the iPhone meter exposure based on that spot, but that was about as far as your adjustments went. Even third-party apps that provided exposure control had to use workarounds to accomplish this, instead of being able to adjust the exposure directly at the camera sensor level.
The good news is that iOS 8 introduced the ability for a whole bunch of camera settings to be manually controlled, although most of these are only available through developer APIs, allowing third-party apps to let you take over and do things in an almost “full-manual” mode. While the built-in Camera app doesn’t provide all of this functionality, Apple has hidden away a manual exposure control option, allowing you to adjust the exposure to your preferences.
To do this, tap on the camera window in much the same way as you would to set the exposure metering point, and then release your finger and tap again in the exposure metering frame that appears. You can then slide your finger up and down to set the exposure value manually — you should see a small starburst to the left of the exposure frame that will slide up and down a vertical line illustrating your exposure settings as you slide. In this case, much like the normal exposure metering feature, the setting will only be maintained for as long as lighting doesn’t dramatically shift, so if you move the iPhone to another scene, the camera will return to the default automatic weighted exposure mode. If you want to lock in your manual exposure settings, however, you can also do this in much the same way as before: tap-and-hold on the camera window until you see “AE/AF Lock” appear at the top of the screen, then release your finger and tap-hold-and-slide to adjust the locked exposure setting.