Pinch to zoom on the screen of your iPhone/iPad in the Camera app to zoom in and out.

If you find yourself in a place where you need to zoom in when taking a photo, you can simply place your thumb and a finger on your iPhone or iPad screen when in the Camera app. Then, with your thumb and finger still on the screen slowly spread them apart. The subject of the photo will get bigger as you zoom in. Keep in mind, this is a digital zoom meaning that the more you zoom in, the less you will get in photo resolution (your photo will become slightly pixelated). To zoom out, simply reverse the process. Alternatively, whenever you begin to pinch and zoom on the screen, a slider with a plus and minus will appear on the screen. You can use this to zoom in and out as well.