The iPhone’s camera has a 0.2 second fixed shutter speed and a fixed aperture of f/2.8. It adjusts for exposure by adjusting the sensitivity of the sensor inside the camera — in effect, adjusting the ISO. This means that the darker your photograph, the greater the chance there will be noise in the image.
You can improve the color and clarity, as well as reduce the noise in your iPhone images by making sure, whenever possible, that you have a strong light source that’s behind you. For example, when taking outdoor photographs, try shooting with the sun at your back whenever you can. Lighting your subject properly will help you get good color, good contrast and reduced noise from your iPhone photos.