The iPhone 15 Pro has a more powerful sensor (2.44 µm quad pixel to the iPhone 15's 2 µm quad pixel) in the main camera, which goes from a ƒ/1.6 aperture in the iPhone 15 to ƒ/1.78 in the Pro.

This is usually expressed in the order of a shutter speed value at a given aperture and ISO - say 1/60 sec at f/8, ISO 200. In the camera's automatic and scene modes, that's about as far as it goes.

An f/1.8 aperture is wider than f/2.8, for example. Aperture has one main effect in photographs, which is depth of field. Using a wider aperture like f/1.8 will create a smaller depth of field.

Creating a harmonious exposure using the aperture, shutter speed and ISO is a juggling act. As soon as you make a decision about one element, you'll need to compromise with another.