The iPhone camera has some limitations compared to an SLR camera. The aperture, with which the amount of light that falls on the sensor is determined, is not adjustable on the iPhone. The shutter speed, with which you control the exposure time, is no longer than 1/3 of a second on my iPhone 8 Plus. An SLR camera can even illuminate indefinitely (Bulb mode). And the sensor of an iPhone is much smaller and gets less light. Therefore you get noise faster and the dynamic range is smaller.
How does the exposure of an iPhone work?
The exposure of a photo is determined by the ISO value, the shutter speed and the aperture. The ISO value indicates the sensitivity of the sensor to light. A high ISO gives more noise. In the standard iPhone camera app, the exposure time and ISO are not adjustable. The iPhone camera app will always try to take photos with a low ISO (minimum ISO 25) and a high shutter speed (maximum 1/8000). Low ISO produces low noise and a high shutter speed prevents motion blur. If the light is reduced, the iPhone will first bring down the shutter speed to an exposure time, with which you still can take pictures hand-holding the iPhone without motion blur. Then the ISO will increase in steps up to a maximum of 2000 ISO. In dark conditions the photo will be taken with a long exposure time and a high ISO.
This video explains until 2:30 how the exposure of the iPhone camera works.
How to change the iPhone shutter speed and ISO
There are camera apps that give you more control over exposure with modes for shutter speed priority and ISO priority. The app automatically selects the correct ISO value at a manually set exposure time and vice versa. Camera apps are also available with which you can set the exposure completely manually.
How to use the shutter speed creatively
You can add special effects to your iPhone photos by using a long exposure time. In the blogs below I will explain how you can set a long exposure time with the iPhone camera app and special apps.
Many limitations of the iPhone camera app with regard to manual exposure settings can be solved with special camera apps. My favorite app is Adobe Lightroom CC Mobile App Store. You can read about the possibilities of this camera and photo editing app in this blog.