There was a very interesting topic that came up today when I had my meeting my thesis supervisor and group. Taking a picture from a modern smartphone shows that the phone will attempt to focus on objects.

How exactly did the camera phone "focus"?

With further research it turns out that until recently, most camera phones would employ a "fixed focus system", but in the recent years, higher end phones used newer camera modules which integrate auto-focus systems within the small camera modules.

Fixed Focus Systems

A fixed focus lens is basically one where the focus distance is decided during manufacturing, and there is no way of changing the focus. However, this means that things could be "out of focus" pretty easily. This system instead attempts to keep a large range of the image in the picture relatively in focus, or achieving a large depth of field. the illustration below shows how aperture and depth of field are related, In addition to the aperture indicated, the size of the sensor also affects the "depth of field".

Picture indicating how aperture and depth of field and other optical parameters affect the depth of field.

By created fixed focus lens with a large depth of field, it was possible to produce cheap camera modules that were relatively sharp for everyday use. However, these systems were rarely used in recent day high end cameras - autofocus was introduced in iphone 3gs, and continues to be used in high end smartphones. The wiki article goes deeper into some specifics of fixed focus lens in camera phones.

Auto Focus Systems

High end phones employ camera modules which have a built in auto focus system (Link to Samsung camera module pr info). Here is an article which highlights mems for AF systems, but also explains the different sub categories of auto-focus systems. It boils down to two main methodologies which are used to change AF for camera phones

Lens Modification AF
- This involves a lens which would change refractive index or other properties to change focus.
- This method tends to be much more costly to manufacture, and are limited in resolution, as such it is rarely used in the cameraphone market.
Lens Motion AF
- This involves the movement of the lens with high precision using a motor to change the focus.
- The most prevalent is to use a voice coil motor(VCM) or a piezo motor, but there are starting to be mems based focus systems like the DigitalOptics Module
- VCM motors tend to draw a relatively large amount of power in comparison to mems, which helps explains the larger power consumption of taking continuous videos on a camera phone.

Final Thoughts

Focus systems are independent of camera sensors, which was confusing when i started researching into this. The camera modules typically use another manufacturer's camera sensor and integrate it with their own focus, lens, and stabilization system.
There is a huge field to improving camera abilities in terms of camera phones, where the size and usage restrictions push for innovation.
Smartphone SDK's tend to support only auto-focusing onto an area, instead of truly supporting a manual focus, which leads to a question of how some camera applications can employ "manual focus". Are there some tricks to enabling manual focus abilities on android? (Link to android camera SDK)
Apps like SynthCam show some really cool innovation in enabling multi-point focusing and DSLR like depth of field effects