Sensor size. Depth of field is based on (1) sensor size (you want big), (2) aperture (you want a low number, i.e. big opening), (3) focus distance to nearest subject, and (4) focal length (I think...).

So on a full frame sensor camera, shooting someone's portrait with an 80mm will give you very shallow DoF, on a sensor the size of an iPhone, it's probably close to infinite.

Edit: I don't think the above is technically exactly correct...Here's a good explanation: http://www.cambridgeincolour.com/tutorials/depth-of-field.ht...

Nit-pick: more exit pupil than sensor size, but they're a rough approximation anyhow.

But you're correct that the short (actual, not equivalent) focal length of camera phones makes an f/1.8 completely irrelevant.

Sensor size doesn't really have an effect - it's about focal length. The reason sensor size seems to have an effect is because for the same field of view with a bigger sensor, you have a longer focal length. For the same focal length, aperture and focus distance, a larger sensor doesn't change DOF; it just adds more of the surroundings to the outside of the picture.

It's a combination of wide-angle focal length and small sensor size. It is still possible to get a natural depth of field effect by getting very close to the subject, and that effect should be more pronounced with the 56mm equivalent telephoto lens.

Because it's a tiny lens with a very very short focal length. Short focal lens tend to make everything in focus, regardless of aperture.

Because the focal length (not the 35mm equivalent) is very short due to the small sensor on the phone.

Because it depends on other factors. See what happens for the 1/3" Sensor Size, f1.8, 29mm focal length, etc..

http://dofsimulator.net/en/

Because sensor size has a large factor in depth of field. The camera sensors in phones are very small. This is why you can get better depth of field with a full frame camera then you can with a crop sensor and same goes for small phone sensors.