Artificially reducing depth of field on portraits in iPhone

Left: artificial aperture set to minimum          Right:  at maximum

Smartphone cameras are incredibly popular and take over 90% of the photos captured on this planet today.  They do have a few weaknesses that their larger brethren don't have, one being the ability to alter depth of field.

To take a portrait on a DSLR or mirrorless camera with the background out of focus, I would pick a prime lens (85 mm) with a low minimum aperture (f/1.8).  With the background a good distance away I would have a nice shot of the subject and the background would be pleasantly blurred and exhibit a pleasing bokah.

You can't do this on a smartphone, although the lenses are primes and the apertures are relatively low (f/1.78 in the above photo).  The problem is that the small digital sensors use very low focal length lenses to obtain decent fields of view.  In this case, the focal length was a paltry 6.76 mm.  There is so much depth of field at this focal length that it is almost impossible to get anything out of focus.  This is where computer enhancements come in.

By pressing the "f" button you bring up the aperture slider bar that lets you determine how much depth of field there is.  By sliding it to the right you create less depth of field and moving it left creates more.  See the image below.

Using portrait mode to alter depth of field on an iPhone

The camera uses the ToF sensor (time of flight) on the camera's front to determine which parts of the image are farther away; it then uses that information to blur the background.  Fine details in the subject can be missed though and the depth of field isn't necessarily uniform.  However, it works well enough for most of us.
Notice how the hair on the right is blurred artificially when it should be sharp.

The bottom line is that, while the image may be acceptable, it is far from perfect.  It is a tool that allows a certain level of simulation but is not as accurate as the real thing.  If you are happy with the results, then go with your phone.  If not, consider using a DSLR or mirrorless system.

Thanks for reading.

Eric Svendsen     www.ericspix.com


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