How can a super tiny iPhone 6 Plus lens produce significant DOF?

Question

According to Apple specs the iPhone 6 Plus thickness is 0.28 inch (7.1 mm) and the lens length is only a part of that. And according to an article I found, Depth of Field is a function of "aperture (i.e. lens diameter), lens size, distance ratios, and print size".

Why is it that a very short lens with a small diameter in an iPhone 6 Plus has a DOF like this, with so much visible bokeh?

Here is the link to the original full size sample, to check the EXIF info. All of iPhone 6 Plus sample images there seem to be f=2.2.

Note: DOF could be added in a software way (similarly to PhotoShop/Gimp "Lens/Focus blur"), provided the software knows what is to be in and what out of focus. I also don't see any artifacts on focus boundaries betraying the filter application without retouching.

Although physical principles are always the same I think it's a bit different to the How can I get dramatic shallow DOF with a kit lens? question as the smartphone lens is much smaller (when compared to an avg. DSLR kit lens), doesn't have an optical zoom feature to play with, and even the aperture size is fixed (based what I've found on the Internet).

The middle of the branch in the picture above (could be a sort of rowan) might be about 30-50 cm (12-20 inches) distant and the closest tree might be about 5m (16 feet). Thus the distance ratio could be about 1:10 or 1:20.

I've just taken a picture with my old Nokia Asha 206 phone where the hand to most-distant-tree ratio could be more than 1:100 and yet — everything is in focus!

To rephrase my question a bit: I'm not interested to get a "cool bokeh". I'm just curious on how can an iPhone 6 Plus produce shallow DOF pictures while a few other smartphones I've seen despite having similar dimensions of the lens take "everything in focus" pictures?

Has the lens construction or an image processor changed?

Answer 1

Many older or cheaper phone cameras use a "fixed focus" lens. ie it is always set to focus a specific distance away from the camera. This is usually set to the "hyperfocal distance", ie everything from half that distance out to infinity is in focus.

This depends on just what is acceptable as 'in focus'. But most photos from these cameras will be sharp enough, they will always have a big depth of field. But they may not be able to focus on things a few centimetres away.

Most newer and better quality phone cameras use a lens with auto-focus. eg for the iPhone, all models since the 3GS have auto-focus (at least for the rear camera). They can focus at a specific distance, which can give much sharper photos. So you can focus on something close to the camera, and have more blur in the background, ie a shallower depth of field.

Also phone cameras have improved in other ways. Specifically, the sensor size. eg the iPhone 6 has a 1/3-inch sensor. This is not that big compared to a DSLR, or some compact cameras, but it is much bigger than many older camera phones. A bigger sensor can allow a shallower depth of field (for an equivalent focal length and aperture).

Answer 2

The simple answer is you can get shallow depth of field (hence bokeh) with any camera system if you focus close enough.

Finite depth of field arises due to the inability to focus light coming in at different angles in the same plane. When light is focused at the wrong distance is appears as a spot the shape of the aperture, instead of a point.

Large apertures create larger spots, which is where aperture comes in. However close focusing causes greater angular variability, which is where that factor comes in. Imagine an object 100m away and another object 101m away. The angle between the top of the object and the camera will be almost the same in both cases, and both objects will be in focus (within the depth of field). Now imagine an object 2m away and an object at 1m away. The angles are now totally different and you wont be able to get both in focus, despite the objects being the same distance from each other in both cases. Focusing closer has reduced depth of field. Now imagine if they're even closer still.

So regardless of your possible aperture values, if you can focus close enough you will always reach a point where your depth of field becomes too small and you get blurred objects/background.

Answer 3

You mentioned four factors from your reading (lens diameter, lens size, distance ratios, and print size), but the only ones that really matter are the first two - or, more specifically, the lens iris (the diameter of the opening that lets the light in, not the physical lens diameter) and the lens focal length (the distance from the center of the lens to the sensor). The ratio of these is the f/stop, and the closer it gets to 1, the more bokeh (out of focus) effect you'll get.

As you mention, the camera EXIF data stored in the jpeg header shows the f/stop for each shot, and at f/2.2, you get a good amount of bokeh (as you see) - as you get above f/4 or f/8 you start to see a more "everything's in focus" effect, and at f/16 there will be little bokeh left.

IOW, it's the ratio of diameter and length that matter, not the actual dimensions. So a small lens is OK as long as it's matched to a small sensor, and is opened up to a large iris setting. That opening is controllable on an SLR, but not so much on an iPhone.

Answer 4

If a camera is focused at infinity, then the size of the blur circle for an object at a given distance is the same as the size of the lens aperture. So if the iPhone camera has a 1 mm diameter lens aperture, and if focus is set to infinity, then every object is blurry at the 1 mm level: which is not detectable on a tree a hundred meters away, but is detectable on the cherries right in front of the lens.

Therefore, to get the cherries sharp, you have to set the focal distance to that range. As a consequence, everything at large distances will have a blur circle that is the same angular size (i.e. number of pixels) as a 1 mm circle at the sharply focussed cherries.

Note that it is not distance ratio that matters (if the Moon is in focus then stars will be in focus, even though they are a billion times farther away) but the ratio of the lens aperture to the objects in question.