Smartphones with bokeh effect: revolutionary or just a gimmick?

Smartphones are covering more and more features that used to be the domain of «real» cameras. There’s RAW, image stabilisers and even mechanical apertures, which are brand new to the Samsung Galaxy S9, to name but a few. But what has caused more of a sensation in the last few years is, in fact, the artificial bokeh effect. Thanks to this feature, you can now make the background of your photo blurry even when you shoot on a smartphone.

For those who aren’t as proficient in photography, let me explain what this means. I’ll also compare the smartphone effect with the real bokeh effect.

Depth of field

I’m afraid there’s no getting around a bit of theory. I’ll try to keep it as brief as possible so you get to grips with the concept. If you already know this stuff, feel free to jump to the next section.

Every photo has a focal point: that’s the distance used to set the focus. Depending on the shooting conditions, hardware and settings, there is a more or less large area in front and behind that is sharp. This area is called depth of field or DOF for short.

When you’re taking snaps with a traditional camera, the depth of field depends on a number of factors. You’ll need to consider the following.

• Aperture: the larger the aperture, the shallower the depth of field. As the aperture is expressed as a fraction, a smaller number means it’s bigger. For example: f/1.4 is a larger aperture than f/4. These numbers are in a logarithmic order. That means the difference between f/2 and f/2.8 is much greater than between f/5.6 and f/6.4.
• Distance from the subject: in close-up, depth of field is much shallower.
• Focal length: with a telephoto lens that has a focal length of 200 mm, the depth of field is shallower than it would be with a wide-angle lens of 30 mm.

Side note: bokeh isn’t the same thing as a blurry background or shallow depth of field. Bokeh means the way out of focus areas look. Wikipedia](https://en.wikipedia.org/wiki/Bokeh) explains the concept really well.

Why smartphones hardly have any natural depth of field

Most smartphones have a focal length of 4 mm. It’s not possible to have any more than that, as the focal length is the distance from the lens to the image sensor. The smartphone would have to be more than 4 mm thick or the lens would have to protrude. But the thing is, people want slim phones these days.

As explained above, a shorter focal length gives deeper depth of field – making everything in the image sharp. A 4 mm focal length is very short. In full-frame cameras, you hardly ever have less than 25 mm – unless you experiment with fisheye lenses, in which case it can drop as low as 8 mm.

These 4 mm are so meagre that it wouldn’t make much difference if the smartphone had a large aperture (e.g. f/1.6). The background still wouldn’t be very blurry.

The only way to make an out-of-focus background look natural would be to photograph the subject really close up. As with the big leaf in the image below.

What’s the point of a shallow depth of field anyway?

When you’re taking snapshots of people, the background is often random and busy. It distracts your eye from the subject of the photograph. A shallow depth of field can solve this problem very easily. Once you have that, you don’t need to worry about the background being «calm».

How the artificial effect works

Smartphones that can make the background blurry artificially (via software) usually have two cameras on the same side. Both cameras take the shot simultaneously. The phone then compares the two pictures to calculate the distance of the individual parts of the image and also to determine how blurry or sharp to make each part of the photograph.

We don’t know exactly how smartphone manufacturers set that up. We just know it works in a similar way to our eyes. We also have two of them so we can see three-dimensionally. What lets our brain work out the distance is the fact that both eyes are looking at the same thing at slightly different angles.

It definitely makes one thing clear: the two cameras alone don’t give reliable distance measurements; the software also relies on other factors in its calculations, such as facial recognition. There are smartphones that create an artificial bokeh effect with just one camera per side. We noticed that in Wiko View 2, for example. In that instance, the camera software only blurred areas of the image where it didn’t detect a face.

Is this artificial effect anything like the real bokeh?

The quality of an artificial bokeh effect obviously depends on the smartphone. Unfortunately, a detailed comparison of smartphones would have been way beyond the scope of this article. As a compromise, I decided to examine the effect on the Huawei Mate 10 Pro to see how it fared.

This smartphone has dual Leica lenses with a 1/1.6 light intensity. There’s even an optical image stabiliser thrown in. The photo hardware is also high quality.

The thing was, the software didn’t seem to be able to keep up. It’s the same type used in other Huawei and Honor devices. Let me show you what I mean with a photo comparison:

This is the image above slightly enlarged.

Here’s what springs to mind:

1. The blurriness isn’t just a simple soft focus lens. It really is imitating the bokeh effect. An easy way to know is if individual points of light become magnified circles. That’s a good thing.
2. There is an artificial glow around the light side of the metal clasp. That’s not so good.
3. The out-of-focus background is distorted at the transitions, and the software is struggling to distinguish the edges of the foreground object. That happens a lot and is really annoying. It also means the foreground is out of focus and the background is «cut out». Ironically, that’s usually the reason the photographer wanted a shallow depth of field in the first place.
4. The way blurriness flows into sharpness isn’t right here. The front part of the lower edge is indeed further away but not so far that you’d notice it as clearly. Also, the area where the curve begins should be in focus, as it’s the closest.

What’s more, there’s no flowing transition from in focus to out of focus in the section on the carpet.

For comparison: this is what it looks like when I took the shot with a single-lens reflex camera (SLR) and open aperture.

Wide angle with bokeh: an unnatural combination

You’ve probably noticed that the guitar appears to be about the same size in the photos taken on the smartphone and SLR but it still doesn’t look the same. The angle of view is different on the instrument and the background. Obviously, in an ideal world we would have taken the exact same photo with the smartphone and single-lens reflex camera, but that’s not so straight forward.

Why? Because, as I mentioned at the start, smartphones usually have a wide-angle field of view. In terms of the Huawei Mate 10 Pro, the field of view is equivalent to a 27 mm full-frame lens. In APS-C, that would be a lens with a focal length of 18 mm.

When I attached a wide-angle lens to a single-lens reflex camera, I did achieve the same composition but I couldn’t create any bokeh effect. Then again, it’s almost impossible to generate that effect in wide angle on large cameras. That’s why I opted for a bog-standard 50 mm lens with f/1.8 aperture for my bokeh comparison shots.

The only thing is, it turned out to be physically impossible to take the same photo. In order to keep the subject the same size when you have a larger focal length, you have to be further away from the subject than you were with the smartphone.

That in turn changes the whole composition. You can see that clearly from my still life of foam Nerf gun shells. From above, they takes on the same shape as the number five on a dice. Here’s what it looks like from the side on a 50 mm camera lens:

And here’s what it looks like when photographed on the smartphone:

The artificial bokeh effect went belly-up here. It’s most noticeable on the red rod tips on the left and right of the image edge. But I don’t care about that; I’m only interested in the geometry.

• The rods at the back are much more centred on the smartphone photo.
• In terms of proportions, they’re much smaller.
• The table edge on the bottom right has a completely different angle.
• You can see a lot more space in the background on the smartphone photo because everything further away is smaller.

In short: the photo looks completely different when the same magnification is shot with a different focal length.

You almost never shoot a photograph with «real» bokeh with a wide angle lens. Because wide angle lenses have a unique geometry, the photos they take are easily distinguishable. That’s why I could recognise from the combination of wide angle and bokeh that the effect was artificial – even if the effect was replicated flawlessly (which it is certainly far from being at the moment).

It doesn’t matter whether you have a trained eye or not, you’d still sense «something wasn’t quite right». You’re unconsciously noticing that something doesn’t gel. You just don’t know what it is.

Incidentally, you can’t solve the problem by just zooming in on your smartphone. That’s just a digital zoom and doesn’t affect the layout of objects in the picture. Very few smartphones have another camera with another field of view to work alongside the wide-angle camera. This set-up would permit an optical zoom rather than a digital zoom and thereby create a composition that works with bokeh.

Although I said most don’t come with two cameras there are exceptions, such as the iPhones with dual cam and the Razer phone.

Videos with artificial bokeh effect

On the Huawei Mate 10 Pro, you can even activate the bokeh effect in video mode. That means the smartphone has to compute the effect in real time. I’d say it’s somewhat successful. However, you can see in the video that there is sometimes a delay as the edges of the glass pane become out of focus.

Another problem I had with videos was that the autofocus kept changing, meaning I kept losing the bokeh effect. You’ll notice that in the video below as well. As soon as the autofocus was on the building behind, everything was more or less in focus. In reality, the window frame should have been out of focus.

Verdict: It almost makes the cut but not quite

In theory, being able to use the bokeh effect on smartphones is a handy tool for getting good compositions. It quietens the image and focuses the viewer’s attention on the most important parts of the image.

However, the effect isn’t comparable to traditional bokeh. For a start, the distance calculation is clearly unreliable and, as a result, the software shows picture errors. Moreover, smartphones usually feature wide-angle lenses, which aren’t suitable for bokeh portraits.

Smartphones with bokeh features (selection)

A pack of Bokeh monsters: light-gathering lenses with decent focal length