The trend of having dual-cameras has established itself in the smartphone world, and it adds one more question to prospect buyers: are dual-camera better or worth the potential additional cost? As usual, the answer to this question depends on how you use the camera. With a clear explanation of what dual-camera system can, or cannot do, you can assess their value to you.
It is necessary to establish that “two” isn’t always better than “one”. Don’t let yourself fooled into that kind of thinking. Adding more cameras does not scale the photo quality linearly, if at all. A secondary camera is often a supplement that creates new functionalities (artsy, effects) instead of increasing image quality (in the strict sense, not the artistic sense).
Dual-camera smartphone systems are truly useful for three things:
- Portrait photo (bokeh/blur).
- Telephoto (zoom)
- Wide-angle photo
Would using a second camera and sensor fusion (merging data from two sensors) beat the best single-lens system? So far, this remains an unproven concept. The best mobile cameras are single-lens ones. More on that later…
Portrait Photography (Bokeh)
Introduced by HTC (HTC M8), perfected by Huawei (Huawei P9) and followed by Apple (iPhone 7+), portrait photography refers mainly to the ability to take photos with a nice background blur (called Bokeh, from a Japanese term). The blur makes the photo subject “pop” and brings to focus to it, making for nice photo art. The photo below shows how the iPhone 7+ dual-lens produces better bokeh than the single-lens Galaxy S7.
With large cameras, this effect happens naturally when the size of the lens and the aperture size are large enough to create an analog blur wherever the pixels are out of focus. The Wikipedia Bokeh page has a more detailed explanation of why this happens if you want to know it all.
Mobile phones have lenses and sensors that are too small for this to happen naturally (in the analog space). Therefore, a software emulation process is devised: as pixels are located farther away from the focus area or focus plane, they are blurred artificially using one of the many blur algorithms commonly used in image processing.
To know how far each pixel is from that focus area, it is best and faster to do it with two photos taken with a ~1cm separation. Because of the fixed separation length and the ability to snap both views at the same time (avoiding motion noise), it is possible to triangulate the depth of every pixel in the photo (using a multi-view stereo algorithm). From there, it is easy to have an excellent estimation of the position of each pixel, relative to the focus area.
Without getting into the details, this is a very complicated thing to do, but dual-camera phones streamline the process because they can snap two photos simultaneously. Single-lens systems have to either the user to snap two photos consecutively (with different angles) or use a lesser depth approximation.
The result is that taking portrait/bokeh photos on dual-camera phones is a much nicer experience (than with an emulation). You almost always have to switch to a different “photo-mode”, which adds a little friction. The iPhone 7+ and the Huawei P10 are the leading contenders for this.
The downside of having a separate “photo mode” is that it adds just enough user-interface friction that the feature is not used very often. Ideally, the switch between cameras should be done automatically, but it is very difficult for the camera to “know” what the user intent is.
Alternatively, the camera could always take a bokeh photo in addition to a normal one. However, that would lead to a huge waste of space and may add processing time (lag) to the camera user experience.
Finally, note that every software variants of Bokeh will have some odd things happen at the edges because there isn’t as much information as with a large camera, sensor, and lens. How odd things get will depend on the methods used to generate the depth map, and the blur itself.
It is fair to say that zooming is a natural thing to do when you want to frame a photo. Every single smartphone can zoom using a digital zoom (magnifying the image). The cost of it is of course that you lose some resolution in the process, but it is not a horrible trade-off for a well-framed image.
With the iPhone 7s, an optical zoom (2X) was introduced as a secondary lens. As such, it is possible to zoom by a factor of two, without losing resolution. As you zoom further, the camera app falls back to magnifying the optically zoomed image, and again you start losing resolution.
Although the image quality of an 8X or 10X zoom image is better than not having an optical 2X zoom, the result is still blurred. Also, a 2X optical zoom is more or less equivalent to moving forward by ~1 yard (1 meter) if you are not zooming further. It is therefore always best to move forward if you can.
Of course, the view angle will not be the same, and the zoom lens can be great for portrait photo because it induces less deformation. However, my opinion is that a 2X zoom is too little to justify a second camera module. The 5X zoom of Oppo would be much more interesting, but it is voluminous. If it is really important to you, it is a good thing that someone is offering it.
Wide-angle photography is the exact opposite of telephoto. Instead of wanting to get closer, the wide angle shows you “more” of what’s in front of you. Wide-angle photography was introduced as a second lens system on the LG G5 and subsequent phones (V20, G6).
LG Nailed this one from day one and remains the only game in town for mobile wide-angle photos. I highly recommend trying this if you have not experienced it. The two photos above were shot with an LG wide lens.
"I HIGHLY RECOMMEND TRYING WIDE-ANGLE MOBILE PHOTOGRAPHY"Wide-angle photography is particularly useful to capture immersive experiences like being in a concert crowd, or a place too large for a narrower lens to capture. It is also usually great for cityscapes, capturing tall buildings and other things that regular lenses simply can’t see.
In my opinion, this is the best, or most valuable, implementation of dual-lens today. Why?
- People use it very often
- You cannot do/emulate this in any other way
There is no need to switch to a specific “mode” because the camera switch happens as people zoom in or out, which is perfectly integrated into the normal camera experience.
According to LG, 50% of its dual-lens cameras users have the wide-angle as their primary camera. It is a huge number, which probably dwarves the usage rate of Portrait mode.
When compared to telephoto photography, one would argue that wide-angle photo brings a lot more value: while an optical zoom increases slightly the quality of a photo that could otherwise have been taking without, wide-angle allows taking a shot that would otherwise be impossible.
Dual Lens and Higher-Quality Photography (Sensor Fusion)
The idea of using multiple lenses to increase the quality of one photo by merging and exploiting the data may intuitively make sense, but there isn’t a single mobile dual-camera implementation that beats the best single-lens mobile cameras.
You may have heard that of the two cameras, one may only capture the light, while the other captures the colors. The “light-only” camera could even have its RGB filter removed to allow more light to come in. It sounds great, but in reality, there are many issues at play:
- Dual-lens system have too often two lower-quality (surface area, pixel size) sensors and lenses
- It is difficult to align the data from both sensors. This could lower sharpness
- Image tuning varies greatly from OEM to OEM
It is not dissimilar to trying to use two small telescopes to beat a larger one. In general, it simply doesn’t work this way. Ideally, you would at least want to have two “top” cameras, but in reality doing that costs much money, so the dual-camera quality leads to hardware quality compromises.
"ADDING MORE CAMERAS DOES NOT SCALE THE PHOTO QUALITY LINEARLY, IF AT ALL"The internal volume of the mobile is also a premium resource is that not infinite. Not only having dual-cameras could be expensive, but it does occupy more space as well. Interestingly enough, dual-camera is used with smaller sensors by many phone-makers to enable designs with cameras flush to the surface. Qualcomm’s Clear Sight is one popular implementation of dual-lens cameras.
Finally, it is possible to take very quick consecutive shots with a single system lens (that’s even more power-efficient). This is not without creating its own set of problems (motion ghosting, image alignment…) but Google has done it successfully with the Google Pixel, leveraging the power of algorithms into making it a top mobile camera. I think that the LG G6 also does something similar to compensate for its relatively small sensor.
Today, the Galaxy S7/S8 and the Google Pixel are the two best mobile cameras available, thus proving that dual-camera system some great advantages (easy portrait mode), but don’t match the quality of the best single-lens systems.
With mobile cameras being severely limited by how big (or thick) they can get, having more lenses is a good idea. In fact, if you ignore the form-factor for a moment, it has been demonstrated that adding many lenses can contribute to increasing the image quality significantly.
However, you can see that the model has needs that simply don’t fit into what people are willing to carry and spend today. When does it become beneficial to smartphones remains to be seen, but it is one thing that I hope all OEMs are exploring.
At the end of the day, it’s a difficult balancing between:
- How much one is willing to spend
- How much room there is to fit the hardware
- How smart one is at using the hardware
Conclusion: multi-lens is very useful, but single lens quality still wins, for now
Two isn’t always better than one, that is for certain. What’s best “for you” depends on how you intend to use the camera and how much “effort” you are willing to make. If you are OK with switching modes (most people laze out most of the time), you can take advantage of dual-cameras’ abilities for Bokeh and Portrait.
If you travel a lot and visit interesting places, a wide-angle camera is definitely worth looking at, even if its nominal quality is not the absolute best.
Finally, if you are a point & shoot person who just want to capture a great moment without going “artsy” or put any extra effort, the best single-camera systems will work perfectly for you.
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