The evolution of mobile autofocus: from contrast to dual pixel

    Hello Giktayms! When shooting on a smartphone (and not only) it is very important that the photos are sharp and clear. To do this, the subject of the picture must be in focus before you click on the "Take photo" button. Recently, many smartphone manufacturers have been working to improve the auto focus technology, and today we will look at the pros and cons of each, and how they differ. As usual, all the details under the cut.

    When choosing a camera phone, many pay attention to the number of megapixels - they say whoever has more of them is cooler. However, it is often more important and useful to look at other factors that have an equally serious impact on the quality of photographs. Among them is the autofocus type of the smartphone’s camera. Apple, Samsung, LG and other manufacturers have actively plunged into this area now, and many really managed to significantly advance.

    What is autofocus, and why do we need it?

    Using the camera’s automatic focusing system, the lens is adjusted to focus directly on the subject, thereby ensuring the difference between a clear shot and a missed opportunity.

    Simplified, the principle of the camera’s operation is that light rays are reflected from photographed objects and then fall on a sensor that converts the photon flux into an electron flux. After that, the current is converted into a set of bits, the data is processed and recorded in the camera’s memory. CMOS sensors are now particularly popular with smartphone manufacturers, which convert the charge into voltage directly in the pixel, subsequently providing direct access to the contents of an arbitrary pixel.

    In theory, everything works like this: lenses focus the light on the sensor, the sensor then creates a digital photo.

    In reality, everything is not so simple. The angle of incoming better light depends on the distance at which the subject is photographed. The diagram below shows the lenses focusing the rays of light on a blue object: the green and red objects are out of focus and will be blurred in the final picture. If we want to focus on a green or red object, we need to change the distance between the lenses and the sensor.

    At the dawn of the camera industry, most devices had a fixed focus. In modern smartphones, it is possible to adjust the distance between the lenses and the sensor. Therefore, you get high-quality detailed images. Now, for the implementation of autofocus in smartphones, three methods are mainly used: contrast, phase and laser.

    Contrast auto focus

    Contrast autofocus refers to a passive type of autofocus. Until now, this solution is used in most smartphones - largely because it is one of the simplest. Using the sensor, the amount of light on the object is measured, after which it moves the lens depending on the contrast. If the contrast is maximum, then the subject is in focus.

    In general, contrast autofocus copes quite well with its task and has a big fat plus - it is quite simple and does not require any kind of complex hardware for its work.

    But he has several drawbacks. In particular, contrast autofocus is slower than the rest - it usually takes about a second to focus on the subject. During this time, you may want to take a picture, or the moment will be missed if you wanted to shoot, for example, a fast-moving object. This is due to the fact that the lion's share of the time is occupied by the process “shift of the focus point / lenses of the lens - assessment of contrast - shift - assessment of contrast”. In addition, contrast AF does not have the ability to track focus, and in poor lighting conditions it is unlikely to impress you. Therefore, this type of autofocus is currently used mainly in budget smartphones, such as Lenovo A536 , ASUS Zenfone Go and others.

    Phase autofocus: a fast and advanced alternative

    One of the pioneers here was Samsung, which borrowed technology from digital SLR cameras and equipped its Galaxy S5 smartphone with phase autofocus. The bottom line is, in this case, special sensors are used - they catch the transmitted light flux from different points of the image using lenses and mirrors. Inside the sensor, light is divided into two parts, each of which enters the hypersensitive sensor. The distance between the streams of light is measured by the sensor, after which it determines how much you need to move the lens for accurate focusing. For example, the Samsung Galaxy S5 takes only 0.3 seconds to focus on the subject.

    Visually, the principle of phase autofocus operation is presented below.

    The first and main advantage of phase autofocus is that it is much faster than contrast, it is simply a must have for shooting moving objects. In addition, the camera can evaluate the movement of the object using sensors, from here we get the possibility of tracking autofocus.

    But there are also disadvantages. Phase autofocus, like contrast, also does not do very well in low light conditions. It also requires more powerful hardware, which is why it is usually available in high-end smartphones. Among them are Huawei Honor 7 , Sony Xperia M5 and Samsung Galaxy Note 5 , which, by the way, can be found in M. Video.

    Some manufacturers went further and decided to use laser autofocus in smartphones (more on that later), while others were actively engaged in improving the technology of phase autofocus. So, for example, Apple uses the so-called “focus pixels” in its iPhone 6s and iPhone 6s Plus - the bottom line is that the technology uses part of the pixels as a phase sensor, and shooting on Apple’s smartphones is really fast. In fact, this is the same phase autofocus, here you already have to pay tribute to marketers.

    But the Dual Pixel technology that Samsung uses in its Galaxy S7 and Galaxy S7 Edge smartphones is really different from phase focusing in the cameras of other smartphones. Although it is a kind of phase autofocus, it still has some differences and subtleties. In smartphones, phase autofocus is somewhat limited - in order to assign a focus sensor to each pixel, you need to greatly reduce it, hence we get noise and blurry photos. Usually about 10% of photosensitive points are equipped with sensors; some manufacturers, however, do not go beyond 5%.

    In Dual Pixel, each pixel is equipped with a separate sensor due to the increase in pixel size. The processor processes the readings of each pixel, but does it so quickly that autofocus still takes tenths of a second. Samsung says the Dual Pixel technology is like focusing with the human eye, but this is again a marketing ploy.

    Nevertheless, it is necessary to recognize the innovativeness of this approach to phase autofocus in modern smartphones. Now it is a true exclusive for the Galaxy S7 and Galaxy S7 Edge .

    Laser autofocus: the most active

    Like phase, laser autofocus refers to the active type of autofocus. LG has been engaged in this area for a long time, which first implemented laser autofocus in its G3 smartphone. The technology is based on the principle of a laser rangefinder: a laser emitter illuminates the object, and the sensor measures the distance to it and the time of arrival of the reflected laser beam.

    One of the main advantages of this autofocus is time. As they say in LG, the entire process of autofocus with a laser takes 0.276 seconds. Significantly faster contrast autofocus and a little faster than phase.

    An obvious plus of laser autofocus is that it is incredibly fast and performs well in low light conditions. But it works only at a certain distance - the best effect is achieved if the distance from the smartphone to the object is less than 0.6 meters. And after five meters - hello, contrast autofocus.

    Laser smartphones are predominantly equipped with LG smartphones - for example, LG G4 . But there are exceptions: the same One Plus 2 or Asus Zenfone 2 Laser . However, the latter has everything clear from the name, and the price is attractive for such a set of features.

    Dual camera: boldly, but not everyone understands

    At some point, the manufacturers realized that it was necessary to do something outlandish, outside of phase or laser autofocus. So dual cameras came into being: to obtain clear pictures, not one, but two lenses are used at once. While one camera with a fixed focus takes a picture of distant objects, the other focuses on objects that are nearby.

    An important advantage of the dual camera is the ability to quickly take a picture, and focus later, just like in the Lytro camera. But if we talk about a more accurate focus, here the dual camera clearly loses phase focus.

    So far, not many smartphones on the market are available with a dual camera - these are devices from HTC (for example, One M9 + ), Honor 6 Plus and others. Rumor has it that Apple in its new iPhone will decide to use a dual camera.

    The infrared autofocus technology that Lenovo showed at MWC last year works essentially like laser autofocus, but in speed it is about twice as fast as contrast. You can test it on the example of Lenovo Vibe Shot .

    What to choose?

    Since everyone chooses a smartphone for their needs, it is difficult to advise something that will suit everyone at once. Someone is delighted with Huawei's auto-focus, which is configured after shooting, while others consider the Dual Pixel to be optimal. If you take it as a whole, at the moment phase autofocus is the right solution for most flagships, and the manufacturers constantly prove this to us.

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