Smartphone focus mode: difference between PDAF / CDAF / LDAF

Phase Detection Auto Focus

The full name of phase focusing is "phase detection auto focusing", which is the mainstream focusing method in the field of mobile phone photography. Compared with the contrast focusing mode, the phase focusing mode has faster focusing speed.

The focusing method based on SLR is based on the fact that some masking pixels are reserved on the sensor to act as autofocusing sensor, which is specially used for phase detection. By comparing the distance between the pixels on the left and the right and their changes, the offset value of focusing can be determined so as to achieve accurate focusing.

After the global phase detection focus position, the lens driver module will move the lens group to close the focus, so it will take less time.

Since the focus mode of PDAF requires detection of individual pixels, it requires higher illumination conditions. In low-light environments, the focusing speed of PDAF tends to slow down due to insufficient light in the sampling area.

Contrast Detection Auto Focus

Most mobile phones now have multiple modes of focusing at the same time. For example, when the PDAF mode is not suitable, the phone will automatically switch to contrast or other modes of focusing.

The principle of contrast focusing is to detect the contrast by focusing the area. At the same time, the focusing motor drives the lens module to move the lens step by step and record the contrast value in real time. After the complete displacement of the lens, the system will get the position with the largest contrast and finally return to the place with the maximum contrast to complete the focusing process.

Even if the focal point is found in the middle, the camera module will continue to complete the movement record, so the whole focusing process will be slower.

This mode of focusing does not need to release phase monitoring pixels as PDAF does, so the cost is lower. And in the case of more obvious contrast, the contrast detection auto focus can also achieve good results.

Laser Detection Auto Focus

The first two types of focusing are also known as passive focusing because they rely on the reflection of light from the object. Laser focus is called active focus, or range-finding focus.

This is because the principle of laser focus is through a separate infrared laser sensor to the object to shoot infrared laser, after reflection by the sensor to receive, in the calculation of the distance between the object to be photographed, the focusing motor will drive the lens group to the corresponding position to complete the focus.

This mode of focusing is very fast and does not rely on reflected light, so it can still ensure excellent focusing performance in dark light environment. However, due to the focus distance and cost, laser focus is still limited in use.

Dual Pixel CMOS AF

The dual-core focus is essentially the same as the PDAF focus, but because a conventional sensor has only one photodiode per pixel, half the pixels are obscured. The full-pixel dual-core sensor has two photodiodes at each pixel point -- photodiode A and photodiode B. When focusing, the two photodiodes detect the new Numbers of image A and image B respectively, and the two images will coincide in the state of focusing. In the case of incomplete focusing, the two images are blurred and staggered.

The principle of focusing is still through relative detection. By detecting the deviation (signal difference) and deviation direction (which side of the center of A image and B image is located respectively), we can calculate the direction and distance that the focusing lens group should move, so the focusing speed will be further improved.

At present, there are many kinds of focusing methods, but most of them are carried out on the basis of the above methods. However, these different ways of focusing have their own advantages and disadvantages. In order to cope with different shooting scenes, mobile phones now have built-in multiple ways of focusing to improve the focusing ability.

In the future, this kind of mixed use of focusing will become the norm, so we don't worry too much about it when we buy products.

At present, a variety of models have adopted the laser focusing mode, and the focusing ability has been improved. To experience the excellent photo effect, you may wish to have a look at the Lenovo Z6 Pro.

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