A brief analysis of contrast-focusing

Reprinted from: http://www.soomal.com/doc/10100003120.htm original Chacunghen

Contrast focus in recent years has been a major technical breakthrough, it has achieved fast and accurate focus performance, and this focus is based on the relatively low cost of physical costs, therefore, its promotion of the camera industry is very unusual, we need more in-depth understanding.

Advantages of contrast-focused focusing

• Low physical cost
  Use the sensor with the image processor, not a separate subsystem, without occupying a separate space
  High Focus accuracy

Focus principle and process

Click here to view the relevant information [ bookmark this chart] [ view related pictures] contrast focus

There is a GIF demo animation of the contrast focus, which we use to sort it out a little bit, and through this picture, we can see the fundamentals of the contrast focus.

Click here to view the relevant information [ bookmark this image] [ view related pictures] A simple relationship between the focus position and the image sharpness of the object being photographed

In order to illustrate the relationship between focus position and image sharpness, we added a picture. Use F1-f8 to represent the relationship of 8 different focal positions to the image sharpness of the subject.

With these two graphs, we can briefly describe the contrast-focusing process:

• The drive lens, which changes the focus along the axis of the subject, and acquires the image on each focal point, similar to a point-by-spot scan
  First, the image obtained from each focus is digitized, the digitized image is actually an integer matrix, which we have mentioned many times in the "Digital Image Introduction"
  To find out the contrast value of the image is actually a simple numerical comparison operation, and the value difference between the maximum and the minimum value is obtained.
  Compare the values of each focus to get the maximum value
  Drive the lens, placing focus on the focus of the highest contrast value, i.e. getting the correct focus, i.e. focusing is done

This is actually a very simple to find the maximum process, there is no secret, the implementation of the program is very simple thing, and the performance requirements of the processor is not very high, but why the different differences in the difference in the focus performance is so big?

Sample frame rate

Comparing data is simple, but getting a dense data source is not so simple. This requires direct support from the sensor. When many camera manufacturers show off parameters, there is one parameter that is always hidden from the public, which is the real-time sampling frame rate of the sensor. Only Panasonic, who is willing to disclose the data, Panasonic claims that its sensors can achieve a 120FPS sampling frame rate. This 120FPS does not mean that it can complete 120 frames per second of static image shooting, it does not mean that it can achieve 120FPS video shooting, but the sensor will be real-time sampling to the image processor frame rate, these sampling data can be used for focusing, white balance, metering and so on.

Click here to view the relevant information [ bookmark this chart] [ view related pictures]Panasonic Panasonic Lumix dmc-gx1 miniature convertible lens digital camera

Panasonic said GX1 focus speed of 0.09 seconds, very simple projection, that is, about 11 frames can be completed one focus, if the algorithm is identical, and the sampling frame rate is only 60FPS, then the focus speed will be reduced to 0.18 seconds, 30FPS, the focus speed is slower to 0.36 seconds, this is the effect of sampling frame rate on the focus efficiency.

The lens also has the important influence to the focus, Panasonic also mentions the frame rate of the lens, also is the FPS this frame rate unit expresses, the newest lens, can realize 120FPS the focus performance, what does this mean? In fact refers to the lens of the driving frequency, its complete expression means that the lens can be driven 120 times per second, that is, the focus can be changed 120 times. Only if the lens frame rate is greater than or equal to the sensor frame rate, each frame of the sensor can be considered a new sample.

The impact of the algorithm

With a dense high-frequency data source, not equal to the focus speed will have an absolute elevation, but also need good algorithm support, some time ago, Sony released the NEX-5N Focus performance upgrade firmware, after the upgrade, hardware or those hardware, but the focus speed in some scenes really become a little faster. How is this achieved? Root in the algorithm.

The contrast-focused process will scan the focus one or more times, and if the scanning point is evenly spaced along the axis, the focus efficiency is minimal. Using the previous diagram as a demonstration, this example uses 8 frames to get the focus. If in the F1, found that the contrast value is low, directly skip the F2 to F3, will detect the contrast is worth the rapid increase, at this time to narrow the "jumping distance" into the F4, F5, which saves a frame, speed naturally accelerated. When entering into the F6, F7, you will find that the contrast value is falling, at this point can be judged to have crossed the correct focus, so as to stop entering the F8 action, so that the entire process saves 2 frames, speed can be increased by 25%. This is the effect of the algorithm on speed.

Of course, this is just a paper, in practical applications, the situation will be more complex, but the basic purpose is to use the minimum number of samples to complete the focus. This article just gives an example to emphasize the effect of algorithm on efficiency.

Stepper Motor Lens

In the process of contrast focusing, the focus of the lens is driven, and this mode of operation allows the lens based on the stepper motor to be useful. Stepping motor is the use of pulse signal to complete the command control, the control accuracy is ideal. The combination of the two can produce a more satisfying fit, and most of the micro-lenses are currently driven by stepper motors.

Present situation

The advantages of contrast focusing, many manufacturers have seen, especially the cost savings is obvious to all, but really achieve high-performance focus of the manufacturers are few. To achieve high performance, behind the development work is very large, is a system engineering, the need for a full update of the sensor, image processor, the need for a new lens design, the need for excellent algorithm support, the formation of excellent algorithms also need long-term accumulation, contrast focus is to solve the dark part of the speed of the focus of the problem. For now, most contrast-focused camera models have the disadvantage of slow focus or decreased focus performance. It will take time for a truly pervasive, high-performance, contrast-focused focus, and the industry's ecology may change once it becomes popular.

A brief analysis of contrast-focusing