What does Pentax K5 AF do? More details~

AF= Auto Focus

Introduction

Auto Focus (Auto Focus) uses the principle of light reflection from objects, and the reflected light is reflected by the camera. The sensor CCD accepts, through computer processing, the method of driving the electric focusing device to focus is called autofocus. It is divided into two categories: one is active, and the other is passive.

Edit this section to classify

1. Active: The infrared generator or ultrasonic generator on the camera emits infrared light or ultrasonic waves to the subject. The receiver on the camera receives the reflected infrared light or ultrasonic waves for focusing. Its optical principle is similar to the triangular ranging focusing method. There is also an energy method in the active type, which is used for the autofocus of low-end popular cameras and is widely used in various heads-up methods. Viewfinder camera. It is difficult for active focusing to focus on inclined and smooth surfaces. It is difficult to focus on bright and distant subjects. This is because the emitted light is reflected in other directions or cannot reach the subject. Because the camera actively emits light or waves, it can focus in low-contrast and low-light conditions. It can automatically focus on thin-line subjects and moving objects. The disadvantage is that it focuses when the subject absorbs light or waves. It is difficult and will be reflected by the glass, so it is difficult to focus through the glass.

2. Passive: that is, a method that directly receives and analyzes the reflection from the scene itself and performs automatic focusing. The advantage of this automatic focusing method is that it does not need to transmit the system itself, so it consumes less energy and is conducive to miniaturization. .It can achieve ideal autofocus on subjects with a certain brightness, and it can also focus well under backlight. It can automatically focus on objects with high brightness in the distance. It can focus through the glass. But the disadvantage is that it is difficult to auto-focus on subjects with thin lines. It is difficult to focus in low contrast and low light. It has poor auto-focusing ability on moving subjects. It has poor auto-focusing ability on subjects with polarized light. Poor. The focusing ability of black objects or mirrors is poor.

Active and passive autofocus methods have their own merits. Fortunately, there are two autofocus methods on general SLR cameras, which can be used complementary and automatically switch to give full play to their strengths and overcome their weaknesses. Passive autofocus methods are mostly used on SLR cameras. Focus method, so its focus is limited by the maximum aperture number. It is difficult to autofocus when the aperture is smaller than F8. For this reason, most SLR cameras have autofocus aidlights emitters that emit beams with infrared stripes to help focus. Auto focus on subjects of different textures. These fill lights don't work when the light is bright enough. What you need to pay attention to when using it is that since the active emission window is on the right side of the camera, do not block the emission window with your hands when holding the camera. You will not be able to focus when you block the emission window. There is no emission window for emitting auxiliary light on the professional camera body. , only when a flash is installed, the emission window on the light is used to emit auxiliary light for active focusing.

Edit features of this section

It has the following three characteristics

1 .Automatically determine the subject photographed by the photographer in some way;

2.Measure the distance between the subject and the camera sensor in some way;

3. Drive The motor pushes the lens's focusing device to the corresponding distance scale.

Edit this paragraph Autofocus principle

From a basic principle, autofocus can be divided into two major categories: one is distance measurement based on the distance measurement between the lens and the photographed target. Autofocus, the other type is focus detection autofocus based on clear images on the focusing screen.

1. Ranging autofocus

Ranging autofocus mainly includes infrared ranging method and ultrasonic ranging method.

Infrared ranging method The principle of this method is that the camera actively emits infrared rays as a ranging light source, and the focusing distance is calculated based on the geometric relationship between infrared light-emitting diodes.

Ultrasonic ranging method This method measures distance based on the time it takes for ultrasonic waves to propagate between the digital camera and the subject. Digital cameras are equipped with ultrasonic transmitting and receiving devices respectively. When working, the ultrasonic vibration generator emits continuous ultrasonic waves. After the ultrasonic waves reach the subject, they immediately return to be sensed by the receiver, and then the integrated circuit calculates and determines them based on the round-trip time of the ultrasonic waves. Focus distance.

Infrared and ultrasonic autofocus use active emission of light waves or sound waves to measure distance, which is called active autofocus.

2. Focus detection autofocus

Focus detection methods mainly include contrast method and phase method

a Contrast method This method is realized by detecting the contour edge of the image Auto focus. The clearer the outline edge of the image, the greater its brightness gradient, or the greater the contrast between the scene and the background at the edge. On the contrary, in an out-of-focus image, the outline edges are blurred, and the brightness gradient or contrast decreases; the farther out of focus, the lower the contrast. Using this principle, two photoelectric detectors are placed at equal distances before and after the CCD. The image of the photographed object is simultaneously split on the two detectors, and the contrast of the imaging is output respectively. When the absolute value of the contrast output of the two detectors is the smallest, it means that the focused image plane is just in the middle of the two detectors, that is, close to the imaging surface of the CCD, and the focusing is completed.

b Phase method This method achieves automatic focusing by detecting the offset of the image.

Place a grid plate composed of parallel lines at the position of the photosensitive CCD.

The lines are successively light-transmitting and opaque. Place two light-receiving elements symmetrically with the optical axis at appropriate positions behind the network board. The network board vibrates back and forth in the direction perpendicular to the optical axis. When the focusing surface coincides with the network board, the light passing through the light-transmitting lines of the grid board reaches the two light-receiving elements behind it at the same time. When out of focus, the light beam can only reach two light-receiving elements one after another, so there is a phase difference between their output signals. After the two signals with phase difference are processed by the circuit, the actuator can be controlled to adjust the position of the objective lens so that the focusing surface coincides with the plane of the grid plate.

3. Characteristics of various autofocus methods

Various autofocus methods have their own limitations. For example, in the focusing methods of infrared ranging and ultrasonic ranging, when the measured target has strong absorption of infrared light or ultrasonic waves, the ranging system will malfunction or the focus will be inaccurate; while the contrast method focusing detection is restricted by lighting conditions. , when the light is dim or the difference between the light and dark of the subject and the background is small, focusing will be difficult or even ineffective.

4. Application analysis

Many consumer-grade digital cameras on the current market use the contrast method for autofocus. From the principle of the contrast method, it can be seen that when the contrast output by the two detectors is different, The optimal state is when the absolute value of the value is minimum. We assume that the absolute value of the contrast difference output by the two detectors is m. To minimize m, the lens must be moved multiple times and then approximated successively using the difference method. Multiple moves The lens takes a lot of time, and digital cameras have certain requirements for focusing time. This is a contradiction in itself, so the compromise is to give a value that satisfies the use. For the time being, we assume that it is Q, As long as m < Q, we consider the focus to be successful.

So we can draw the following conclusions:

a The smaller the Q value is set, the higher the accuracy of autofocus and the slower the focusing speed. On the contrary, the larger the Q value, the lower the focusing accuracy and the faster the focusing speed.

b The greater the contrast of the image and the stronger the light, the faster the successive approximation speed of the difference method, and it is easier to meet the focusing conditions.

c The smaller the contrast of the image and the weak light, the slower the difference method's successive approximation speed, making it difficult to focus. When the light is very weak, it is impossible to complete focus.

Thus we can know that under different circumstances, we can set this Q value according to our needs to meet the requirements. The focus speed of current digital cameras is not adjustable and has been fixed in the fireware, but we can see the difference in focus speed from different settings of the camera.

We can simply divide the application of digital cameras into the following levels:

a High-precision level This level focuses the slowest and has high light requirements.

b Normal precision level This level has the most general focus and is not too demanding on light.

c sub-precision level This level focuses slightly faster, but the accuracy is reduced.

d Low-precision gear This gear has the fastest focusing speed, but the focusing accuracy is very low.

5. Examples

Let’s analyze the application of different focus speeds based on FZ10:

As a digital camera application, it is easy for us to Classify the various curing modes of the FZ10:

The macro mode is the small flower mode of the FZ10 and should be a high-precision mode. Generally, the light is good when shooting. It doesn’t matter if the autofocus is slower. The main thing is to obtain the highest clarity. image.

The modes of FZ10 such as A/S/M should belong to the normal precision level. This is a compromise mode. Although it is not the highest precision, it can get a good autofocus speed.

The tracking focus mode is the sports shooting mode of the FZ10, and the focusing speed is slightly faster.

The FZ10 video mode has very low accuracy, and requires fast focusing speed, so the low-precision mode is suitable for it.

The following are two test pictures, using the small flower mode and M mode of the FZ10 respectively. The exposure parameters are exactly the same. They were shot with a tripod and autofocus and self-timer. The distance is about 6m and the focal length is 432mm.

From the actual test of FZ10, the focusing speed of macro mode is significantly slower than that of normal mode, but the focusing accuracy is higher than that of normal mode.

Edit this paragraph's autofocus methods

DSLR generally includes the following three autofocus methods:

1. Single autofocus

2. Continuous autofocus

3. Automatic switching/artificial intelligence servo/nearest subject-first dynamic autofocus

First, let’s take a look at the most commonly used single-shot autofocus. The working process is started by pressing the shutter halfway, and the focusing process continues until the focus is accurate. Once the processor determines that the focus is accurate, the shooting process is completed by fully pressing the shutter button, and the autofocus system stops working.

If the subject moves after the focus completes beep but before the shutter button is fully pressed. Since it is "single-shot" autofocus, you may see a blurry picture after fully pressing the shutter button. Of course, this is an exaggeration. It is said to better pave the way for explaining continuous autofocus.

Due to the characteristics of single-shot autofocus, it is the most suitable choice when shooting stationary objects (such as landscapes, macro photography, group photos of people, etc.). This method of automatically locking the focus after focusing, and you can recompose the shot by pressing the shutter halfway and holding it down, is very easy to operate.

Second, let’s take a look at continuous autofocus, which is most suitable for shooting moving objects. Since the single-shot autofocus method mentioned above cannot "track" moving objects very well, it brings a lot of trouble to some shots, so the continuous autofocus method was born.

Unlike single autofocus, continuous autofocus continues to work after the processor "thinks" the focus is accurate, and the focus is not locked. The purpose is that when the subject moves, the autofocus system can drive the lens adjustment in real time according to changes in focus, so that the subject remains clear. Of course, the camera's focus frame must also be aligned with the subject in real time, so that you don't have to worry about the subject being out of focus when you fully press the shutter.

Continuous autofocus is mostly used for shooting objects in motion, such as shooting athletes in sports competitions, shooting spokespersons in press conferences, and capturing wonderful moments of animals in motion, etc. Moreover, taking into account the advantage of DSLR, which does not require film, as long as it is combined with the high-speed continuous shooting function, it is relatively easy to take a set of wonderful photos.

Finally, we have to take a look at how automatic switching/artificial intelligence servo/recent subject-first dynamic autofocus works (for convenience of description, hereafter referred to as smart focus).

Theoretically, with single-shot autofocus and continuous autofocus, it should be able to meet the needs of various shooting scenarios. However, in the long-term actual shooting process, some problems will still be discovered, such as the problem of relatively large power consumption of DSLRs that are in continuous autofocus for a long time. Of course, the most important thing is to worry about a subject that may move at any time changing from a relatively static state to a moving state, or vice versa.

The emergence of smart focus is a good compromise to solve the above-mentioned problems. This method of combining single-shot autofocus and continuous autofocus is more suitable for use in scenes where the subject is constantly switching between motion and stillness. The DSLR automatically selects the focus method based on the moving speed of the subject. The internal distance measurement component continuously measures the image in the autofocus area and transmits it to the processor in real time. Select Single AF when the subject is stationary, and Continuous AF when the subject is moving. Since the switching is done by the processor, all you have to do is press the shutter.

It should be noted that the first two mentioned autofocus methods are the most common and commonly used, and no matter which DSLR manufacturer they are, they basically name them according to the above names. No matter what the name of the third mentioned method is, its working principle is basically the same. It selects which point to autofocus based on which focus point is closest to the subject. The more autofocus points there are, the corresponding The probability of the subject being accurately focused increases. Auto-switching focus is mostly used by Komei, artificial intelligence servo focus is used in Canon products, and recently subject-first dynamic autofocus is equipped on Nikon's high-end DSLRs.