Camera autofocus how does it work

The process of autofocusing generally works as follows: 1 An autofocus processor AFP makes a small change in the focusing distance. This entire process is usually completed within a fraction of a second. For difficult subjects, the camera may fail to achieve satisfactory focus and will give up on repeating the above sequence, resulting in failed autofocus.

This is the dreaded "focus hunting" scenario where the camera focuses back and forth repeatedly without achieving focus lock. This does not, however, mean that focus is not possible for the chosen subject. Whether and why autofocus may fail is primarily determined by factors in the next section. The photographic subject can have an enormous impact on how well your camera autofocuses—and often even more so than any variation between camera models, lenses or focus settings.

The three most important factors influencing autofocus are the light level, subject contrast and camera or subject motion. An example illustrating the quality of different focus points has been shown to the left; move your mouse over this image to see the advantages and disadvantages of each focus location. Note that each of these factors are not independent; in other words, one may be able to achieve autofocus even for a dimly lit subject if that same subject also has extreme contrast, or vice versa.

This has an important implication for your choice of autofocus point: selecting a focus point which corresponds to a sharp edge or pronounced texture can achieve better autofocus , assuming all other factors remain equal. In the example to the left we were fortunate that the location where autofocus performs best also corresponds to the subject location.

The next example is more problematic because autofocus performs best on the background, not the subject. Move your mouse over the image below to highlight areas of good and poor performance.

In the photo to the right, if one focused on the fast-moving light sources behind the subject, one would risk an out-of-focus subject when the depth of field is shallow as would be the case for a low-light action shot like this one. Alternatively, focusing on the subject's exterior highlight would perhaps be the best approach, with the caveat that this highlight would change sides and intensity rapidly depending on the location of the moving light sources.

If one's camera had difficulty focusing on the exterior highlight, a lower contrast but stationary and reasonably well lit focus point would be the subject's foot, or leaves on the ground at the same distance as the subject. What makes the above choices difficult, however, is that these decisions often have to be either anticipated or made within a fraction of a second.

Additional specific techniques for autofocusing on still and moving subjects will be discussed in their respective sections towards the end of this tutorial. The robustness and flexibility of autofocus is primarily a result of the number, position and type of autofocus points made available by a given camera model. One is Contrast Detection and the other is Phase Detection.

Autofocus is a highly sophisticated and useful tool. Understanding which mode to use and how to choose and move an active AF Point will help you to be more selective and creative with your photography. Phase-detection auto-focus is a newer and more advanced method of focussing an image. The technique is more commonly seen on DSLRs rather than smartphones or compact cameras.

Recently, the technology has appeared on high-end smartphones from Apple and Samsung. With phase-detection auto-focus, your camera analyses the light as it passes through the lens.

It compares light from two apertures on opposite sides of the lens. If the image is properly focussed, the two light rays will give an overlapping intensity profile. According to Samsung, the Galaxy S5 can focus in an image in only 0. More importantly, when the image is out-of-focus, a comparison of the light intensity profiles will tell you how much the focus will need to be adjusted. It takes only around 0. This is approximately 3 times faster than contrast-detection. In fast-moving scenes e.

You can do continuous auto-focus. With contrast-detection and phase-detection auto-focus, we analyse the light that comes in to the camera. Collectively, the two methods are known as passive auto-focus. In contrast, laser auto-focus is a very different system. Instead, the phone will emit its own beam of infrared light towards the object.

It calculates the time for the light to be reflected and is then able to calculate the distance to the object:. After the distance to the object has been determined, the lens is moved to the relevant position. According to LG, the whole process only takes 0. This makes it a much quicker method than contrast-detection auto-focus. It uses the information to quickly focus the camera. According to LG, it only takes 0. It works in low-light conditions.

See more cool camera stuff pictures. Panasonic and Matsushita Electric Corporation of America. Contents What is Autofocus? Is Autofocus Always Accurate and Faster? Autofocus Video Cameras. What is Autofocus? These features include: Advertisement. Automatic film advance Automatic flash Automatic exposure.

Looking for a Tripod to Steady your Images? Active Autofocus In , the Polaroid Corporation used a form of sound navigation ranging SONAR , like a submarine uses underwater, to bounce a sound wave off the subject. Triangulation Amount of infrared light reflected from the subject Time. A source of infrared light from an open flame birthday cake candles, for instance can confuse the infrared sensor. A black subject surface may absorb the outbound infrared beam. The infrared beam can bounce off of something in front of the subject rather than making it to the subject.

Passive Autofocus " ". This type of lens has its focus set at the factory, and it typically works best with a subject distance of about 8 feet. Four feet is about as close as you can get to the subject with a fixed-focus camera. When you look through a fixed-focus camera, you typically do not see the square brackets or circles found in an autofocus camera. However, you may see a "flash ready" indicator. SLR cameras with interchangeable lenses typically use the passive autofocus system. Cameras without interchangeable lenses typically use active infrared , and you can see the emitter and the sensor on the front of the camera.

Go outdoors and aim the viewfinder at an area of the sky with no clouds, power lines or tree limbs. Press the shutter button halfway down. If you get a "focus okay" indication, it's an active autofocus system. If you get a "focus not okay" indication, it's a passive autofocus system. The CCD cannot find any contrast in a blue sky, so it gives up.

The two main causes of blurred pictures taken via autofocus cameras are: Mistakenly focusing on the background Moving the camera while pressing the shutter button. Compose the picture so that the subject is either in the left third or the right third of the picture. This makes for pleasing pictures. You will come back to this position. Move the camera right or left so the square brackets in the center of the viewfinder are over the actual subject.

Press and hold the shutter button halfway down so the camera focuses on the subject. Keep your finger on the button. Slowly move your camera back to where you composed the picture in step 1. Press squeeze the shutter button all the way down. It may take some practice to do it right, but the results will be great!

On most consumer-grade autofocus cameras, use manual focus when: You have a zoom lens on an active autofocus camera, and your subject is more than 25 feet away.