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Exactly how Phase Detection Autofocus Works

When it comes to DSLR technology, there seems to become quite a bit of confusion on how exactly stage detection autofocus works. While for many individuals this might not be a topic of great curiosity, if you are wondering how and exactly why a camera could have an autofocus problem, this article will shed some light into what goes on inside the camera in terms of autofocus if a picture is taken. There is an overpowering amount of negative feedback on autofocus issues on such fine equipment as the Canon 5D Mark 3, Nikon D800 , Pentax K-5 as well as other digital SLR cameras and it seems like the majority of photographers do not seem to understand that the actual problem is not necessarily with a specific design or type of a camera, but instead with the specific way these digital cameras acquire focus. If you search on the Internet, you will discover thousands of autofocus reports on all sorts of DSLRs dating back 10+ years. Therefore, the front focus and back concentrate issues we see in contemporary cameras are not anything new – they have been there ever since the first DIGITAL SLR with a phase detect sensor was developed.

How DSLR Digital cameras Work

To understand this problem in more detail, it is important to get to know what sort of DSLR digital camera works first. The normal DSLR illustrations only show just one reflex mirror positioned at a 45-degree angle. What they don’ t display, is that there is a secondary mirror at the rear of the reflex mirror that demonstrates a portion of light into a stage detect sensor. Take a look at the beneath simplified illustration that I made from an example Nikon D800 image:

Here is the description of each number proven in the above illustration:

  1. Ray of light
  2. Main/Reflex Mirror
  3. Secondary Mirror, also known as “ Sub-Mirror”
  4. Camera Shutter plus Image Sensor
  5. Odd pin (1. 5mm hex) just for adjusting the Main Mirror
  6. Eccentric pin (1. 5mm hex) for adjusting the Secondary Reflection
  7. Phase Detect Messf¨¹hler (AF Sensor)
  8. Pentaprism
  9. Viewfinder

Let’ s take a look at what goes on inside the camera when a picture is usually taken. Light rays enter the zoom lens (1) and make it into the digital camera. A partially transparent main looking glass (2) is positioned at a 45 degree position, so it reflects most of the light vertically into the pentaprism (8). The pentaprism magically converts the vertical lighting back into horizontal and reverts this, so that you see exactly what you get once you look through the viewfinder (9). Some of light goes through the main looking glass and gets reflected by the supplementary mirror (3) that is also tilted at an angle (54 degrees on several modern Nikon cameras, as highlighted above). Next, the light reaches the particular Phase Detect / AF Messf¨¹hler (7), which redirects it to some group of sensors (two sensors for each AF point). The camera after that analyzes and compares images out there sensors (similar to how concentrate is evaluated on a rangefinder) and when they do not look identical, it advices the lens to make proper changes (see below for more details).

While the above process appears more or less straightforward, there is one significant problem with this approach. The Phase Identify sensor is the one that instructs the zoom lens to make proper adjustments, while the picture is captured by a completely different gadget – the sensor on the back again of the camera. Why is this an issue? Remember, when you take a picture, both main and the secondary mirrors switch up, the shutter is opened up and the light from the lens straight hits the camera sensor (4). For phase detection autofocus to operate correctly, the distance between the lens install and the camera sensor, as well as the range between the lens mount and the Stage Detect sensor should be identical . If there is even a minor deviation, autofocus will be incorrect. Besides this, if the angle of the secondary reflection is not precisely what it should be, it will also lead to autofocus issues.

Exactly how Phase Detect Sensor Works

As I have already said over, the phase detect system functions similarly as rangefinder cameras. Gentle that gets bounced off the supplementary mirror is received by several small image sensors (depending about how many focus points an AF system has) with microlenses over them. For each focus point the thing is in a viewfinder, there are two small sensors for phase difference – one for each side of the zoom lens, as shown in the illustration along with the page (7) (the representation over-exaggerates this behavior by displaying two separate light beams reaching 2 separate sensors. In fact , there are far more sensors than two on a contemporary phase detect device and these detectors are located very closely to every other). When the light reaches both of these sensors, if an object is in concentrate, light rays from the extreme edges of the lens converge right in the heart of each sensor (like they would with an image sensor). Both sensors could have identical images on them, indicating that the thing is indeed in perfect focus. In case an object is out of focus, the light might no longer converge and it would strike different sides of the sensor, because illustrated below (image courtesy of Wikipedia):

Figure 1 in order to 4 represent conditions where the zoom lens is focused (1) too near, (2) correctly, (3) too far and (4) way too far. It can be seen through the graphs that the phase difference involving the two profiles can be used to determine not merely in which direction, but how much to alter the focus to achieve optimal focus. Remember that in reality, the lens moves rather than the sensor.

Since the stage detect system knows if a subject is front focused or back again focused, it can send exact guidelines to the camera lens on which method to turn its focus and by just how much. Here is what happens when a camera receives focus on a subject (closed-loop AF operation):

  1. The light that will passes through the extreme sides from the lens is evaluated by 2 image sensors
  2. Depending upon how the light reaches the image sensors, the particular AF system can determine if a subject is front or back concentrated and by how much
  3. The particular AF system then instructs the particular lens to adjust its focus
  4. The above is repeated as often as needed until perfect concentrate is achieved. If focus can not be achieved, the lens resets plus starts reacquiring focus, resulting in concentrate “ hunting”
  5. As soon as perfect focus is achieved, the particular AF system sends a verification that the object is in focus (a green dot inside the viewfinder, the beep, etc)

All this happens in a fraction of your time, which is why phase detection system is considerably faster than contrast detection system (which relies on changing focus back and forth till focus is achieved, with lots of picture data analysis happening on the picture sensor level).

The particular phase detection/AF system is a very complicated system that sees improvements almost every time when a higher end camera series is refreshed. Over the years, the number of autofocus points have been increasing, as well as the variety of more reliable cross-type autofocus factors. For example , the Canon 1D X and the Canon 5D Tag III have a whopping 61 concentrate points, 41 of which are cross-type. Take a look at this complex matrix associated with autofocus sensors on the camera:

Not only have the number of AF points increased, but also their dependability. Most modern professional cameras today include extremely fast and highly configurable autofocus systems that can continuously track topics and acquire focus.

DIGITAL SLR Autofocus Problems

From this article you can see above, the phase detection autofocus system is very complex and needs high precision to get accurate outcomes. Most importantly, the phase detect/AF program must be properly installed and in-line during the manufacturing process. If there is a slight deviation, which does occur quite a bit in manufacturing, autofocus will be off. This is the main reason why stage detect has been the source of difficulties pretty much since the first DSLR having a phase detect sensor came out. Knowing these possible deviations, all DIGITAL SLR manufacturers developed a high precision calibration system that takes this into consideration and allows for individual camera calibration during the inspection and quality reassurance (QA) process. If a phase identify sensor alignment problem is detected, the machine performs automatic computerized testing that will goes through each and every focus point plus manually adjusts it in the digital camera. The points that are off are usually re-calibrated and re-adjusted, then the payment values are written into the digital camera firmware. Think of this as a similar procedure to AF Fine Tune / AF Micro Adjust that happens on the stage detection level, except it is finished each AF focus point individually.

In my next post, I will talk about the Nikon D800 asymmetric focus issue (the still left AF issue), why it occurred and what Nikon is doing to fix this.

If you have appreciated this article, please check out our in-depth Level 1 Digital photography Basics Course , where we all explore all the basics of digital photography in much more detail. It is a rigorous, 5+ hour course with sufficient material to not only get you began today, but also to serve as the reference material in the future.

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