CamCyclopedia
Contents
- What is AF?
- Camera category based on focus positioning
- Types of AF based on focus principles
- Category of AF based on the target of focus
- Why does AF not work in these scenarios?
- AF in Galaxy Camera
What is AF?
AF (Auto Focus) [1], the camera feature, automatically changes the position of the focusing lens to make sure the subject comes out clear in a frame.
Camera category based on focus positioning
Cameras can be differentiated as below based on how they focus.
AF -Auto Focus- Camera
You will find auto-focus in most professional cameras and cell phone cameras today.
MF -Manual Focus- Camera
Manual focus cameras were older film cameras where the user had to manually position the focus.
FF -Fixed Focus- Camera
Fixed focus cameras, such as a car dash-cam, can only focus on subjects at a certain distance and make sure they are clearly visible.
Types of AF based on focus principles
AF types can be distinguished as follows according to the principles of focus they use.
Active AF
A camera with an active AF emits a signal with a specific wavelength (laser, IR, ultrasound, etc.) and measures the distance from the subject by the time taken (or angle) for the signal to reflect back to the subject.
When Active AF was first installed in cameras, the distance from the subject was measured using the rule of triangulation, where a camera-generated ray would hit the subject and bounce back at a certain angle.
With advances in technology, now we can measure the distance from the subject with great accuracy using the time of flight (ToF) of the light wave from the camera reflected back to the subject.
Strictly speaking, it is more of a distance-measuring technology than an AF technology. But, modern cameras use it as an AF distance-measuring aid.
Advantages
AF can measure distances accurately.
It can measure distances even when you are shooting in the dark.
Disadvantages
Optical signals (Laser, IR) are absorbed by black bodies and account for low distance‑measurement accuracy.
Accuracy also drops when there are transparent obstructions such as glass between the camera and the subject.
Changes in lens refractive index with temperature also cause a decline in accuracy.
Passive AF
Passive AF finds the focus-locked lens position by analyzing the light (image) entering the camera.
It is currently the mainstream AF technique adopted by most cameras and is categorized into Range Finder AF/Contrast AF/Phase AF/Hybrid AF according to its working principle.
1. Range Finder AF
The camera houses a fixed reflective glass and lens and is also equipped with a secondary glass with adjustable angles. The camera focuses by matching the image formed on both these reflective surfaces.
This is the oldest version of passive AF. The focus is less accurate because it resorts to a different light path from the lens to autofocus.
2. Contrast AF
This AF was often used in early digital cameras.
It changes the lens position slightly from frame to frame and analyzes the image formed in the camera to figure out the lens position where the subject is at its sharpest, as shown in the image below.
Advantages
AF accuracy is extremely high because it analyzes the image formed in the camera to find the lens position where the subject is the sharpest.
Disadvantages
The slight movement of the lens from frame to frame, requires the use of many frames (time) to focus, resulting in a slow AF speed.
3. Phase AF
This AF method focuses just like the human eye which perceives a particular image by taking it apart (as seen by each eye) and then conjoining the two into one. It separates the image incident on the lens to the left and right and then matches the position of the subject in both to auto-focus.
Most AF cameras in existence use this method.
It analyzes just one frame of the image footage and moves the lens directly to the focus position, as shown in the image below.
Advantages
AF is very fast because it only needs one frame to find the lens position to focus.
Disadvantages
It is slightly less accurate than contrast AF because it doesn't directly analyze the sharpness of the image.
4. On-Sensor Phase Detection AF
Take a look at the structure of a traditional digital single-lens reflex camera (DSLR camera) in the figure below.
In a DSLR (or SLR) camera, the phase AF sensor located under a translucent mirror (Quick Return Mirror) inside the camera body was used for phase AF.
However, in digital compact still cameras (Digital CSC) and mirrorless cameras, the quick return mirror is removed to make the camera smaller and thinner. The lack of a quick return mirror makes it difficult to embed a phase detection AF in them.
This problem is solved by putting AF pixels on the image sensor. This circumvention helps apply the phase detection AF in the absence of an AF sensor.
Initially, the AF was not as accurate because AF pixels formed a very small part of the sensor as shown on the left side of the image below. But, since the development of the dual-pixel (or 2PD) sensor, where all pixels are composed of AF pixels (as shown on the right side of the image below), the accuracy of phase detection AF is as good as the contrast AF.
Hybrid AF
Hybrid AF simultaneously uses two or more of the AF modes described above, such as active AF, contrast AF, phase AF, etc.
Phase-detection AF is very fast but has the disadvantage of being somewhat inaccurate. Hybrid AF uses phase-detection AF to move to an in-focus position and then use contrast AF to find the accurate focus.
In very dark environments, it also uses active AF to quickly move the focusing lens to a position according to the distance measured, and then the phase AD or contrast AF takes over to find precise focus.
Category of AF based on the target of focus
AF methods can be distinguished as follows depending on how you decide to focus on the subject.
Continuous AF
Continuous AF detects any movement in the camera, the subject or any change in the scenery and automatically focuses on a specific area of the screen (ROI: Region of interest).
Depending on how you set up your ROI, it can be categorized as follows
- Center AF: Focuses at the centre of the frame.
- Multi AF: Divides the entire screen into several areas to obtain AF data and focuses on areas where the subject is the closest.
- Face Detection/Eye/Pet AF: Detects a person (or a person’s eye or animal) on the screen and focuses on that area.
- Tracking AF: Tracks the movement of an object that the user points out by touching and continuously focusing on that object.
Normal AF
For flash and timer shots, the camera focuses once when the shutter is pressed and does not autofocus again before the shot is complete.
Touch AF
The camera focuses once on the area touched by the user and does not auto-focus again until the camera shakes or the screen flips.
Focus Lock
You can long-press and use this to force lock AF after the autofocus is complete.
Why does AF not work in these scenarios?
In very dark environments
Cameras today use multi-frame processing technology to combine several frames to create a bright picture, even if the picture is clicked against a very dark setup almost without light.
However, AF only has a single, unprocessed dark image before the photo is clicked, to rely on for focus. This makes it difficult to focus in very dark conditions.
Subject without pattern
AF is the process of finding the lens position that renders the sharpest image of the subject in the area you want to focus on.
If there are no patterns on the screen to gauge the sharpness of the image, such as a white wall or a clear, cloudless sky, it gets difficult to find the lens position to focus.
Subject with fine/subtle patterns
If you are trying to focus on a subject with a very fine pattern, such as a monitor, a TV, or a mosquito net, you will experience moiré [2], optical illusions of strange patterns as shown in the image below.
Moiré can create anomalous AF data. The AF confuses an out-of-focus lens position with the in-focus position, making the focus awry.
Super-thin subject
Typically, AF analyzes the sharpness about a quarter of the way around the centre of the screen.
So if you're trying to focus on something thin or small, like a finger or a ballpoint pen, the AF area will contain more background than your target subject, and the background will come into focus.
AF in Galaxy camera
2010 (Galaxy S)
The Galaxy camera began with contrast AF embedded in it.
2011 (Galaxy S2)
Touch is first introduced to the Galaxy camera and now you can focus wherever you want.
2013 (NX300)
This change is not pertinent to the Galaxy camera. The on-sensor phase detection AF is applied to the Samsung Electronics Mirrorless Camera NX300.
2014 (Galaxy S5)
The on-sensor phase detection AF is applied to a mobile phone camera for the first time in the world and the AF speed increases drastically. (0.6 seconds -> 0.4)
2015 (Galaxy S6)
Tracking AF is applied to the Galaxy camera for the first time and the camera can now focus on a moving subject.
2016 (Galaxy S7)
The world's first dual-pixel AF sensor in a mobile phone camera dramatically improves AF speed (0.4 sec. -> 0.2 sec.)
A manual focus that allows users to adjust the focus as they want is also added in Pro mode.
2017 (Galaxy S8)
The advent of clearer selfies with the first-ever application of AF to the front camera.
2017 (Galaxy Note8)
For the first time, the Galaxy camera has a telephoto (2x optical) camera, allowing for clearer shots from a greater distance.
2018 (Galaxy S9)
For the first time in the world, apertures are fitted onto cell phone cameras, allowing for brighter and clearer shots in low-light conditions.
2019 (Galaxy S10)
The Galaxy camera gets an ultra-wide-angle (0.5x optical) camera for the first time, allowing for more expansive landscape photography.
2020 (Galaxy S20)
The Galaxy camera now has a high-megapixel (108M) sensor for clearer shots.
Since the high-megapixel sensor is not a dual-pixel phase detection sensor, a TOF camera is added to achieve the same level of AF performance as a dual-pixel phase detection sensor.
Also, the world's first super-telephoto (10x optical) camera is added to a cell phone camera, enabling powerful zoom shots like moon photos.
2021 (Galaxy S21)
The Galaxy camera features AI AF for the first time. The AI analyzes the frame and focuses wherever you want it to focus.
Also, for the first time, the Galaxy Camera has two telephoto lenses, enabling sharp/clear photos even at different zoom levels.
■ Related Content
Quad Pixel PD AF - Samsung Members
Reference
[1] AF reference documents: https://ko.wikipedia.org/wiki/자동_초점
[2] Shortcut to Moiré - Samsung Members
6 Comments
