Focus Methods, Modes & Areas

January 03, 2019  •  Leave a Comment

Focus Methods, Modes & Areas

By Marc F Alter

 

 

Automatic Focus Modes & Focus Areas:

Today’s digital cameras have several different options for how and when your camera will automatically focus. Obtaining a sharp image involves the capabilities of your camera (ie; can the camera automatically focus in low light) as well as making several different decisions. Some of these decisions are based on your Subject (is your subject stationary or moving), some based on your Exposure Settings (effected by Shutter Speed & F-Stop’s Depth of Field), and some based on your camera’s Focus Settings at the time when you take the picture.

 

For Focus Settings, the first decision you need to make is how you want your camera to focus; Manually or Automatically. Depending on the camera, the setting for this is usually found either on the camera, on the lens, in the camera’s Settings Menu and/or any combination of these.

  1. (M) Manual Focus – This setting tells the camera not to auto-focus. When set, you will need to manually focus, usually by turning the Focus Ring on the lens until your image is sharp in the viewfinder. Many lenses also have distance markers just above the Focus Ring which allows you to manually set the focus on the distance from your camera to your subject. Manual Focus is usually most beneficial when your subject is stationary and your want to ensure a really sharp image (and you have good eyesight). This method is also used when you have a fast-moving object and you don’t want to lose precious seconds waiting for your camera to Focus Lock on your subject. Extremely sharp focus can be obtained using the camera’s Live View and then Zooming on the area where you want to focus.

 

  1. (A or AF) Automatic Focus - This setting tells the camera to initiate its auto-focus functions. With this setting turned on, you would typically (most camera’s default setting) point your camera to your subject, press your Shutter Release Button ½ way down to focus and then all the way down to take the picture.

 

If you are using Auto-Focus there are several additional settings you must make decisions on so your camera knows how you want it to function. These settings are known as Focus Modes and Focus Areas.

 

  1. Focus Modes tell the camera how often you want it to automatically focus on your subject. There are **usually three types of Focus Modes; (S) for Single Shot Focus and (C) for Continuous Shot Focus and (A) for Auto Select.

 

  1. (AF-S or S) Auto-Focus Single Shot Focus (Also known as Single-Area Focus Mode or One Shot AF”) - This setting tells the camera to Focus once after it Locks onto the Subject. This setting is best used when you are taking a picture of a stationary object.

 

  1. (AF-C or C) Auto-Continuous Shot Focus (Also known as AI Servo Focus Mode) - This setting tells the camera to constantly Focus and is best used when you are taking a picture of a moving object. As your object moves, the camera will cause the lens to maintain its focus.  In many of today’s cameras, the focus calculations can include a prediction of where the subject will be when the picture is taken and will automatically move the lens focus area to capture this.

 

  1. Auto (also known as Single/Continuous Hybrid Mode) – Some cameras have an Automatic Focus Selection mode that acts as a hybrid between Single Shot and Continuous Shot Focusing. With this mode, the camera detects if the subject is stationary and if so, uses Single Shot Focus. If the Subject moves, the camera will automatically switch to Continuous Shot Focus.

 

** Some newer more advanced DSLRs have similar functions using different names as well as some additional advanced and/or hybrid auto-focus features. Some examples are:

  • Canon - One-Shot AF, Predictive AI Servo AF (AI Servo AF III), AI Focus AF, Manual focus
  • Nikon - Single-servo AF (AF-S), Full-time-servo AF (AF-F), Continuous-servo AF (AF-C): predictive focus tracking automatically activated according to subject status, Manual focus (M)
  • Sony - AF-A (Automatic AF), AF-S (Single-shot AF), AF-C (Continuous AF), DMF (Direct Manual Focus), Manual Focus
    1. Focus Areas (Also known as Focus Points) tell the camera how much of the image displayed in the viewfinder you want it to automatically focus on. Focus points are laid out in certain parts of the frame. The number of Focus Points available for use will vary from camera to camera (less expensive DSLRs typically have fewer Focus Points and simple Auto-Focus Systems while more expensive DSLRs typically have more Focus Points with more complex configuration options).

 

It is important to note the number of Focus Points is not the only factor in the Auto-Focus calculations but also the Type of Focus Sensors used (See below titled “Types of Auto-Focus Points “).

 

  1. Depending on your camera and model, there are usually several different options for choosing the Focus Area setting. The three most common types of Focus Areas are Single Point, Multi-Point and Automatic:

 

  1. Single Point Focus Area – the camera uses only a single stationary focus point to determine accurate focus. This mode is typically used when the scene has many different objects that might confuse the camera’s automatic Focus and when you want to manually identify the Subject to make sure it is as sharp as possible. Typically used for stationary objects such as landscapes, architecture and macro photography.

 

  1. Multi-Point Focus Area (Also known as Group Area) – the camera uses a set number of focus points that are grouped together. When using this option, you typically have the ability of selecting the number of focus points you wish to use. This number will vary based on the camera model but may allow for groups of 9, 21, 51. This mode is typically used for small Subjects that move fast erratically.

 

  1. Auto Area (also known as Automatic AF Point) –the camera detects the scene and determines the number of Focus Points used. This is typically used in many DSLRs, Point-And-Shoot and/or Cell Phone camera. In many of these cases, there may be a “Scene Setting” or “Face Detection” that will help determine how the camera focuses. The biggest issue with this is the camera, not the photographer, is determining where the Subject is and what should (and should not be) in Focus.

 

  1. Some newer more advanced DSLRs have similar functions using different names as well as some additional advanced and/or hybrid Focus Point features.

 

  1. Dynamic Single Point Area (Also known as AF Point Expansion) – typically the camera uses only a single focus point to determine accurate focus but if the subject moves, the camera will attempt to maintain focus on that subject. This is typically used for very fast moving &/or objects moving unpredictably.

 

  1. 3D Tracking – the photographer manually selects the focus point(s). When the shutter-release button is kept pressed halfway after the camera has focused, the photographer can change the composition and the camera will automatically choose a focus point(s) to keep the Subject in focus. Somewhat counter-intuitively, this mode is typically used not to capture moving objects, but rather to capture stationary objects while the photographer moves and/or changes his/her composition.

 

Note: When 3D Tracking is used in Nikon DSLR cameras, 11 Focus Point are used and the camera uses the Subject’s color to track the Subject. This mode may be problematic when the Subject’s color is the same as other objects and/or background color.

Separating Exposure & Focus (Back Button Focus)

The typical default setting on most cameras is combing Exposure and Focus. For most cameras, you point your camera to your subject, press your Shutter Release Button ½ way down. The camera then determines the Exposure and Focus. You then press the Shutter Release all the way down to take the picture. Using this method, your Subject is Exposed and Focus established with the simple press of the Shutter Release. This method is quick, simple and intuitive. The downside to this approach is that your Exposure and Focus are taken at the same time, causing you to lock your Focus and then recompose for Exposure.

 

Although this works for many images, there may be times when you wish to separate Exposure and Focus. This is especially important when taking pictures of fast-moving objects (such as birds in flight) or when your Subject and scene Exposure are vastly different. A method to address this situation is called “Back Button Focus” (BBF). This method allows you to use one button (usually a button located on the back of the camera) to Focus and then another button (Shutter Release) to create the Exposure and take your picture.

 

The disadvantages of using “Back Button Focus” is that it requires using two separate buttons to take a picture which is not simple or intuitive. You will need to “relearn” taking your pictures from using just the Shutter Release to using both the Shutter Release and the BBF Button. The advantages of using “Back Button Focus” however are worth the learning curve. Separating Exposure and Focus Controls allows you to have much better control over both your Focus and your Exposure.

 

Using BBF is accomplished in the camera’s Settings Menu. Not all cameras have this function and depending on your camera’s capabilities, there may be several different options to choose from. Some of these may include using both the Shutter Release and BBF and/or selecting the camera’s AF-On/AF-Off Button for BBF or selecting another button for BBF. Before deciding to try using Back Button Focus, you should read your camera’s manual and/or watch Internet videos about your camera and its BBF capabilities.

More Information Than You Really Want Or Need To Know

 

Auto-Focus: How Does The Camera Do This

Today’s DSLR (Digital Single Lens Reflex) cameras are really highly advanced mini-computers, performing vast calculations at incredible speeds. Most DSLRs will automatically change how it performs auto-focus functions based on using either Active or Passive Methods:

  1. Active Auto-Focus (Also known as Active AF) – The camera “shoots” a red beam to your subject. This beam bounces back to the camera’s sensors, the camera calculates the distance to the subject and then adjusts the lens to focus at this distance. This function typically works for stationary subjects that are within about 15-20 feet, in either bright or dim light conditions. 

 

  1. Passive Auto-Focus (Also known as Passive AF or Phase Detection) – The camera uses its sensors to detect contrasts within the image and then moves the lens back and forth until it detects sharpness. There are two types of methods used; Phase Detection and Contrast Detection:

 

  1. Phase Detection - The camera uses special sensors to detect contrast from the light that goes through the lens. Phase Detection is typically very fast as it uses one or more “Focus Points” and not the entire image. As a result, this method is usually used for moving subjects.

 

  1. Contrast Detection - The camera uses special sensors to detect contrast in the image itself. This method can only be done in Live View mode as it requires light to directly reach the sensor and therefore DSLRs must have their mirrors raised for this to work. As a result of these mechanics, this method is usually use for stationary subjects. This method can use different parts of the image to focus and is often more accurate than Phase Detection, especially when used in low-light conditions.

Auto-Focus Assist (AF Assist) – A feature on some cameras that helps the camera to autofocus by emitting a light onto the subject so the camera can better detect the contrasts. Usually used during Passive Auto-Focus.

 

Types of Auto-Focus Points

There are two main different types of Auto-Point Sensors used in most DSLRs; Vertical and Cross-Type. Vertical sensors detect contrast but only on vertical lines. Cross-Type sensors detect contrast on both vertical and horizontal lines. As a result, Cross-Type Sensors are considered more accurate and thus the more Cross Type Sensors your camera has, the more accurate your auto-focus will be.  When marketed, a camera’s manufacturer will usually list both the total auto-focus point sensors as well as the number of Cross-Type Sensors (ie; Nikon’s D5 has 153 Auto-Focus Points of which 99 are Cross-Type Sensors. The Canon 1DX has 61 Auto-Focus Points of which 41 are Cross-Type Sensors). Some of the newer Mirrorless camera (ie; Sony a6500 or Sony A7RIII, do not employ Cross Type Sensors).

 


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