Saturday, September 24, 2011

Understanding Nikon's Autofocus, AF-Area, and Release Modes

The following article is presented in a generic way for most newer Nikon DSLR users. Some mode names may vary with older Nikons. This type of information is found in my Mastering the Nikon DSLR series books on Amazon.com—only the books have full color graphics, step-by-step mode setup information, and much deeper detail. Please consider buying whichever of my books (or eBooks) support your Nikon. I try very hard to make things understandable for my readers. See if you agree:

Autofocus and release modes are active settings that you’ll deal with each time you use your camera. Unlike adjusting settings in the menus, which you’ll do from time to time, you’ll use autofocus and release modes every time you make an image or movie.

To take pictures and make movies you need to be very familiar with these settings, so this is a very important chapter for mastery of your Nikon. Grab your camera and let’s get started!

Nikon DSLRs have two types of autofocus built in, with different parts of the camera controlling AF in different shooting modes. Taking pictures through the Viewfinder has one type of autofocus, and shooting a picture or movie using Live View has a different type. They are as follows:

  • TTL phase detection autofocus – Through-the-lens (TTL) phase detection autofocus uses the Multi-CAM autofocus module with all AF points in a grid-like array in the central area of the Viewfinder. This type of AF is known simply as phase-detection AF. It is a very fast type of autofocus and is used by the camera only when you are taking pictures through the Viewfinder.
  • Focal plane contrast AF – Focal plane contrast AF uses pixel-level contrast detection directly from the camera’s imaging sensor. A simple name for this is contrast-detect AF. It can use the entire surface of the imaging sensor to detect contrast between light and dark boundaries to provide autofocus. This is a relatively slow form of autofocus, but it is extremely accurate since it is done at the pixel level. This form of autofocus is used only while shooting in Live View and Movie modes.


Three Mode Groups

There are three specific mode groups that you should fully understand: Autofocus modes, AF-area modes, and Release modes.

Many people get these modes confused and incorrectly apply functions from one mode to a completely different mode. It is a bit confusing at times, but if you read this carefully and try to wrap your head around the different functionalities provided, you’ll have much greater control of your camera later.

The three mode groups for Viewfinder shooting are as follows (may vary with different Nikon DSLRs):

Autofocus modes:

  • Auto-servo AF (AF-A)
  • Single-servo (AF-S)
  • Continuous-servo (AF-C)

AF-area modes:

  • Single-point AF
  • Dynamic-area AF (multiple patterns of AF points)
  • 3D-tracking AF
  • Auto-area AF

Release modes:

  • Single frame (S)
  • Continuous low speed (CL)
  • Continuous high speed (CH)

Note: There are other release modes than the three I've listed above—such as Self-timer, Remote control, Mup, and Quiet mode—however, they are not directly related to using autofocus and shooting rapidly so I won't consider them in this article (they are considered in my books).

What’s the difference between these modes? Think of them like this:

  • Focus modes are how it focuses
  • AF-area modes are where the AF module focuses
  • Release modes control how often a picture is taken

These mode types work together to make a Nikon’s autofocus and subject tracking system one of the world’s best.


Autofocus Modes


The focus modes allow you to control how the autofocus works with static and moving subjects. They allow your camera to lock focus on a subject that is not moving or is moving very slowly. They also allow your camera to follow focus on an actively moving subject. Let’s consider the three servo-based focus modes to see when and how you might use them best.



Auto-Servo AF Mode (AF-A)

Auto-servo AF (AF-A) is an automatic mode that pays attention to your subject’s movement. It is rather simple to use because it senses whether your subject is static or moving.
  • Subject is not moving – If the subject is not moving, the camera automatically uses AF-S mode. In this mode the focus locks on the subject and does not update as long as the subject remains still. However, the focus can unlock if the camera detects subject movement, and it will switch to AF-C mode.
  • Subject is moving – If the subject it moving, the camera automatically sets itself to AF-C mode. It detects the movement across the AF sensors and automatically starts focus tracking the subject.

Single-Servo AF Mode (AF-S)

Single-servo AF (AF-S) works best when your subject is stationary—like a house or landscape. You can use AF-S on slowly moving subjects if you’d like, but you must be careful. The two scenarios listed next may help you decide:
  • Subject is not moving – When you press the Shutter-release button halfway down, the AF module quickly locks focus on your subject and waits for you to fire the shutter. If your subject starts moving and you don’t release pressure on the Shutter-release button to refocus, the focus will be obsolete and useless. When you have focus lock, take the picture quickly. This mode is perfect for stationary subjects or, in some cases, very slowly moving subjects.   
  • Subject is regularly moving – This will require a little more work on your part. Since the AF system locks focus on your subject, if the subject moves even slightly, the focus may no longer be good. You’ll have to lift your finger off of the Shutter-release button and reapply pressure halfway down to refocus. If the subject continues moving, you’ll need to continue releasing and pressing the Shutter-release button halfway down over and over to keep the focus accurate. If your subject never stops moving, is moving erratically, or stops only briefly, AF-S is probably not the best mode to use. In this case, AF-C is better because it never locks focus and the camera is able to track your subject’s movement, keeping it in constant focus.

Continuous-Servo AF Mode (AF-C)

Using Continuous-servo AF (AF-C) is slightly more complex since it is a focus tracking function. The camera looks carefully at whether the subject is moving, and it even reacts differently if the subject is moving from left to right, up and down, or toward and away from you. Read these three scenarios carefully:
  • Subject is not moving – When the subject is standing still, Continuous-servo AF acts a lot like Single-servo AF with the exception that the focus never locks. If your camera moves, you may hear your lens chattering a little as the autofocus motor makes small adjustments in the focus position. Since focus never locks in this mode, you’ll need to be careful that you don’t accidentally move the AF point off of the subject because it may focus on something in the background instead.
  • Subject is moving across the Viewfinder – If your subject moves from left to right, right to left, or up and down in the Viewfinder, you’ll need to keep your AF point on the subject when you are using Single-point AF area mode. If you are using Dynamic-area AF or Auto-area AF modes, your camera can track the subject across a few or all of the 39 AF points.
  • Subject is moving toward or away from the camera – If your subject is coming toward you, another automatic function of the camera kicks in. It is called predictive focus tracking, and it figures out how far the subject will move before the shutter fires. After you’ve pressed the Shutter-release button all the way down, predictive focus tracking moves the lens elements slightly to correspond to where the subject should be when the shutter fires a few milliseconds later. In other words, if the subject is moving toward you, the lens focuses slightly in front of your subject so that the camera has time to move the mirror up and get the shutter blades out of the way. It takes several milliseconds for the camera to respond to a press of the Shutter-release button.



AF-Area Modes


The AF-area modes are designed to let you control how many Viewfinder AF points—the area of focus attention—are in use at any one time. Three of the four modes will track subject movement.


You can use 1 AF point in Single-point AF mode; multiple AF points in Dynamic-area AF mode; and you can even use 3D tracking mode (all AF points), which uses the color of the subject to help track it, keeping it in focus while it moves around. If you don’t want to think about the autofocus area, you can let the camera automatically control the AF-area by using the Auto AF-area mode.

Single-Point AF

This mode uses a single AF point out of the array of all AF points to acquire good focus. As mentioned before, you can control which AF point is used by selecting it with the Multi Selector.

Dynamic-Area AF

This mode is best used when your subject is moving. Instead of a single AF point used alone for autofocus, several sensors surrounding the one you have selected with the Multi Selector are also active. The AF point you can see in the Viewfinder provides the primary autofocus; however, the surrounding points in the pattern you’ve selected are also active (see user's manual for pattern information for your Nikon). If the subject moves and the primary AF point loses its focus, one of the surrounding points will quickly grab the focus.

Using Dynamic-area AF, you can more accurately track and photograph all sorts, sizes, and speeds of moving subjects. The initial focus reaction speed of the AF system is somewhat slower when you use all of the camera's AF points since the camera needs to process a lot more information. Take that into consideration when you are shooting events.

3D-Tracking AF

The mode called 3D-tracking (shown as 3D on the Control panel) adds color-detection ability to the tracking system. The camera will not only track by subject area, it will also remember the color of the subject and use it for tracking.
  
3D-tracking works like the largest AF-point pattern except that it is more intelligent. Often your subject will be a different color from the background, and the Nikon’s color-based system will provide more accuracy in difficult conditions. Be careful if the subject is a similar color to the background because this may reduce the autofocus tracking accuracy.

3D-tracking is a good mode for things like action sports, air shows, races, etc. It allows the camera to become a color-sensitive, subject-tracking machine. Try it and see if it works for you.

Auto-Area AF

Auto-area AF turns your Nikon into an expensive point-and-shoot camera. Use this mode when you simply have no time to think and would still like to get great images. The AF module decides what the subject is and selects the AF points it thinks will work best.

According to Nikon, if you are using a D or G lens with a newer Nikon, there is a bit of “human recognition technology” built into this mode, similar to the Nikon Coolpix. Since most of us will use Auto-area AF only when we want to shoot for fun, a human subject that is closest to the camera is the most likely subject anyway. Using Auto-area AF, your camera can usually detect a human and help you avoid shots with perfectly focused backgrounds and blurry human subjects.


Release Modes

Nikons have several Release modes, which apply to both the Viewfinder and Live View photography. Whether you place your eye up to the Viewfinder or use the Monitor in LV mode to shoot images, all these modes apply.

Release modes decide how many images can be taken and how fast. In the good-old film days, the following release modes would have been called motor-drive settings since they are concerned with how fast the camera is allowed to take pictures.

Single Frame (S) Release Mode

This is the simplest frame rate since it takes a single picture each time you press the Shutter-release button fully. This is no speed here. This is for those shooting a few frames at a time. Nature shooters often use this mode since they are more concerned with correct depth of field and excellent composition.


Continuous Low Speed (CL) Release Mode

This mode allows you to select a frame rate between one and the maximum number of frames per second (fps) your camera can shoot. The default frame rate from the factory is three fps, which seems about right for most of us. If you want more or less speed, simply open Custom Setting Menu > Custom setting d > CL mode shooting speed and select your favorite frame speed.

Continuous High Speed (CH) Release Mode

This high-speed mode is designed for when you want to go fast! The camera will attempt to capture six frames per second every time you hold down the Shutter-release button.

The internal buffer memory of the camera limits how many frames you can take. When shooting in JPEG mode you may be able to shoot as many as 100 frames in one burst. You can control this maximum for JPEGs only by adjusting Custom Setting Menu > Custom setting d > Max. continuous release.

However, in lossless compressed NEF (RAW) mode you’ll be able to shoot only 10 to 15 frames before the buffer memory is full. You’ll have to wait for the camera to offload images to the memory card before you can shoot another long burst. 


Summary

With the controls built into the camera’s body, you’ll be able to select whether the AF module uses one or many of its AF points to find your subject. You’ll also select whether the camera grabs the focus and locks on a static subject or whether it continuously seeks new focus if your subject is moving, and how fast (in frames per second) it captures the images.

My Recommendation: If you are having trouble remembering what all these modes do—join the club! I’ve written multiple books about Nikon cameras and I still get confused about what each mode does. I often refer back to my own books to remember all the details. I have both the print and e-book versions of my books so they are always nearby (I love my Kindle). You’ll become familiar with the modes you use most often, and that is usually sufficient. Try to associate the type of mode with its name, and that will make it easier. Learn the difference between an AF-area mode (focus where), a focus mode (focus how), and a release mode (how often). A Nikon DSLR has amazing power, quality, and flexibility—at the cost of sometimes overwhelming complexity.

Keep on capturing time...
Darrell Young

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