Nikon DSLR Focusing

1. Introduction

This brief note is intended as an introduction the auto-focus system found in Nikon's DSLRs as a supplement to the manual that came with your camera. While most of the information that you'll find here is in the manual, it is rather terse and assumes that the reader is familiar with the fundamental principles of auto-focus systems. This note's approach is more of a tutorial than a manual.

2. Terminology

Before we dive down into the nitty-gritty, I want to introduce some of the terminology that I use when discussing auto-focus.

Priority

Auto-focus systems may operate in either focus priority or release priority mode. Focus priority means that the shutter will not fire unless focus is locked (as reported by the focus indicator). Release priority means that the camera will fire as soon as you press the shutter, whether focus is locked or not.

On Nikon's professional bodies (i.e. from the D300 and up), you can set this priority directly. Nikon's entry-level and prosumer bodies (i.e. from the D90 and down) don't let the user set AF priorities directly. In most of its modes, Nikon entry level and prosumer cameras are set to focus priority. The exception is the AF-C (Continuous-servo AF) mode (also used by the Sports vari-program), where the camera becomes release priority.

Initial Focus Point

The initial focus point is the focus point that is selected (by you) as the initial focus point in the context of the above modes. You select the initial focus point by unlocking the Focus Area Select (the L switch on the back of the camera) and using the direction pad.

On lower end Nikon bodies, only the centre focus point is of the cross-type. This means that the centre is more sensitive than the other focus points. If you want to focus on something off-centre, it is much better to focus on it the centre in single servo mode (AF-S), and then recompose by moving the camera, than to shift the initial focus point to one of the off-centre sensors.

3. Auto-focus Modes and Settings

All Nikon DSLRs offer these three basic AF modes: AF-S, AF-C, and AF-A. For cameras that let you record video, there may also be video-releated auto-focus settings.

There will be additional settings that are accessible via custom settings. Below is a summary of all those settings.

Basic AF modes: AF-S, AF-C, AF-A, and AF-F

The basic AF modes let you control how (and if) the camera locks focus on a subject. On higher end cameras, you set these with the AF-switch located beside the lens at the five o'clock position. On the lower end cameras, you sett these via menus, typically through the INFO screen.

• AF-S – Single-servo AF. Upon halfway press of the shutter-release, the camera will try to lock focus. If the auto-focus system sees subject movement it does not lock the focus until the subject stops moving. When the subject stops, the focus locks. Once this lock takes place, the little round green light (the focus indicator) comes on in the viewfinder, and auto-focus activity ceases. To refocus, you must reactivate auto-focus by lifting your finger and reapply pressure. The focus is truly locked and will not try to follow your subject unless you refocus. To follow a moving subject requires you to tap the shutter button as the subject moves. Also, the camera will not fire unless focus indicator indicates that focus is obtained (i.e. focus priority). This does not necessarily mean that focus is locked on the subject that you think focus should be locked on, so your main subject may still be out of focus. Your main subject will also be out of focus if it moves to a different distance between the time you lock focus (by halfway press) and release the shutter (by full press).
• AF-C – Continuous-servo AF. Camera focuses continuously while shutter-release button is pressed halfway. When using this mode the auto-focus never locks. The camera will focus continuously as the subject moves, or you recompose. Camera will fire whether focus is locked or not (i.e. release priority).
• AF-A – Automatic AF. The camera tries to judge when the subject moves, and refocus with the subject as it moves. Camera will not fire unless focus is locked (i.e. focus priority). It's a solution halfway between AF-S and AF-C.
• AF-F – Full time AF during video recording. This mode was introduced with the Nikon D3100. When it is enabled, the camera focuses continuously during movie recording without the need for holding the shutter-release button down halfway.

AF-Area Mode: Single, Dynamic, Auto

The AF-area mode controls which of the focus sensors are used to detect subjects and lock focus. It is controlled with a switch on higher end cameras, and via a custom setting on lower end models.

• Single – you select one focus sensor, and that's it. No others are used. Use this mode with stationary subjects, and when you need to be in control of what the focus locks on.
• Dynamic – This mode only makes sense in AF-C and AF-A mode. The camera starts with the initial focus point, but moves to another sensor with the subject if it detects subject movement. Use with erratically moving subjects. Higher end cameras let you select the number of focus points the system should monitor to track focus. Dynamic mode is the default setting for the Sports vari-program.
• 3D-Tracking – This mode only makes sense in AF-C and AF-A mode. 3D-Tracking is only available on a few models (e.g. the D3100, D5000, D90). It is closely related to Dynamic Area, but 3D Tracking also exploits colour information to help track a subject. Nikon warn that it may become confused if the subject is the same colour as the background, the system will become confused. Nikon recommends using Dynamic for erratically moving subjects, and 3D-Tracking when recomposing after obtaining focus with relatively static subjects.
• Auto – This mode used to be known as “closest subject priority”, but was renamed “auto area” for the D80. In this mode, you can not select the (initial) focus point, but leave it to the camera to select the main subject and select focus point. If you use G- or D-type lenses, older models will pick the subject closest to the camera that has enough contrast for AF. With newer models, this mode tries to detect human subjects and lock focus on them. Use this when you do not want to be bothered with focus point selection. This is the AF-area mode you are locked into when the camera is in Auto mode and most of the other vari-programs.

Centre AF-area Zone

Lower end cameras do not let you select the number of focus points to monitor in dynamic AF-Area Mode. Instead the size of the centre focus area can be controlled with a custom setting. The two options are:

• Normal zone – Focus on a specific subject in a small area without other nearby objects interfering with focus. This is the default setting.
• Wide zone – Focus on moving subjects and other objects that are difficult to track. Not available when AF-area mode is set to Auto Area.

4. Learning to Use AF

On a lower end camera like the Nikon D80, there are 18 possible combinations of all the modes and settings discussed in the previous section (AF mode, AF-area mode, and Zone). On higher end cameras, there is even a bigger selection. While not all combinations make sense, you need to learn which does, and how they work.

To learn how the different modes and auto-focus settings work, you need to practice.

Make sure you practice with all the different modes and settings. Make notes, and decide for yourself how the different modes and settings work.

Here are the combinations I currently use on the D80/D700:

1. AF-S, Single AF-area, (Normal zone). This is the setting I usually use.
2. AF-A, Auto AF-area, (Normal zone). This is the setting that fits me best in a point & shoot type situation.
3. AF-C, Dynamic AF-area, Wide zone/9 points. When tracking most moving subjects.
4. AF-C, Dynamic AF-area, Wide zone/51 points. When tracking birds in flight.

5. Anatomy of the Nikon AF system

The auto-focus modules used in Nikon DSLRs are known as Multi-CAM followed by a number. The number indicates the number of CCD contrast-sensing elements in the module. The speed and accuracy of a camera's auto-focus system is determined by the number of contrast sensing elements, the number of focus points, and the number of extra sensitive cross-type sensors.

Below is a list of all the AF-modules used in Nikon DSLRs, with the number of focus points, followed by the number of cross-type sensors in parenthesises, plus a list of the camera models that uses this particular module:

• Multi-CAM 530 &ndash 3 (1). D50, D40, D40x, D60.
• Multi-CAM 900 &ndash 5 (1). D70, D100.
• Multi-CAM 1000 &ndash 11 (1). D3000, D3100, D5000, D80, D90, D200.
• Multi-CAM 1300 &ndash 5 (1). D1-series.
• Multi-CAM 2000 &ndash 11 (9). D2-series.
• Multi-CAM 3500DX &ndash 51 (15). D300-series.
• Multi-CAM 3500FX &ndash 51 (15). D700, D3-series.

All modules operate between EV -1 to +19 (ISO 100, normal temperature).

The image below shows the D80 viewfinder and the position of the 11 focusing points of the Multi-CAM 1000. They become illuminated red when active.

Only the centre focus point of the Multi-CAM 1000 is of the cross-type. There are 2 horizontal (top and bottom in the centre column) and 8 vertical focus points (the rest).

6. Other Focus Features

Focus Lock

Focus lock can be used to change the composition after focusing, making it possible to focus on a subject, lock focus, and recompose.

It is usually done by pressing the shutter button halfway down, but unless you change the default, you can also lock focus by pressing the AE-L/AF-L button after focus has been obtained (by halfway press of the shutter button).

Focus will remain locked as long as the shutter-release button is kept pressed halfway, or the AE-L/AF-L button is pressed. This allows several photographs in succession to be taken at the same focus setting.

The operation of the AE-L/AF-L-button can be changed with a custom setting. The default is to lock both exposure and focus when the button is pressed, but it can be used for a number of other things. See your cameras manual for a list.

Focus Area Wrap

This it toggled via a custom setting (e.g. #20 in the D80, a7 in the D700). It can be used to toggle focus area wrap. The default is not to wrap around from top to bottom, or from left to right, when the multi-selector is pressed when at one of the outer focusing points.

Focus Selector Lock

When you you use the Single or Dynamic AF-Area Mode, the multi-selector lets you choose the (initial) focus point. Some people have problems with their nose, etc. accidentally bumping the multi-selector after they've set this up, changing the selection. Moving he focus selector lock switch to “L” disables the multi-selector.

Manual Focus

You select manual focus by setting the focus mode selector switch to “M”.

When using manual focus on lenses with a maximum aperture of f/5.6 and faster, the viewfinder focus indicator should indicate when the subject is in focus when the shutter button is pressed halfway down.

AF-Assist Illuminator

If the subject is poorly lit, the AF-assist illuminator will light automatically to assist the AF operation when the shutter-release button is pressed halfway. Unless a Speedlight with a built in AF-assist illuminator is mounted, the AF-assist illuminator on the camera will be used.

The illuminator has a range of about 0.5-3.0 meters. The illuminator works best with a lens with a focal length of 24-200 mm. Remove the lens hood if the lens hood blocks the light from the AF-assist illuminator.

The AF-assist illuminator will not light in Landscape, Sports, or Night Landscape vari-programs.

You can toggle the AF-assist illuminator On/Off with custom setting #4.

Register Distance

The register distance (i.e. the distance between the lens mounting flange and the sensor focal plane) on a Nikon SLR or DSLR is 46.5 mm.

The position of the sensor focal plane is indicated by a small focal plane mark just behind the mode wheel. You can use this mark to measure the exact distance between the subject and the sensor.

7. Subject Tracking

To track a moving subject, you must use continuous-servo AF (AF-C). In this mode, the camera will keep refocusing as long as you keep the shutter half-pressed. In addition, select dynamic AF-area, to make sure that the AF zone moves with the subject.

If you use a professional model, also select the number of focus points to monitor for subject tracking. The D700 manual suggests you select 9 for subjects moving predictably (e.g. runners and race cars), 21 for subjects moving unpredictably (e.g. football players), and all (51) for moving subjects that can not be easily framed in the viewfinder (e.g. birds in flight). If you use one of the lower end bodies, instead select wide zone (for best transfer of focus monitoring to next zone). As initial focus point, select the one in the centre.

To learn how these settings work, you need to practice. Go somewhere where there is a lot of moving subjects (e.g. cars and/or people), and try out how the camera's AF react to moving subjects.

First, just hold the camera steady and let the subjects move. Make sure you try out different movement patterns (stationary, passing, advancing, receding, etc.). Then, practice tracking different movement patterns with the camera's AF.

Finally practice with the types of moving subjects you want to photograph (e.g. kids, pets, sports, etc.), until you are completely familiar with the various settings and how the camera respond to them.

Below is a list of things to consider when in order to get a sharp picture of moving subjects:

1. AF-setting. Find the AF-setting combination that suits you best. Refer to this section for a summary of AF-settings.
2. The ambient light. AF is more efficient if there is a lot of light. The quality of light matters as well, high contrast light is better for efficient AF (but diffuse light is often better for the overall image quality).
3. A fast lens. I believe (but have not been able to confirm this) that the cross-type sensor in the D80 needs a lens with f/2.8 or larger to work well.
4. Focus motor. A lens with a fast focus motor (i.e. AF-S – silent wave type) is the best choice for tracking moving subjects.
5. Depth of Field (DOF). If you shoot wide open, the DOF is very shallow, in particular at short distances. This may help make the subject stand out from the background, but also makes focus more critical, with less room for error.
6. Pre-focusing. In advance, you set the focus manually at the distance you expect your subject to appear. Them, when you half-press the shutter button, the AF system will respond faster than it would if it has to hunt from a distance far away.
7. Shutter time. For fast moving subjects, (unless you use flash) the shutter time should be fast enough to eliminate any (undesirable) motion blur. For ball games (soccer, rugby), and for dogs running and jumping, you need at least 1/1000, maybe as fast as 1/4000. Make test exposures and examine them for motion blur on the LCD to find the shutter time to use.
8. ISO. You may have to use high ISO to get a fast enough shutter time. I consider auto ISO an excellent tool for action photography.

To have your camera's auto-focus track the subject is a powerful tool, but it is not magic. To get sharp images of a fast moving subject, in particular at short range, requires planning, skill and experience.