C. Autofocus | Nikon D5100 guide

The D5100's autofocus works differently when shooting through the viewfinder than in Live View mode, when you see a preview of the image on the monitor.

This section applies to shooting through the viewfinder.

What | The autofocus must be switched on at the D5100 and at the lens, otherwise it is not active. Then you can only focus by turning the distance ring on the lens.

Where | The focus mode in the shooting information must start with AF, i.e. be AF-A, AF-S or AF-C.

Your lens probably has a switch labelled M/A or A and M. It must be set to A or M/A for autofocus to be active.

The M/A setting means that when the autofocus is on, you can intervene at any time by turning the distance ring. Then the autofocus is switched off until you tap the shutter release again.

Reminder: The camera mode is also important for the autofocus, only in the advanced camera modes P, S, A, M you can use all the settings described in this chapter.

The start signal for the autofocus to begin focusing is normally given by tapping the shutter button. In Nikon's standard settings for the D5100, this is the only way.

You can arrange that the AE-L/AF-L button on the back iniates the autofocus, in the custom settings menu f2 Assign AE-L/AF-L button by selecting AF-ON.

Then the D5100 will not refocus when you tap the shutter button. This way you can focus at a certain distance, keep it unchanged and later release the shutter at the right moment without delay by refocusing.

What | Focus tracking controls whether the autofocus stops once it can focus correctly or whether it continuously tracks moving subjects.

Where | It's the focus mode in the shooting information that decides about focus tracking:

• AF-A: The D5100 automatically decides whether autofocus will lock once proper focus is achieved or if it will continuously track for moving subjects.

The subject must be moving at the moment of focusing for the autofocus to switch to focus tracking.

• AF-S (single auto focus): The D5100 focuses once and does not change focus until you release the shutter button and press it again.
• AF-C (continuous auto focus): The D5100 continuously adjusts focus as long as you keep the shutter button tapped.

What | The D5100 has 39 autofocus sensors distributed over the centre of the frame. The focus point selection decides which of these are used to focus.

Automatic selection from all 39 points, most of the time the D5100 will focus on the foreground.

Single focus point − you select one sensor for focusing.

: Dynamic − You select a sensor for focusing and try to track your moving subject with it. If you lose it momentarily, the D5100 will use the surrounding focus points as supplementary. Not available with AF-S.

You select the metering point for initial focus and the camera tracks your subject with all the metering points as it moves within the viewfinder. Not available with AF-S.

Where & How | You set the AF point control with option AF-area mode in the shooting information.

What | With all AF focus point selection options except AUTO, it is up to you to select a point in the viewfinder for initial focusing. 

Here is how:

• You select a metering field with the arrow keys, a corresponding marker moves around in the viewfinder.
• The OK button resets the selected focus point to the centre.

The following settings apply to static autofocus AF-S and when automatic autofocus AF-A does not detect motion:

Beep | As soon as the autofocus is able to focus successfully, a beep sounds, unless you stop it in the custom settings menu d1 Beep. The signal also sounds when the self-timer is running. It can only be switched on and off for autofocus and self-timer at the same time.

AF-assist light | In low light, the D5100 switches on an assisting light for focusing when the focus point selection is AUTO or the central focus point is selected. You can turn it off in the custom settings menu a2 Built-in AF-assist illuminator if it bothers you.

There is only one: Focus and shutter priority | Nikon has also preset shutter priority for AF-C, you can change it in the custom settings menu a1 AF-C priority selection.

What | Autofocus lock means that the current focus is frozen and you can take pictures without any delay. The D5100 will not attempt to focus again while the autofocus lock is active.

Where & How | The autofocus lock is assigned to the AE-L/AF-L button in Nikon's default settings. AF-L stands for autofocus lock and AE-L stands for auto exposure lock − the exposure is also frozen, at least in Nikon's default settings.

To use autofocus lock

• press the shutter button lightly
• press the AE-L/AF-L button while keeping the shutter release half pressed

• compose the final image or wait for the perfect moment, then press the shutter button completely.

You can assign different behaviour to the AE-L/AF-L button in the custom settings menu f2 Assign AE-L/AF-L button.

Instead, you can assign the autofocus lock to the preview or Fn button on the front − if you select AE lock in the custom settings menu f1 Assign Fn button.

Switching to manual focus | To focus manually, the focus mode in the shooting information must be set to MF or the focus switch on the lens must be in the M position. Either one will do.

Then, to focus, turn the focus ring on the lens as best suits your shot; the D5100 will not intervene.

Manual focusing | The obvious and easiest way is to estimate in the viewfinder image when the part of the picture that is important to you has the best possible sharpness. But there is help from the electronics:

• If your focus matches the autofocus, a green dot appears at the bottom left in the viewfinder, like a confirmation from the autofocus when it has been able to focus.
  
• However, this is still not much help, the display reacts sensitively and gives no indication in which direction to turn the focus ring.
• In the custom settings menu a3 Rangefinder you can switch on an assistant. At the bottom of the viewfinder you will then see a display that compares your manual focus with the autofocus. An arrow pointing to the left indicates that the focus ring is too far to the left and you should turn it to the right, and vice versa.
• The D5100 uses the central autofocus point for assistance, but you can use the directional buttons to move to a different one. Tapping the shutter button will illuminate the currently active field in red if you are unsure.

Another option is to switch to Live View mode for manual focus. You can then zoom into the live image with the magnification button and directly judge the focus on the enlarged image, better than on the smaller viewfinder image.