Article: Autofocus Areas

Where autofocus mode chooses when to autofocus, the Autofocus Area parameter tells your Nikon D3100 how to choose which of the 11 focus points in the viewfinder should be used to evaluate and lock in focus.

Ordinarily, your camera would like to be able to choose among the available AF points itself. In fact, that’s the default behavior, and when AF-Area mode for Viewfinder is set to Auto-Area, the D3100 chooses the focus point automatically in Auto, No-Flash, Portrait, Landscape, Night Portrait, and PAS (Program, Aperture-priority, and Shutter-priority) exposure modes. Giving the D3100 free rein in selecting a focus point works well much of the time, and you can use this default mode with confidence.

If you want to choose a focus point yourself, you must do two things.

1- When the focus point is unlocked, you can use the multi selector pad to shift the active point to any of the 11 focus points seen in the viewfinder. The available points are shown in Figure 1. The currently active focus point is highlighted in red.

Figure 1: There are 11 possible focus points shown in the viewfinder.

2- The second thing to do is to switch Focus Area mode in the Shooting menu from Autoarea (which always chooses the focus point automatically) to Single-point, Dynamic area, or 3D-tracking (11 Points). These modes change the D3100’s behavior as follows:

1. Single-point.
   You choose which of the 11 points are used, and the Nikon D3100 sticks with that focus bracket, no matter what.
   
   "This mode is best for stationary subjects, and is used automatically in Close-up scene mode."
   
   In this mode, you always select the focus point manually, using the multi selector button. The D3100 evaluates focus based solely on the point you select, making this a good choice for subjects that don’t move much.
   
   
2. Dynamic-area.
   You can select the focus point, but the D3100 can use other focus points as well.
   
   "You’d want to use this mode when photographing subjects that are moving unpredictably, but want the flexibility of being able to choose one of the 11 focus zones yourself."
   
   Once you’ve specified the focus bracket you want using the multi selector’s buttons, the D3100 will use that area exclusively in Single-servo autofocus mode (AF-S). If you’ve chosen Continuous-autofocus mode (AF-C) or Automatic-autofocus mode (AF-A), if the subject begins moving after autofocus is activated, the D3100 will focus based on information from one of the other focus zones.
   
   Well suited for sports photography, this mode is applied automatically with the Sports scene setting, and can be used with other types of moving subjects, such as active children.
   
   
3. Auto-area.
   This default mode chooses the focus point for you, and can use distance information when working with a G or D lens that supplies that data to the camera.
   
   
4. 3D-tracking (11 points).
   In this mode, you select the focus point using the multi selector, but if you subsequently reframe the picture slightly, the D3100 uses distance information when in AF-C (Continuous Autofocus) or AF-A (Automatic Autofocus) modes to refocus on the original subject if necessary.
   
   When using AFS (Single Autofocus), this mode functions the same as Single-point focus area mode.
   
   "This mode is useful if you need to reframe a relatively static subject from time to time." If your subject leaves the frame entirely, you’ll need to release the shutter button and refocus.
   

Article: Autofocus Modes

Your Autofocus Mode Options.

Choosing the right autofocus mode and the way in which focus points are selected is your key to success. Using the wrong mode for a particular type of photography can lead to a series of pictures that are all sharply focused—on the wrong subject.

There are two main autofocus options you need to master to make sure you get the best possible automatic focus with your Nikon D3100: Autofocus mode and Autofocus Area. I’ll explain each of them separately in different articles.

Autofocus Mode
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This choice determines when your D3100 starts to autofocus, and what it does when focus is achieved. Automatic focus is not something that happens all the time when your camera is turned on.

To save battery power, your D3100 generally doesn’t start to focus the lens until you partially depress the shutter release. (You can also use the AE-L/AFL button to start autofocus) Autofocus isn’t some mindless beast out there snapping your pictures in and out of focus with no feedback from you after you press that button. There are several settings you can modify that return at least a modicum of control to you.

Your first decision, if you’ll be composing your image through the optical viewfinder, should be whether you set the D3100 to AF-S, AF-C, AF-A, or Manual.

To change to any of the automatic focus modes, use the information edit menu and select the focus mode (AF is fifth from the bottom of the screen). With the camera set for one of the scene modes, AF-S will be used automatically, except when using the Sports/Action scene mode. To switch to manual mode, slide the AF/M or M-A/M switch on the lens to M.

AF-S
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In this mode, also called Single Autofocus, focus is set once and remains at that setting until the button is fully depressed, taking the picture, or until you release the shutter button without taking a shot. You can also use the AE-L/AF-L button, if you’ve set that button to lock focus when pressed.

For non-action photography, this setting is usually your best choice, as it minimizes out-of-focus pictures (at the expense of spontaneity).

The drawback here is that you might not be able to take a picture at all while the camera is seeking focus; you’re locked out until the autofocus mechanism is happy with the current setting. AF-S/Single Autofocus is sometimes referred to as focus priority for that reason. Because of the small delay while the camera zeroes in on correct focus, you might experience slightly more shutter lag. This mode uses less battery power.

When sharp focus is achieved, the focus confirmation light at the lower left will remain green, without flashing. By keeping the shutter button depressed halfway, you’ll find you can reframe the image while retaining the focus (and exposure) that’s been set.

AF-C
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This mode, also known as Continuous Autofocus, is the one to use for sports and other fast-moving subjects.

In this mode, once the shutter release is partially depressed, the camera sets the focus but continues to monitor the subject, so that if it moves or you move, the lens will be refocused to suit. Focus and exposure aren’t really locked until you press the shutter release down all the way to take the picture. You’ll often see Continuous Autofocus referred to as release priority.

If you press the shutter release down all the way while the system is refining focus, the camera will go ahead and take a picture, even if the image is slightly out of focus. You’ll find that AF-C produces the least amount of shutter lag of any autofocus mode: press the button and the camera fires. It also uses the most battery power, because the autofocus system operates as long as the shutter release button is partially depressed.

AF-A
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This setting is actually a combination of the first two. When selected, the camera focuses using AF-S AF and locks in the focus setting. But, if the subject begins moving, it will switch automatically to AF-C and change the focus to keep the subject sharp.

AF-A is a good choice when you’re shooting a mixture of action pictures and less dynamic shots and want to use AF-S when possible. The camera will default to that mode, yet switch automatically to AF-C when it would be useful for subjects that might begin moving unexpectedly. However, as with AF-S, the shutter can be released only when the subject at the selected focus point is in focus.

Manual Focus
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In this mode, or when you’ve set the lens autofocus switch to Manual (or when you’re using a non AF-S lens, which lacks an internal autofocus motor), the D3100 always focuses manually using the rotating focus ring on the lens barrel.

However, if you are using a lens with a maximum aperture of at least f/5.6, the focus confirmation light in the viewfinder will glow a steady green when the image is correctly manually focused.

In manual focus mode, you can use the rangefinder feature to help you achieve sharp focus when you’re shooting in Program, Aperture-priority, or Shutter-priority mode. The rangefinder supplements the focus confirmation indicator at the left edge of the viewfinder by using the analog exposure indicator as a focusing “scale.”

In Figure 1, you can see that the focus indicator has illuminated all the bars to the right of the “zero” point. That means that the current focus is significantly behind the correct focus for the area in the red-highlighted focus point. To focus on the left-most battery, instead, you’d need to adjust the focus forward.

Figure 1: The manual focus scale in the viewfinder shows that the current focus is
significantly behind the subject at the selected focus point (highlighted in red). Focus more closely.

To summarize the instructions for using the rangefinder:
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Turn the rangefinder On with this option if you want an additional manual focusing aid. With a manual focus lens and the rangefinder operating, the analog exposure display at bottom center in the viewfinder will be replaced by a rangefinder focusing scale. Indicators on the scale like those in Figure 2 show when the image is in sharp focus, as well as when you have focused somewhat in front of, or behind the subject.

Follow these steps to use the rangefinder:

1. Activate. Use the Setup menu’s Rangefinder entry to turn on the rangefinder.
   
   
2. Select a focus point. Use the multi selector to move the highlighting around in the frame.
   
   
3. Rotate the lens focus ring. Zoom lenses will have two rings; there’s no fixed convention as to whether the wider or narrower ring is the focus ring. Choose the one farthest from the zoom scale.
   
   
4. Watch the rangefinder. If the indicator is pointing towards the left, focus farther away. If the scale points towards the right, focus more closely.
   
   
5. Achieve sharp focus. When the subject you’ve selected with the focus zone bracket is in sharp focus, only two bars will appear, centered under the 0, and the focus confirmation indicator will stop blinking. If no 0 appears, the camera cannot determine focus.
   

Figure 2: Upper left: Correct focus;
Upper right: focus is grossly incorrect;
Center left: focus slightly in front of the subject;
Center right: focus slightly behind the subject;
Bottom left: focus significantly in front of the subject;
Bottom right: focus significantly behind the subject.

Article: Choosing the right Exposure Method - Aperture-Priority

The Nikon D3100’s scene modes choose an exposure method for you.

But there are three semi-automated methods (plus manual) that you can use to choose the appropriate shutter speed and aperture. You can choose among them by rotating the mode dial until the one you want is selected. Your choice of which is best for a given shooting situation will depend on things like your need for lots of (or less) depth-of-field, a desire to freeze action or allow motion blur, or how much noise you find acceptable in an image. Each of the D3100’s exposure methods emphasizes one aspect of image capture or another.

Coming articles will explain and introduces you to all four methods.

Aperture-Priority

In A mode (don’t confuse this with Auto; some point-and-shoot cameras use the letter A to represent automatic mode), you specify the lens opening used, and the D3100 selects the shutter speed.

Aperture-priority is especially good when you want to use a particular lens opening to achieve a desired effect. Perhaps you’d like to use the smallest f/stop possible to maximize depth-of-field in a close-up picture. Or, you might want to use a large f/stop to throw everything except your main subject out of focus, as in Figure 1, a chilling portrait of a menacing lizard. Maybe you’d just like to “lock in” a particular f/stop because it’s the sharpest available aperture with that lens. Or, you might prefer to use, say, f/2.8 on a lens with a maximum aperture of f/1.4, because you want the best compromise between speed and sharpness.

Aperture-priority can even be used to specify a range of shutter speeds you want to use under varying lighting conditions, which seems almost contradictory. But think about it. You’re shooting a soccer game outdoors with a telephoto lens and want a relatively high shutter speed, but you don’t care if the speed changes a little should the sun duck behind a cloud.

Figure 1: Use Aperture-priority to “lock in” a large f/stop when you want to blur the background

Set your D3100 to A, and adjust the aperture until a shutter speed of, say, 1/1,000th second is selected at your current ISO setting. (In bright sunlight at ISO 400, that aperture is likely to be around f/11.) Then, go ahead and shoot, knowing that your D3100 will maintain that f/11 aperture (for sufficient depth-of-field as the soccer players move about the field), but will drop down to 1/750th or 1/500th second if necessary should the lighting change a little.

A Lo or Hi indicator in the viewfinder, accompanied by a Subject Is Too Dark or Subject Is Too Bright warning on the LCD indicates that the D3100 is unable to select an appropriate shutter speed at the selected aperture and that over- and underexposure will occur at the current ISO setting.

That’s the major pitfall of using A: you might select an f/stop that is too small or too large to allow an optimal exposure with the available shutter speeds.

For example, if you choose f/2.8 as your aperture and the illumination is quite bright (say, at the beach or in snow), even your camera’s fastest shutter speed might not be able to cut down the amount of light reaching the sensor to provide the right exposure. Or, if you select f/8 in a dimly lit room, you might find yourself shooting with a very slow shutter speed that can cause blurring from subject movement or camera shake.

Aperture-priority is best used by those with a bit of experience in choosing settings. Many seasoned photographers leave their D3100 set on A all the time.

Summary

As I said earlier, the best knowledge comes from experiment and experience. However, in general, if you want a blurred background, open up the lens aperture (small f-stop numbers) and get close to the focus object. To make the background sharp do just the opposite.

So when you want to create a softer, blurrier background in your photos, use a wider (lower) aperture value, switch to a longer lens, and get in close to your subject.

See also

1- Controlling Depth of Field (DOF).
2- Depth of Field: One of the most important elements in photography.
3- Book by David D. Busch (NIKON D3100 GUIDE TO DIGITAL SLR PHOTOGRAPHY).