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.

Photo: Salah El Dien Citadel

This photo is taken from El Azhar park, Cairo, Egypt to Salah El Dien Citadel, at when i was in photography field trip at evening.

Shoot Info:
Photo taken with Nikon D3100.
Mode: Manual.
Exposure: 1/500 sec @ f/5.
Focal length: 70mm (70.0-300.0mm Lens).
ISO 400.
Flash: Did not fire.
Tripod: Yes.

Article: Using Autofocus with the Nikon D3100

Autofocus can sometimes be frustrating for the new digital SLR photographer, especially those coming from the point-and-shoot world. That’s because correct focus plays a greater role among your creative options with a dSLR, even when photographing the same subjects.

Most non-dSLR digital cameras have sensors that are much tinier than the sensor in the D3100. Those smaller sensors require shorter focal lengths, which have, effectively, more depth-of-field.

The bottom line is that with the average point-and-shoot camera, everything is in focus from about one foot to infinity and at virtually every f/stop. Unless you’re shooting close-up photos a few inches from the camera, the depth-of-field is prodigious, and autofocus is almost a non-factor.

The D3100, on the other hand, uses longer focal length lenses to achieve the same field of view with its larger sensor, so there is less depth-of-field.

That’s a good thing, creatively, because you have the choice to use selective focus to isolate subjects. But it does make the correct use of autofocus more critical.

To maintain the most creative control, you have to choose three attributes:

• How much is in focus.
  Generally, by choosing the f/stop used, you’ll determine the range of sharpness/amount of depth-of-field. The larger the DOF, the “easier” it is for the autofocus system’s locked-in focus point to be appropriate (even though, strictly speaking, there is only one actual plane of sharp focus). With less depth-of-field, the accuracy of the focus point becomes more critical, because even a small error will result in an out-of-focus shot.
  
  
• What subject is in focus?
  The portion of your subject that is zeroed in for autofocus is determined by the autofocus zone that is active, and which is chosen either by you or by the Nikon D3100.
  For example, when shooting portraits, it’s actually okay for part of the subject—or even part of the subject’s face—to be slightly out of focus as long as the eyes (or even just the nearest eye) appear sharp.
  
  
• When focus is applied.
  For static shots of objects that aren’t moving, when focus is applied doesn’t matter much. But when you’re shooting sports, or birds in flight (see Figure 1), or children, the subject may move within the viewfinder as you’re framing the image. Whether that movement is across the frame or headed right towards you, timing the instant when autofocus is applied can be important.
  

Figure 1: When capturing moving subjects, such as birds in flight, timing the instant when autofocus is applied can be important.

Photo: The Lonely Moon at Night

This photo is taken from Egypt Cairo at 4/6/2012 when moon is in its full size, and it was very shiny.

Shoot Info:
Photo taken with Nikon D3100.
Mode: Aperture Priority.
Exposure: 1/250 sec @ f/11.
Focal length: 300mm (70.0-300.0mm Lens).
ISO 400.
Flash: Did not fire.
Tripod: Yes.

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).