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
--------------------
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
-----
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
----------------
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:
-----------------------------------------------------------------------

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).

Article: Fine-Tuning Camera Exposure

Getting a Handle on Exposure

Exposure determines the look, feel, and tone of an image, in more ways than one. Incorrect exposure can impair even the best composed image by cloaking important tones in darkness, or by washing them out so they become featureless to the eye. On the other hand, correct exposure brings out the detail in the areas you want to picture, and provides the range of tones and colors you need to create the desired image.

However, getting the perfect exposure can be tricky, because digital sensors can’t capture all the tones we are able to see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, the sensor may not be able to capture them all. Sometimes, we must settle for an exposure that renders most of those tones—but not all—in a way that best suits the photo we want to produce. You’ll often need to make choices about which details are important, and which are not, so that you can grab the tones that truly matter in your image. That’s part of the creativity you bring to bear in realizing your photographic vision.

For example, look at the two typical tourist snapshots presented side by side in Figure 1. The camera was mounted on a tripod for both, so the only way you can really see that they are two different images is by examining the differences in the way the water flows over the rocks.

However, the pair of pictures does vary in exposure. The version on the left was underexposed, which helps bring out detail in the ridges and sky in the background, but makes the water and foreground look murky and dark. The overexposed version on the right offers better exposure for the foreground area, but now the ridges and sky are too light.

Figure 1: At left, the image is exposed for the background highlights, losing shadow detail. At right, the exposure captures detail in the shadows, but the background highlights are washed out.

With digital camera sensors, it’s tricky to capture detail in both highlights and shadows in a single image, because the number of tones, the dynamic range of the sensor, is limited. (The solution in this particular case was to combine the two photos using Photoshop.) For the image on the left, the camera calculated exposure based—mostly—on the subject matter in the background. The camera’s sensor simply can’t capture detail in both dark areas and bright areas in a single shot.

The solution, in this particular case, was to resort to a technique called High Dynamic Range (HDR) photography, in which the two exposures from Figure 1 were combined in an image editor such as Photoshop, or a specialized HDR tool like Photomatix (about $100 from www.hdrsoft.com). The resulting shot is shown in Figure 2. I’ll explain more about HDR photography later.

To understand exposure, you need to understand the six aspects of light that combine to produce an image. Start with a light source—the sun, an interior lamp, or the glow from a campfire—and trace its path to your camera, through the lens, and finally to the sensor that captures the illumination.

Figure 2: Combining the two exposures produces the best compromise image.

Here’s a brief review of the things within our control that affect exposure.

1. Light at its source.
   Our eyes and our cameras are most sensitive to visible light. That light has several important aspects that are relevant to photography, such as color, and harshness (which is determined primarily by the apparent size of the light source as it illuminates a subject). But, in terms of exposure, the important attribute of a light source is its intensity. We may have direct control over intensity, as might be the case with an interior light. Or, we might have only indirect control over intensity, as with sunlight, which can be made to appear dimmer by introducing translucent light-absorbing or reflective materials in its path.
   
   
2. Light's duration
   We tend to think of most light sources as continuous. But, as you’ll learn, the duration of light can change quickly enough to modify the exposure, as when the main illumination in a photograph comes from an intermittent source, such as an electronic flash.
   
   
3. Light reflected, transmitted, or emitted.
   Once light is produced by its source, either continuously or in a brief burst, we are able to see and photograph objects by the light that is reflected from our subjects towards the camera lens; transmitted (say, from translucent objects that are lit from behind); or emitted (by a candle or television screen). When more or less light reaches the lens from the subject, we need to adjust the exposure. This part of the equation is under our control to the extent we can increase the amount of light falling on or passing through the subject (by adding extra light sources or using reflectors), or by pumping up the light that’s emitted (by increasing the brightness of the glowing object).
   
   
4. Light passed by the lens.
   Not all the illumination that reaches the front of the lens makes it all the way through. Filters can remove some of the light before it enters the lens. Inside the lens barrel is a variable-sized diaphragm called an aperture that dilates and contracts, producing a variable-sized aperture to control the amount of light that passes through the lens. You, or your camera auto exposure system, can control exposure by varying the size of the aperture. The relative size of the aperture is called the f/stop.
   
   
5. Light passing through the shutter. Once lightpasses through the lens, the amount of time the sensor receives it is determined by camera shutter, which can remain open for as long as 30 seconds (or even longer if you use the Bulb setting) or as briefly as 1/4,000th second.
   
   
6. Light captured by the sensor.
   Not all the light falling onto the sensor is captured. If the number of photons reaching a particular photo site doesn’t pass a set threshold, no information is recorded. Similarly, if too much light illuminates a pixel in the sensor, then the excess isn’t recorded or, worse, spills over to contaminate adjacent pixels. We can modify the number of pixels that contribute to image detail by adjusting the ISO setting. At higher ISOs, the incoming light is amplified to boost the effective sensitivity of the sensor.
   

These factors—the quantity of light produced by the light source; the amount reflected or transmitted towards the camera; the light passed by the lens; the amount of time the shutter is open; and the sensitivity of the sensor—all work proportionately and reciprocally to produce an exposure.
That is, if you double the amount of light that’s available, increase the aperture by one stop, make the shutter speed twice as long, or boost the ISO setting 2X, you’ll get twice as much exposure. Similarly, you can increase any of these factors while decreasing one of the others by a similar amount to keep the same exposure.

F/STOPS AND SHUTTER SPEEDS
If you’re really new to more advanced cameras, you might need to know that the lens aperture, or f/stop, is a ratio, much like a fraction, which is why f/2 is larger than f/4, just as 1/2 is larger than 1/4. However, f/2 is actually four times as large as f/4. (If you remember your high school geometry, you’ll know that to double the area of a circle, you multiply its diameter by the square root of two: 1.4.)

Lenses are usually marked with intermediate f/stops that represent a size that’s twice as much/half as much as the previous aperture. So, a lens might be marked: f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, with each larger number representing an aperture that admits half as much light as the one before, as shown in Figure 3.

Shutter speeds are actual fractions (of a second), but the numerator is omitted, so that 60, 125, 250, 500, 1,000, and so forth represent 1/60th, 1/125th, 1/250th, 1/500th, and 1/1,000th second. To avoid confusion, Nikon uses quotation marks to signify longer exposures: 2", 2"5, 4", and so forth represent 2.0, 2.5, and 4.0 second exposures, respectively.

Figure 3: Top row (left to right): f/2, f/2.8, and f/4; bottom row, f/5.6, f/8, f11.

Most commonly, exposure settings are made using the aperture and shutter speed, followed by adjusting the ISO sensitivity if it’s not possible to get the preferred exposure (that is, the one that uses the “best” f/stop or shutter speed for the depth-of-field or action-stopping we want). Table 1 shows equivalent exposure settings using various shutter speeds and f/stops.

Table 1: Equivalent Exposures
Shutter speed	f/stop	Shutter speed	f/stop
1/30th second	f/22	1/500th second	f/5.6
1/60th second	f/16	1/1000th second	f/4
1/125th second	f/11	1/2000th second	f/2.8
1/250th second	f/8	1/4000th second	f/2

When the D3100 is set for P mode, the metering system selects the correct exposure for you automatically, but you can change quickly to an equivalent exposure by holding down the shutter release button halfway (“locking” the current exposure), and then spinning the command dial until the desired equivalent exposure combination is displayed.

Rotate the dial to the right to increase the size of the aperture and make the shutter speed faster (for less depth-of-field/more action-stopping power) or to the left to use smaller apertures and slower shutter speeds (to increase depth-of-field while potentially adding some blur from subject or camera motion). Nikon calls this ability Flexible Programming and the camera displays a star-like symbol adjacent to the P indicator that lets you know it is in effect.

In Aperture-priority (A) and Shutter-priority (S) modes, you can change to an equivalent exposure, but only by adjusting either the aperture (the camera chooses the shutter speed) or shutter speed (the camera selects the aperture). I’ll cover all these exposure modes later in another post.

Article: Set the correct white balance for your shoot

Color balance correction is the process of rendering accurate colors in your final image.

Most people don’t even notice that light has different color characteristics because the human eye automatically adjusts to different color temperatures—so quickly, in fact, that everything looks correct in a matter of milliseconds.

When color film ruled the world, photographers would select which film to use depending on what their light source was going to be. The most common film was balanced for daylight, but you could also buy film that was color balanced for tungsten light sources. Most other lighting situations had to be handled by using color filters over the lens. This process was necessary for the photographer’s final image to show the correct color balance of a scene.

Your camera has the ability to perform this same process automatically, but you can also choose to override it and set it manually. Guess which method we are going to use? That’s right, once again your photography should be all about maintaining control over everything that influences your final image.

Luckily, you don’t need to have a deep understanding of color temperatures to control your camera’s white balance. The choices are given to you in terms that are easy to relate to and that will make things pretty simple. Your white balance choices are:

1. Auto:
The default setting for your camera. It is also the setting used by all of the automatic scene modes.
2. Daylight:
Most often used for general daylight/sun-lit shooting.
3. Shade:
Used when working in shaded areas that are still using sunlight as the dominant light source.
4. Cloudy:
The choice for overcast or very cloudy days. This and the Shade setting will eliminate the blue color cast from your images.
5. Incandescent:
Used for any occasion where you are using regular household-type bulbs for your light source. Tungsten is a very warm light source and will result in a yellow/orange cast if you don’t correct for it.
6. Cool-White Fluorescent:
Used to get rid of the green-blue cast that can result from using regular fluorescent lights as your dominant light source. Some fluorescent lights are actually balanced for daylight, which would allow you to use the Daylight white balance setting.
7. Flash:
Used whenever you’re using the built-in flash or a flash on the hot shoe. You should select this white balance to adjust for the slightly cooler light that comes from using a flash. (The hot shoe is the small bracket located on the top of your camera, which rests just above the eyepiece. This bracket is used for attaching a more powerful flash to the camera).
8. Pre:
Indicates that you are using a customized white balance that is adjusted for a particular light source. This option can be adjusted using an existing photo you have taken or by taking a picture of something white or gray in the scene.

Article: Photography Composition, Top Rules

There are no fixed rules in photography, but there are guidelines which can often help you to enhance the impact of your photos.

The only rule in photography is that there are no rules. However, there are number of established composition guidelines which can be applied in almost any situation, to enhance the impact of a scene.

These guidelines will help you take more compelling photographs, lending them a natural balance, drawing attention to the important parts of the scene, or leading the viewer's eye through the image.

Once you are familiar with these composition tips, you'll be surprised at just how universal most of them are. You'll spot them everywhere, and you'll find it easy to see why some photos "work" while others feel like simple snapshots.

Rule of Thirds
Imagine that your image is divided into 9 equal segments by 2 vertical and 2 horizontal lines. The rule of thirds says that you should position the most important elements in your scene along these lines, or at the points where they intersect.
Doing so will add balance and interest to your photo. Some cameras even offer an option to superimpose a rule of thirds grid over the LCD screen, making it even easier to use.

Balancing Elements
Placing your main subject off-centre, as with the rule of thirds, creates a more interesting photo, but it can leave a void in the scene which can make it feel empty. You should balance the "weight" of your subject by including another object of lesser importance to fill the space.

Leading Lines
When we look at a photo our eye is naturally drawn along lines. By thinking about how you place lines in your composition, you can affect the way we view the image, pulling us into the picture, towards the subject, or on a journey "through" the scene. There are many different types of line - straight, diagonal, curvy, zigzag, radial etc - and each can be used to enhance our photo's composition.

Symmetry and Patterns.
We are surrounded by symmetry and patterns, both natural and man-made., They can make for very eye-catching compositions, particularly in situations where they are not expected. Another great way to use them is to break the symmetry or pattern in some way, introducing tension and a focal point to the scene.

Viewpoint
Before photographing your subject, take time to think about where you will shoot it from. Our viewpoint has a massive impact on the composition of our photo, and as a result it can greatly affect the message that the shot conveys. Rather than just shooting from eye level, consider photographing from high above, down at ground level, from the side, from the back, from a long way away, from very close up, and so on.

Background
How many times have you taken what you thought would be a great shot, only to find that the final image lacks impact because the subject blends into a busy background? The human eye is excellent at distinguishing between different elements in a scene, whereas a camera has a tendency to flatten the foreground and background, and this can often ruin an otherwise great photo. Thankfully this problem is usually easy to overcome at the time of shooting - look around for a plain and unobtrusive background and compose your shot so that it doesn't distract or detract from the subject.

Depth of field
Because photography is a two-dimensional medium, we have to choose our composition carefully to conveys the sense of depth that was present in the actual scene. You can create depth in a photo by including objects in the foreground, middle ground and background. Another useful composition technique is overlapping, where you deliberately partially obscure one object with another. The human eye naturally recognises these layers and mentally separates them out, creating an image with more depth.

Framing the photo
The world is full of objects which make perfect natural frames, such as trees, archways and holes. By placing these around the edge of the composition you help to isolate the main subject from the outside world. The result is a more focused image which draws your eye naturally to the main point of interest.

Cropping
Often a photo will lack impact because the main subject is so small it becomes lost among the clutter of its surroundings. By cropping tight around the subject you eliminate the background "noise", ensuring the subject gets the viewer's undivided attention.

Experimentation
With the dawn of the digital age in photography we no longer have to worry about film processing costs or running out of shots. As a result, experimenting with our photos' composition has become a real possibility; we can fire off tons of shots and delete the unwanted ones later at absolutely no extra cost. Take advantage of this fact and experiment with your composition - you never know whether an idea will work until you try it.

Final Comment
Composition in photography is far from a science, and as a result all of the "rules" above should be taken with a pinch of salt. If they don't work in your scene, ignore them; if you find a great composition that contradicts them, then go ahead and shoot it anyway. But they can often prove to be spot on, and are worth at least considering whenever you are out and about with your camera.

References
1- 10 Top Photography Composition Rules.
2- 25 Photo Composition Tips.