Digital Phoptography

A camera's optical device system showing intelligence adjusts the lense to get target the topic, and may mean the distinction between a pointy ikon and a incomprehensible  chance. Despite a on the face of it straightforward goal—sharpness at the main target point—the inner workings of however a camera focuses area unit sadly not as simple. This tutorial aims to boost your photos by introducing however optical device works—thereby facultative you to each create the foremost of its assets and avoid its shortcomings.

Note: optical device (AF) works either by mistreatment distinction sensors among the camera (passive AF) or by emitting a symptom to illuminate or estimate distance to the topic (active AF). Passive AF will be performed mistreatment either the distinction detection or part detection ways, however each accept distinction for achieving correct autofocus; they'll so be treated as being qualitatively similar for the needs of this AF tutorial. Unless otherwise expressed, this tutorial can assume passive optical device. we are going to conjointly discuss the AF assist beam technique of active optical device towards the top.
CONCEPT: optical device SENSORS

A camera's optical device sensor(s) area unit the $64000 engine behind achieving correct focus, and area unit arranged  move into varied arrays across your image's field of read. every detector measures relative focus by assessing changes in distinction at its various purpose within the image — wherever maximal  distinction is assumed to correspond to maximal  sharpness.

Change Focus Amount:
   

Blurred
   

Partial
   

Sharp

   

400%
   

Sensor bar graph

Please visit the tutorial on image histograms for a background on image distinction.

Note: several compact digital cameras use the image detector itself as a distinction detector (using a technique known as distinction detection AF), and don't essentially have multiple distinct optical device sensors (which area unit additional common mistreatment the part detection technique of AF).

Further, the on top of diagram illustrates the distinction detection technique of AF;

phase detection is another technique, however this still depends on distinction for correct optical device.

The process of autofocusing typically works as follows:

(1) associate optical device processor (AFP) makes alittle amendment within the focusing distance.

(2) AFP reads the AF detector to assess whether or not and by what proportion focus has improved.

(3) mistreatment the data from (2), the AFP sets the lens to a replacement focusing distance.

(4) The AFP could iteratively repeat steps 2-3 till satisfactory focus has been achieved.

This entire method is typically completed among a fraction of a second. For tough subjects, the camera could fail to attain satisfactory focus and can hand over on continuation the on top of sequence, leading to unsuccessful optical device. this is often the dread "focus hunting" situation wherever the camera focuses back and forth repeatedly while not achieving focus lock. This doesn't, however, mean that focus isn't doable for the chosen subject. whether or not and why optical device could fail is primarily determined by factors within the next section.
FACTORS touching optical device PERFORMANCE

The photographic subject will have a massive impact on however well your camera autofocuses—and usually even additional thus than any variation between camera models, lenses or focus settings. The 3 most vital factors influencing optical device area unit the sunshine level, subject distinction and camera or subject motion.

comparison of optical device purpose quality

An example illustrating the standard of various focus points has been shown to the left; move your mouse over this image to check the benefits and downsides of every focus location.

Note that every of those factors don't seem to be independent; in different words, one {may be|could conjointly be|is also} ready to reach optical device even for a dimly lit subject if that very same subject also has extreme distinction, or the other way around. This has a vital implication for your selection of optical device purpose: choosing attention point that corresponds to a pointy edge or pronounced texture can do higher optical device, assumptive all different factors stay equal.

In the example to the left we tend to were lucky that the placement wherever optical device performs best conjointly corresponds to the topic location. successive example is additional problematic as a result of optical device performs best on the background, not the topic. Move your mouse over the image below to focus on areas of fine and poor performance.

example of image with poor optical device performance

In the ikon to the proper, if one targeted on the fast-moving lightweight sources behind the topic, one would risk associate out-of-focus subject once the depth of field is shallow (as would be the case for a low-light action shot like this one).

Alternatively, that specialize in the subject's exterior highlight would maybe be the most effective approach, with the caveat that this highlight would amendment sides and intensity apace betting on the placement of the moving lightweight sources.

If one's camera had problem that specialize in the outside highlight, a lower distinction (but stationary and fairly well lit) focus purpose would be the subject's foot, or leaves on the bottom at constant distance because the subject.

What makes the on top of selections tough, however, is that these selections usually have to be compelled to be either anticipated or created among a fraction of a second. further specific techniques for autofocusing on still and moving subjects are mentioned in their various sections towards the top of this tutorial.
NUMBER & variety of optical device POINTS

The lustiness and suppleness of optical device is primarily a results of the quantity, position and sort of optical device points created out there by a given camera model. High-end SLR cameras will have forty five or additional optical device points, whereas different cameras will have as few collectively central AF purpose. 2 example layouts of optical device sensors area unit shown below:

Max f/#:
   

f/2.8
   

f/4.0
   

f/5.6
   

f/8.0
   


   

f/2.8
   

f/4.0
   

f/5.6
   

   

high-end SLR optical device detector array
   

midrange SLR optical device detector array

   

High-End SLR
   

Entry to Midrange SLR

Cameras used for left and right examples area unit the Canon 1D MkII and Canon 20D, severally.

For these cameras optical device isn't doable for apertures smaller than f/8.0 and f/5.6.


   

Two varieties of optical device sensors area unit shown:

+
   

cross-type sensors (two-dimensional distinction detection, higher accuracy)

l
   

vertical line sensors (one-dimensional distinction detection, lower accuracy)

Note: The "vertical line sensor" is barely known as this as a result of it detects distinction on a vertical line.

Ironically, this kind of detector is so best at police investigation horizontal lines.

For SLR cameras, the quantity and accuracy of optical device points also can amendment betting on the utmost aperture of the lens getting used, as illustrated on top of. this is often a vital thought once selecting a camera lens: though you are doing not arrange on employing a lens at its most aperture, this aperture should facilitate the camera reach higher focus accuracy. Further, since the central AF detector is nearly continuously the foremost correct, for eccentric  subjects it's usually best to 1st use this detector to attain attention lock (before recomposing the frame).

Multiple AF points will work along for improved reliableness, or will add isolation for improved specificity, betting on your chosen camera setting. Some cameras even have associate "auto depth of field" feature for cluster photos that ensures that a cluster of focus points area unit all among an appropriate level of focus.
AF MODE: CONTINUOUS & AI SERVO vs. ONE SHOT

The most wide supported camera focus mode is one-shot focusing, that is best for still subjects. The unit of ammunition mode is liable to focus errors for fast paced subjects since it cannot anticipate subject motion, additionally to probably conjointly creating it tough to check these moving subjects within the optical device. unit of ammunition focusing needs attention lock before the photograph will be taken.

Many cameras conjointly support associate optical device mode that frequently change the main target distance for moving subjects. Canon cameras visit this as "AI Servo" focusing, whereas Nikon cameras visit his as "continuous" focusing. It works by predicting wherever the topic are slightly within the future, supported estimates of the topic rate from previous focus distances. The camera then focuses at this expected distance beforehand to account for the shutter lag (the delay between pressing the shutter button and also the begin of the exposure). This greatly will increase the likelihood of correct focus for moving subjects.

Example most trailing speeds area unit shown for varied Canon cameras below:

AI servo optical device performance chart

Values area unit for ideal distinction and lighting, and use the Canon 300mm f/2.8 IS L lens.

The on top of plot ought to conjointly give a rule of thumb estimate for different cameras still. Actual most trailing speeds conjointly depend upon however erratic the topic is moving, the topic distinction and lighting, the kind of lens and also the range of optical device sensors getting used to trace the topic. even be warned that mistreatment focus trailing will dramatically scale back the battery lifetime of your camera, thus use only necessary.
AUTOFOCUS ASSIST BEAM

Many cameras come back equipped with associate AF assist beam, that may be a technique of active optical device that uses an understandable or infrared beam to assist the optical device sensors notice the topic. this could be terribly useful in things wherever your subject isn't adequately lit or has low distinction for optical device, though the AF assist beam conjointly comes with the disadvantage of abundant slower optical device.

Most compact cameras use a inbuilt infrared emission supply for the AF assist, whereas digital SLR cameras usually use either a inbuilt or external camera flash to illuminate the topic. once employing a flash for the AF assist, the AF assist beam could have bother achieving focus lock if the topic moves appreciably between flash firings. Use of the AF assist beam is so solely suggested for still subjects.
IN PRACTICE: ACTION PHOTOS

Autofocus can nearly always perform best with action photos once mistreatment the AI servo or continuous modes. Focusing performance will be improved dramatically by guaranteeing that the lens doesn't have to be compelled to search over an outsized vary of focus distances.

Perhaps the foremost universally supported approach of achieving this is often to pre-focus your camera at a distance close to wherever you anticipate the moving subject to experience. within the biker example to the proper, one may pre-focus close to the facet of the road since one would expect the biker to travel at close to that distance.

Some SLR lenses even have a minimum focus distance switch; setting this to the best distance doable (assuming the topic can ne'er be closer) also can improve performance.

Be warned, however, that in continuous optical device mode shots will still be taken though the main target lock has not however been achieved.
IN PRACTICE: PORTRAITS & different STILL PHOTOS

Still photos area unit best taken mistreatment the one-shot optical device mode, that ensures that attention lock has been achieved before the exposure begins. the same old focus purpose necessities of distinction and robust lighting still apply, though one has to guarantee there's little subject motion.

For portraits, the attention is that the best focus point—both as a result of this is often a regular and since it's sensible distinction. though the central optical device detector is typically most sensitive, the foremost correct focusing is achieved mistreatment the eccentric  focus points for eccentric  subjects. If one were to instead use the central AF purpose to attain attention lock (prior to recomposing for associate eccentric  subject), the main target distance can continuously be behind the particular subject distance—and this error will increase for nearer subjects. correct focus is particularly vital for portraits as a result of these usually have a shallow depth of field.

Since the foremost common variety of AF detector is that the vertical line detector, it should even be value considering whether or not your focus purpose contains primarily vertical or horizontal distinction. In low-light conditions, one is also ready to reach attention lock not otherwise doable by rotating the camera 90° throughout optical device.

In the example to the left, the steps area unit comprised primarily of horizontal lines. If one were to focus close to the rear of the foreground stairs (to maximize apparent depth of field mistreatment the hyperfocal distance), one may avoid a unsuccessful optical device by 1st dimensioning their camera in landscape mode throughout optical device. later one may rotate the camera back to portrait orientation throughout the exposure, if thus desired.

UNDERSTANDING CAMERA AUTOFOCUS

A camera's autofocus system intelligently adjusts the camera lens to obtain focus on the subject, and can mean the difference between a sharp photo and a missed opportunity. Despite a seemingly simple goal—sharpness at the focus point—the inner workings of how a camera focuses are unfortunately not as straightforward. This tutorial aims to improve your photos by introducing how autofocus works—thereby enabling you to both make the most of its assets and avoid its shortcomings.

Note: Autofocus (AF) works either by using contrast sensors within the camera (passive AF) or by emitting a signal to illuminate or estimate distance to the subject (active AF). Passive AF can be performed using either the contrast detection or phase detection methods, but both rely on contrast for achieving accurate autofocus; they will therefore be treated as being qualitatively similar for the purposes of this AF tutorial. Unless otherwise stated, this tutorial will assume passive autofocus. We will also discuss the AF assist beam method of active autofocus towards the end.

CONCEPT: AUTOFOCUS SENSORS

A camera's autofocus sensor(s) are the real engine behind achieving accurate focus, and are laid out in various arrays across your image's field of view. Each sensor measures relative focus by assessing changes in contrast at its respective point in the image — where maximal contrast is assumed to correspond to maximal sharpness.

Change Focus Amount:		Blurred	Partial	Sharp
	400%	Sensor Histogram

Please visit the tutorial on image histograms for a background on image contrast.
Note: many compact digital cameras use the image sensor itself as a contrast sensor (using a method called contrast detection AF), and do not necessarily have multiple discrete autofocus sensors (which are more common using the phase detection method of AF).
Further, the above diagram illustrates the contrast detection method of AF;
phase detection is another method, but this still relies on contrast for accurate autofocus.

The process of autofocusing generally works as follows:
(1) An autofocus processor (AFP) makes a small change in the focusing distance.
(2) AFP reads the AF sensor to assess whether and by how much focus has improved.
(3) Using the information from (2), the AFP sets the lens to a new focusing distance.
(4) The AFP may iteratively repeat steps 2-3 until satisfactory focus has been achieved.
This entire process is usually completed within a fraction of a second. For difficult subjects, the camera may fail to achieve satisfactory focus and will give up on repeating the above sequence, resulting in failed autofocus. This is the dreaded "focus hunting" scenario where the camera focuses back and forth repeatedly without achieving focus lock. This does not, however, mean that focus is not possible for the chosen subject. Whether and why autofocus may fail is primarily determined by factors in the next section.

FACTORS AFFECTING AUTOFOCUS PERFORMANCE

The photographic subject can have an enormous impact on how well your camera autofocuses—and often even more so than any variation between camera models, lenses or focus settings. The three most important factors influencing autofocus are the light level, subject contrast and camera or subject motion.

An example illustrating the quality of different focus points has been shown to the left; move your mouse over this image to see the advantages and disadvantages of each focus location.
Note that each of these factors are not independent; in other words, one may be able to achieve autofocus even for a dimly lit subject if that same subject also has extreme contrast, or vice versa. This has an important implication for your choice of autofocus point: selecting a focus point which corresponds to a sharp edge or pronounced texture can achieve better autofocus, assuming all other factors remain equal.
In the example to the left we were fortunate that the location where autofocus performs best also corresponds to the subject location. The next example is more problematic because autofocus performs best on the background, not the subject. Move your mouse over the image below to highlight areas of good and poor performance.

In the photo to the right, if one focused on the fast-moving light sources behind the subject, one would risk an out-of-focus subject when the depth of field is shallow (as would be the case for a low-light action shot like this one).
Alternatively, focusing on the subject's exterior highlight would perhaps be the best approach, with the caveat that this highlight would change sides and intensity rapidly depending on the location of the moving light sources.
If one's camera had difficulty focusing on the exterior highlight, a lower contrast (but stationary and reasonably well lit) focus point would be the subject's foot, or leaves on the ground at the same distance as the subject.
What makes the above choices difficult, however, is that these decisions often have to be either anticipated or made within a fraction of a second. Additional specific techniques for autofocusing on still and moving subjects will be discussed in their respective sections towards the end of this tutorial.

NUMBER & TYPE OF AUTOFOCUS POINTS

The robustness and flexibility of autofocus is primarily a result of the number, position and type of autofocus points made available by a given camera model. High-end SLR cameras can have 45 or more autofocus points, whereas other cameras can have as few as one central AF point. Two example layouts of autofocus sensors are shown below:

Max f/#:	f/2.8	f/4.0	f/5.6	f/8.0		f/2.8	f/4.0	f/5.6
	High-End SLR					Entry to Midrange SLR

Cameras used for left and right examples are the Canon 1D MkII and Canon 20D, respectively.
For these cameras autofocus is not possible for apertures smaller than f/8.0 and f/5.6.

	Two types of autofocus sensors are shown:
+	cross-type sensors (two-dimensional contrast detection, higher accuracy)
l	vertical line sensors (one-dimensional contrast detection, lower accuracy)

Note: The "vertical line sensor" is only called this because it detects contrast along a vertical line.
Ironically, this type of sensor is therefore best at detecting horizontal lines.

For SLR cameras, the number and accuracy of autofocus points can also change depending on the maximum aperture of the lens being used, as illustrated above. This is an important consideration when choosing a camera lens: even if you do not plan on using a lens at its maximum aperture, this aperture may still help the camera achieve better focus accuracy. Further, since the central AF sensor is almost always the most accurate, for off-center subjects it is often best to first use this sensor to achieve a focus lock (before recomposing the frame).
Multiple AF points can work together for improved reliability, or can work in isolation for improved specificity, depending on your chosen camera setting. Some cameras also have an "auto depth of field" feature for group photos which ensures that a cluster of focus points are all within an acceptable level of focus.

AF MODE: CONTINUOUS & AI SERVO vs. ONE SHOT

The most widely supported camera focus mode is one-shot focusing, which is best for still subjects. The one shot mode is susceptible to focus errors for fast moving subjects since it cannot anticipate subject motion, in addition to potentially also making it difficult to visualize these moving subjects in the viewfinder. One shot focusing requires a focus lock before the photograph can be taken.
Many cameras also support an autofocus mode which continually adjust the focus distance for moving subjects. Canon cameras refer to this as "AI Servo" focusing, whereas Nikon cameras refer to his as "continuous" focusing. It works by predicting where the subject will be slightly in the future, based on estimates of the subject velocity from previous focus distances. The camera then focuses at this predicted distance in advance to account for the shutter lag (the delay between pressing the shutter button and the start of the exposure). This greatly increases the probability of correct focus for moving subjects.
Example maximum tracking speeds are shown for various Canon cameras below:

Values are for ideal contrast and lighting, and use the Canon 300mm f/2.8 IS L lens.

The above plot should also provide a rule of thumb estimate for other cameras as well. Actual maximum tracking speeds also depend on how erratic the subject is moving, the subject contrast and lighting, the type of lens and the number of autofocus sensors being used to track the subject. Also be warned that using focus tracking can dramatically reduce the battery life of your camera, so use only when necessary.

AUTOFOCUS ASSIST BEAM

Many cameras come equipped with an AF assist beam, which is a method of active autofocus that uses a visible or infrared beam to help the autofocus sensors detect the subject. This can be very helpful in situations where your subject is not adequately lit or has insufficient contrast for autofocus, although the AF assist beam also comes with the disadvantage of much slower autofocus.
Most compact cameras use a built-in infrared light source for the AF assist, whereas digital SLR cameras often use either a built-in or external camera flash to illuminate the subject. When using a flash for the AF assist, the AF assist beam may have trouble achieving focus lock if the subject moves appreciably between flash firings. Use of the AF assist beam is therefore only recommended for still subjects.

IN PRACTICE: ACTION PHOTOS

Autofocus will almost always perform best with action photos when using the AI servo or continuous modes. Focusing performance can be improved dramatically by ensuring that the lens does not have to search over a large range of focus distances.

Perhaps the most universally supported way of achieving this is to pre-focus your camera at a distance near where you anticipate the moving subject to pass through. In the biker example to the right, one could pre-focus near the side of the road since one would expect the biker to pass by at near that distance.
Some SLR lenses also have a minimum focus distance switch; setting this to the greatest distance possible (assuming the subject will never be closer) can also improve performance.
Be warned, however, that in continuous autofocus mode shots can still be taken even if the focus lock has not yet been achieved.

IN PRACTICE: PORTRAITS & OTHER STILL PHOTOS

Still photos are best taken using the one-shot autofocus mode, which ensures that a focus lock has been achieved before the exposure begins. The usual focus point requirements of contrast and strong lighting still apply, although one needs to ensure there is very little subject motion.
For portraits, the eye is the best focus point—both because this is a standard and because it has good contrast. Although the central autofocus sensor is usually most sensitive, the most accurate focusing is achieved using the off-center focus points for off-center subjects. If one were to instead use the central AF point to achieve a focus lock (prior to recomposing for an off-center subject), the focus distance will always be behind the actual subject distance—and this error increases for closer subjects. Accurate focus is especially important for portraits because these typically have a shallow depth of field.

Since the most common type of AF sensor is the vertical line sensor, it may also be worth considering whether your focus point contains primarily vertical or horizontal contrast. In low-light conditions, one may be able to achieve a focus lock not otherwise possible by rotating the camera 90° during autofocus.
In the example to the left, the stairs are comprised primarily of horizontal lines. If one were to focus near the back of the foreground stairs (to maximize apparent depth of field using the hyperfocal distance), one could avoid a failed autofocus by first orienting their camera in landscape mode during autofocus. Afterwards one could rotate the camera back to portrait orientation during the exposure, if so desired.

CAMERA EXPOSURE

UNDERSTANDING EXPOSURE

Achieving the correct exposure is a lot like collecting rain in a bucket. While the rate of rainfall is uncontrollable, three factors remain under your control: the bucket's width, the duration you leave it in the rain, and the quantity of rain you want to collect. You just need to ensure you don't collect too little ("underexposed"), but that you also don't collect too much ("overexposed"). The key is that there are many different combinations of width, time and quantity that will achieve this. For example, for the same quantity of water, you can get away with less time in the rain if you pick a bucket that's really wide. Alternatively, for the same duration left in the rain, a really narrow bucket can be used as long as you plan on getting by with less water.
In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to the width, time and quantity discussed above. Furthermore, just as the rate of rainfall was beyond your control above, so too is natural light for a photographer.

EXPOSURE TRIANGLE: APERTURE, ISO & SHUTTER SPEED

Each setting controls exposure differently:
Aperture: controls the area over which light can enter your camera
Shutter speed: controls the duration of the exposure
ISO speed: controls the sensitivity of your camera's sensor to a given amount of light
One can therefore use many combinations of the above three settings to achieve the same exposure. The key, however, is knowing which trade-offs to make, since each setting also influences other image properties. For example, aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.
The next few sections will describe how each setting is specified, what it looks like, and how a given camera exposure mode affects their combination.

SHUTTER SPEED

A camera's shutter determines when the camera sensor will be open or closed to incoming light from the camera lens. The shutter speed specifically refers to how long this light is permitted to enter the camera. "Shutter speed" and "exposure time" refer to the same concept, where a faster shutter speed means a shorter exposure time.
By the Numbers. Shutter speed's influence on exposure is perhaps the simplest of the three camera settings: it correlates exactly 1:1 with the amount of light entering the camera. For example, when the exposure time doubles the amount of light entering the camera doubles. It's also the setting that has the widest range of possibilities:

Shutter Speed	Typical Examples
1 - 30+ seconds	Specialty night and low-light photos on a tripod
2 - 1/2 second	To add a silky look to flowing water Landscape photos on a tripod for enhanced depth of field
1/2 to 1/30 second	To add motion blur to the background of a moving subject Carefully taken hand-held photos with stabilization
1/50 - 1/100 second	Typical hand-held photos without substantial zoom
1/250 - 1/500 second	To freeze everyday sports/action subject movement Hand-held photos with substantial zoom (telephoto lens)
1/1000 - 1/4000 second	To freeze extremely fast, up-close subject motion

How it Appears. Shutter speed is a powerful tool for freezing or exaggerating the appearance of motion:

Slow Shutter Speed

Fast Shutter Speed

With waterfalls and other creative shots, motion blur is sometimes desirable, but for most other shots this is avoided. Therefore all one usually cares about with shutter speed is whether it results in a sharp photo — either by freezing movement or because the shot can be taken hand-held without camera shake.
How do you know which shutter speed will provide a sharp hand-held shot? With digital cameras, the best way to find out is to just experiment and look at the results on your camera's rear LCD screen (at full zoom). If a properly focused photo comes out blurred, then you'll usually need to either increase the shutter speed, keep your hands steadier or use a camera tripod.
For more on this topic, see the tutorial on Using Camera Shutter Speed Creatively.

APERTURE SETTING

A camera's aperture setting controls the area over which light can pass through your camera lens. It is specified in terms an f-stop value, which can at times be counterintuitive, because the area of the opening increases as the f-stop decreases. In photographer slang, when someone says they are "stopping down" or "opening up" their lens, they are referring to increasing and decreasing the f-stop value, respectively.

By the Numbers. Every time the f-stop value halves, the light-collecting area quadruples. There's a formula for this, but most photographers just memorize the f-stop numbers that correspond to each doubling/halving of light:

Aperture Setting	Relative Light	Example Shutter Speed
f/22	1X	16 seconds
f/16	2X	8 seconds
f/11	4X	4 seconds
f/8.0	8X	2 seconds
f/5.6	16X	1 second
f/4.0	32X	1/2 second
f/2.8	64X	1/4 second
f/2.0	128X	1/8 second
f/1.4	256X	1/15 second

The above aperture and shutter speed combinations all result in the same exposure.

Note: Shutter speed values are not always possible in increments of exactly double or half another shutter speed, but they're always close enough that the difference is negligible.

The above f-stop numbers are all standard options in any camera, although most also allow finer adjustments, such as f/3.2 and f/6.3. The range of values may also vary from camera to camera (or lens to lens). For example, a compact camera might have an available range of f/2.8 to f/8.0, whereas a digital SLR camera might have a range of f/1.4 to f/32 with a portrait lens. A narrow aperture range usually isn't a big problem, but a greater range does provide for more creative flexibility.

Technical Note: With many lenses, their light-gathering ability is also affected by their transmission efficiency, although this is almost always much less of a factor than aperture. It's also beyond the photographer's control. Differences in transmision efficiency are typically more pronounced with extreme zoom ranges. For example, Canon's 24-105 mm f/4L IS lens gathers perhaps ~10-40% less light at f/4 than Canon's similar 24-70 mm f/2.8L lens at f/4 (depending on the focal length).

How it Appears. A camera's aperture setting is what determines a photo's depth of field (the range of distance over which objects appear in sharp focus). Lower f-stop values correlate with a shallower depth of field:

Wide Aperture
f/2.0 - low f-stop number
shallow depth of field

Narrow Aperture
f/16 - high f-stop number
large depth of field

ISO SPEED

The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, it also correlates 1:1 with how much the exposure increases or decreases. However, unlike aperture and shutter speed, a lower ISO speed is almost always desirable, since higher ISO speeds dramatically increase image noise. As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren't otherwise obtainable.

Low ISO Speed
(low image noise)

High ISO Speed
(high image noise)

note: image noise is also known as "film grain" in traditional film photography

Common ISO speeds include 100, 200, 400 and 800, although many cameras also permit lower or higher values. With compact cameras, an ISO speed in the range of 50-200 generally produces acceptably low image noise, whereas with digital SLR cameras, a range of 50-800 (or higher) is often acceptable.

CAMERA EXPOSURE MODES

Most digital cameras have one of the following standardized exposure modes: Auto (), Program (P), Aperture Priority (Av), Shutter Priority (Tv), Manual (M) and Bulb (B) mode. Av, Tv, and M are often called "creative modes" or "auto exposure (AE) modes."
Each of these modes influences how aperture, ISO and shutter speed are chosen for a given exposure. Some modes attempt to pick all three values for you, whereas others let you specify one setting and the camera picks the other two (if possible). The following charts describe how each mode pertains to exposure:

Exposure Mode	How It Works
Auto ()	Camera automatically selects all exposure settings.
Program (P)	Camera automatically selects aperture & shutter speed; you can choose a corresponding ISO speed & exposure compensation. With some cameras, P can also act as a hybrid of the Av & Tv modes.
Aperture Priority (Av or A)	You specify the aperture & ISO; the camera's metering determines the corresponding shutter speed.
Shutter Priority (Tv or S)	You specify the shutter speed & ISO; the camera's metering determines the corresponding aperture.
Manual (M)	You specify the aperture, ISO and shutter speed — regardless of whether these values lead to a correct exposure.
Bulb (B)	Useful for exposures longer than 30 seconds. You specify the aperture and ISO; the shutter speed is determined by a remote release switch, or by the duration until you press the shutter button a second time.

In addition, the camera may also have several pre-set modes; the most common include landscape, portrait, sports and night mode. The symbols used for each mode vary slightly from camera to camera, but will likely appear similar to those below:

Exposure Mode	How It Works
Portrait	Camera tries to pick the lowest f-stop value possible for a given exposure. This ensures the shallowest possible depth of field.
Landscape	Camera tries to pick a high f-stop to ensure a large depth of field. Compact cameras also often set their focus distance to distant objects or infinity.
Sports/Action	Camera tries to achieve as fast a shutter speed as possible for a given exposure — ideally 1/250 seconds or faster. In addition to using a low f-stop, the fast shutter speed is usually achieved by increasing the ISO speed more than would otherwise be acceptable in portrait mode.
Night/Low-light	Camera permits shutter speeds which are longer than ordinarily allowed for hand-held shots, and increases the ISO speed to near its maximum available value. However, for some cameras this setting means that a flash is used for the foreground, and a long shutter speed and high ISO are used expose the background. Check your camera's instruction manual for any unique characteristics.

However, keep in mind that most of the above settings rely on the camera's metering system in order to know what's a proper exposure. For tricky subject matter, metering can often be fooled, so it's a good idea to also be aware of when it might go awry, and what you can do to compensate for such exposure errors (see section on exposure compensation within the camera metering tutorial).

DIGITAL CAMERA SENSORS

A digital camera uses an array of millions of tiny light cavities or "photosites" to record an image. When you press your camera's shutter button and the exposure begins, each of these is uncovered to collect and store photons. Once the exposure finishes, the camera closes each of these photosites, and then tries to assess how many photons fell into each. The relative quantity of photons in each cavity are then sorted into various intensity levels, whose precision is determined by bit depth (0 - 255 for an 8-bit image).

Cavity Array

Light Cavities

However, the above illustration would only create grayscale images, since these cavities are unable to distinguish how much they have of each color. To capture color images, a filter has to be placed over each cavity that permits only particular colors of light. Virtually all current digital cameras can only capture one of three primary colors in each cavity, and so they discard roughly 2/3 of the incoming light. As a result, the camera has to approximate the other two primary colors in order to have full color at every pixel. The most common type of color filter array is called a "Bayer array," shown below.

Color Filter Array

Photosites with Color Filters

A Bayer array consists of alternating rows of red-green and green-blue filters. Notice how the Bayer array contains twice as many green as red or blue sensors. Each primary color does not receive an equal fraction of the total area because the human eye is more sensitive to green light than both red and blue light. Redundancy with green pixels produces an image which appears less noisy and has finer detail than could be accomplished if each color were treated equally. This also explains why noise in the green channel is much less than for the other two primary colors (see "Understanding Image Noise" for an example).

Original Scene
(shown at 200%)

What Your Camera Sees
(through a Bayer array)

Note: Not all digital cameras use a Bayer array, however this is by far the most common setup. The Foveon sensor used in Sigma's SD9 and SD10 captures all three colors at each pixel location. Sony cameras capture four colors in a similar array: red, green, blue and emerald green.

BAYER DEMOSAICING

Bayer "demosaicing" is the process of translating this Bayer array of primary colors into a final image which contains full color information at each pixel. How is this possible if the camera is unable to directly measure full color? One way of understanding this is to instead think of each 2x2 array of red, green and blue as a single full color cavity.

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This would work fine, however most cameras take additional steps to extract even more image information from this color array. If the camera treated all of the colors in each 2x2 array as having landed in the same place, then it would only be able achieve half the resolution in both the horizontal and vertical directions. On the other hand, if a camera computed the color using several overlapping 2x2 arrays, then it could achieve a higher resolution than would be possible with a single set of 2x2 arrays. The following combination of overlapping 2x2 arrays could be used to extract more image information.

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Note how we did not calculate image information at the very edges of the array, since we assumed the image continued on in each direction. If these were actually the edges of the cavity array, then calculations here would be less accurate, since there are no longer pixels on all sides. This is no problem, since information at the very edges of an image can easily be cropped out for cameras with millions of pixels.
Other demosaicing algorithms exist which can extract slightly more resolution, produce images which are less noisy, or adapt to best approximate the image at each location.

DEMOSAICING ARTIFACTS

Images with small-scale detail near the resolution limit of the digital sensor can sometimes trick the demosaicing algorithm—producing an unrealistic looking result. The most common artifact is moiré (pronounced "more-ay"), which may appear as repeating patterns, color artifacts or pixels arranges in an unrealistic maze-like pattern:

Second Photo at ↓ 65% of Above Size

Two separate photos are shown above—each at a different magnification. Note the appearance of moiré in all four bottom squares, in addition to the third square of the first photo (subtle). Both maze-like and color artifacts can be seen in the third square of the downsized version. These artifacts depend on both the type of texture and software used to develop the digital camera's RAW file.

MICROLENS ARRAYS

You might wonder why the first diagram in this tutorial did not place each cavity directly next to each other. Real-world camera sensors do not actually have photosites which cover the entire surface of the sensor. In fact, they often cover just half the total area in order to accommodate other electronics. Each cavity is shown with little peaks between them to direct the photons to one cavity or the other. Digital cameras contain "microlenses" above each photosite to enhance their light-gathering ability. These lenses are analogous to funnels which direct photons into the photosite where the photons would have otherwise been unused.

Well-designed microlenses can improve the photon signal at each photosite, and subsequently create images which have less noise for the same exposure time. Camera manufacturers have been able to use improvements in microlens design to reduce or maintain noise in the latest high-resolution cameras, despite having smaller photosites due to squeezing more megapixels into the same sensor area.