8/4/2019 Understanding Camera Autofocus

http://slidepdf.com/reader/full/understanding-camera-autofocus 1/5

rstanding Camera Autofocus

/www.cambridgeincolour.com/tutorials/camera-autofocus.htm[3/10/2011 3:26:46 PM]

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.

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 morecommon 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) The 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.

The Ultimate ND FilterControl Exposure, Aperture, Blur With the

Original Fader Filter

Singh-Ray FiltersProfessional grade camera filters for

photography and video

ShareThis

home   gallery   techniques   tutorials   forums  

Follow via:

Receive updates wh

new articles are add

search

enter your email here

Submit

8/4/2019 Understanding Camera Autofocus

http://slidepdf.com/reader/full/understanding-camera-autofocus 2/5

rstanding Camera Autofocus

/www.cambridgeincolour.com/tutorials/camera-autofocus.htm[3/10/2011 3:26:46 PM]

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 thesame 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.

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:

8/4/2019 Understanding Camera Autofocus

http://slidepdf.com/reader/full/understanding-camera-autofocus 3/5

rstanding Camera Autofocus

/www.cambridgeincolour.com/tutorials/camera-autofocus.htm[3/10/2011 3:26:46 PM]

+ 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.

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.

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.

8/4/2019 Understanding Camera Autofocus

http://slidepdf.com/reader/full/understanding-camera-autofocus 4/5

rstanding Camera Autofocus

/www.cambridgeincolour.com/tutorials/camera-autofocus.htm[3/10/2011 3:26:46 PM]

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.

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 thecamera 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.

Note that the emphasis in this tutorial has been on *how* to focus — not necessarily

*where* to focus. For further reading on this topic please visit the tutorials on depth of field

and the hyperfocal distance.

Want to learn more? Discuss this and other articles in our digital photography forums.

Program Of Interest.

Select One:

Program Of Interest.

First Name: Phone Number:

Last Name: Select One:

Email Address: Zip:

II LII L

8/4/2019 Understanding Camera Autofocus

http://slidepdf.com/reader/full/understanding-camera-autofocus 5/5

rstanding Camera Autofocus

/ b id i l /t t i l / t f ht [3/10/2011 3 26 46 PM]

-Back to Photography Tutorials-

about  contact us  copyright notice