How Does A Camera Focus

UNDERSTANDING CAMERA AUTOFOCUS

A photographic camera's autofocus system intelligently adjusts the camera lens to obtain focus on the subject, and tin mean the departure between a precipitous 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 equally straightforward. This tutorial aims to improve your photos by introducing how autofocus works—thereby enabling you to both brand the most of its avails and avoid its shortcomings.

Note: Autofocus (AF) works either past using contrast sensors within the camera (passive AF) or by emitting a indicate to illuminate or estimate distance to the subject field (active AF). Passive AF can be performed using either the dissimilarity detection or stage detection methods, but both rely on dissimilarity 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 likewise discuss the AF aid beam method of agile autofocus towards the end.

CONCEPT: AUTOFOCUS SENSORS

A photographic camera'due south autofocus sensor(s) are the real engine behind achieving accurate focus, and are laid out in various arrays beyond 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 causeless to stand for to maximal sharpness.

Change Focus Amount:
Blurred	Partial	Sharp

Please visit the tutorial on paradigm histograms for a background on image contrast.
Note: many compact digital cameras utilize the image sensor itself as a dissimilarity sensor (using a method chosen dissimilarity detection AF), and exercise non necessarily take multiple discrete autofocus sensors (which are more common using the phase detection method of AF). Furthermore, 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 equally follows:
(1) An autofocus processor (AFP) makes a small change in the focusing distance.
(2) AFP reads the AF sensor to appraise whether and by how much focus has improved.
(3) Using the information from (2), the AFP sets the lens to a new focusing altitude.
(4) The AFP may iteratively repeat steps 2-iii until satisfactory focus has been achieved.

This entire process is normally completed within a fraction of a 2d. For difficult subjects, the camera may fail to achieve satisfactory focus and will give up on repeating the to a higher place 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, yet, mean that focus is non possible for the chosen subject field. Whether and why autofocus may fail is primarily determined by factors in the next department.

FACTORS AFFECTING AUTOFOCUS Performance

The photographic subject tin take an enormous affect on how well your camera autofocuses—and often even more than so than any variation between camera models, lenses or focus settings. The three most important factors influencing autofocus are the light level, bailiwick dissimilarity and camera or field of study motion.

comparison of autofocus point quality

An instance 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 non independent; in other words, one may be able to attain autofocus even for a dimly lit bailiwick if that same subject besides has extreme contrast, or vice versa. This has an important implication for your choice of autofocus point: selecting a focus betoken which corresponds to a sharp edge or pronounced texture tin achieve better autofocus, assuming all other factors remain equal.

In the instance to the left we were fortunate that the location where autofocus performs best besides corresponds to the subject location. The side by side instance is more problematic because autofocus performs best on the background, not the subject field. Move your mouse over the image below to highlight areas of proficient and poor performance.

example of image with poor autofocus performance

In the photo to the right, if one focused on the fast-moving light sources backside the subject, one would gamble an out-of-focus subject when the depth of field is shallow (equally would be the case for a low-light activeness shot similar this one).

Alternatively, focusing on the subject's exterior highlight would peradventure exist the best arroyo, with the caveat that this highlight would change sides and intensity rapidly depending on the location of the moving low-cal sources.

If one's camera had difficulty focusing on the exterior highlight, a lower contrast (merely stationary and reasonably well lit) focus point would be the subject's foot, or leaves on the basis at the same distance as the discipline.

What makes the above choices difficult, even so, is that these decisions often have to exist either anticipated or fabricated 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 cease of this tutorial.

NUMBER & TYPE OF AUTOFOCUS POINTS

The robustness and flexibility of autofocus is primarily a result of the number, position and blazon of autofocus points made available by a given camera model. Loftier-end SLR cameras can take 45 or more autofocus points, whereas other cameras can have as few every bit 1 key AF signal. Two example layouts of autofocus sensors are shown below:

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/viii.0 and f/five.6.

Two types of autofocus sensors are shown:
+cross-type sensors (2-dimensional contrast detection, higher accuracy)
lvertical line sensors (ane-dimensional contrast detection, lower accuracy)

Note: The "vertical line sensor" is but called this considering information technology detects contrast forth a vertical line.
Ironically, this blazon of sensor is therefore best at detecting horizontal lines.

For SLR cameras, the number and accuracy of autofocus points tin can also change depending on the maximum aperture of the lens being used, as illustrated above. This is an important consideration when choosing a photographic camera lens: even if you do not plan on using a lens at its maximum aperture, this aperture may still assist the camera achieve meliorate focus accuracy. Furthermore, since the key AF sensor is almost e'er the virtually accurate, for off-center subjects it is oft best to kickoff use this sensor to attain a focus lock (earlier recomposing the frame).

Multiple AF points can piece of work together for improved reliability, or can piece of work in isolation for improved specificity, depending on your chosen camera setting. Some cameras likewise have an "auto depth of field" characteristic 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 manner is one-shot focusing, which is best for notwithstanding subjects. The i shot mode is susceptible to focus errors for fast moving subjects since it cannot conceptualize subject movement, 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 as well support an autofocus style 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. Information technology works by predicting where the subject will exist slightly in the future, based on estimates of the discipline velocity from previous focus distances. The camera and so focuses at this predicted distance in advance to account for the shutter lag (the delay betwixt pressing the shutter push 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:

AI servo autofocus performance chart

Values are for platonic contrast and lighting, and use the Canon 300mm f/2.8 IS 50 lens.

The above plot should also provide a dominion of thumb estimate for other cameras as well. Actual maximum tracking speeds also depend on how erratic the field of study is moving, the subject contrast and lighting, the type of lens and the number of autofocus sensors being used to runway the subject. Likewise exist warned that using focus tracking can dramatically reduce the bombardment life of your photographic camera, so use simply 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 field of study. This can be very helpful in situations where your subject area is not fairly lit or has insufficient contrast for autofocus, although the AF assist beam too comes with the disadvantage of much slower autofocus.

Well-nigh compact cameras utilise a built-in infrared light source for the AF assist, whereas digital SLR cameras frequently use either a built-in or external camera flash to illuminate the discipline. When using a wink for the AF aid, the AF assist axle may have problem achieving focus lock if the subject field moves appreciably between wink firings. Use of the AF aid beam is therefore only recommended for still subjects.

IN PRACTICE: Action PHOTOS

Autofocus volition almost e'er perform all-time with activeness 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 big range of focus distances.

Perhaps the nigh universally supported way of achieving this is to pre-focus your camera at a distance near where yous conceptualize the moving subject area to laissez passer through. In the biker case to the right, one could pre-focus nigh the side of the road since i would expect the biker to pass by at near that altitude.

Some SLR lenses besides have a minimum focus distance switch; setting this to the greatest distance possible (bold the subject will never exist closer) tin can also improve functioning.

Be warned, however, that in continuous autofocus mode shots tin can withal be taken even if the focus lock has not yet been achieved.

IN PRACTICE: PORTRAITS & OTHER STILL PHOTOS

Still photos are all-time taken using the 1-shot autofocus way, which ensures that a focus lock has been accomplished before the exposure begins. The usual focus betoken requirements of contrast and strong lighting still apply, although one needs to ensure there is very piffling subject field motion.

For portraits, the middle is the best focus bespeak—both because this is a standard and because it has good contrast. Although the key autofocus sensor is ordinarily nigh sensitive, the most accurate focusing is achieved using the off-centre focus points for off-eye subjects. If one were to instead employ the central AF signal to achieve a focus lock (prior to recomposing for an off-center subject), the focus distance volition ever exist behind the actual discipline 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, i may be able to achieve a focus lock not otherwise possible by rotating the photographic camera 90° during autofocus.

In the example to the left, the stairs are comprised primarily of horizontal lines. If 1 were to focus near the dorsum of the foreground stairs (to maximize apparent depth of field using the hyperfocal distance), 1 could avoid a failed autofocus by first orienting their photographic camera in landscape mode during autofocus. Afterwards 1 could rotate the photographic camera back to portrait orientation during the exposure, if and so desired.

Annotation that the emphasis in this tutorial has been on *how* to focus — non necessarily *where* to focus. For further reading on this topic delight visit the tutorials on depth of field and the hyperfocal distance.