|
Autofocus
Autofocus systems
Over recent years, a variety of different autofocus (AF) systems have been offered with many different makes
and types of camera.
Active AF systems work by means of a light beam (often infra-red) that is emitted by the camera and reflected
back from the subject. These systems may be fast and accurate, but can be confused when you try to shoot through a window – they may
focus on the glass rather than on the subject behind it.
Some more modern passive AF systems work using contrast
measurement
on one or more parts of the subject and are not bothered by an intervening pane of glass. These systems tend to work well in good light,
but may be slow or inaccurate, or fail totally, in low light.
There are also hybrid AF systems designed to combine the speed of an active
system with the reliability of a passive system.
If your camera has an AF system, you should at least be aware of how it works and what
its limitations may be.
Using autofocus
Autofocus at its best can be quite fast and accurate, though not consistently as accurate as careful manual focusing.
It is especially useful in good light with fast-moving subjects. For some cameras, autofocus accuracy depends on the maximum lens aperture,
with faster lenses giving better results. In poor light and with slower lenses, most AF systems become slower and less
accurate, and may not achieve focus at all.
The flip side of these facts is that autofocus should not be used all the time under all
circumstances for best possible results.
Many cameras provide some or all of the following AF ‘modes’:
-
AF off – use manual focus instead.
-
One-shot AF mode – AF is activated once, usually by half-pressing the shutter release. You can then recompose, if you
wish, holding that same focus point. When the shot is taken, AF is deactivated.
-
Continuous (or ‘Servo’) AF mode – AF operates continuously once activated, holding focus on whatever was the original
focus point, even if the subject or camera moves. If you remove pressure on the shutter release without taking a picture, AF is deactivated.
With subjects moving towards (or away from) the camera, some cameras can anticipate their motion during the shutter delay (just a short
fraction of a second with better cameras).
-
Auto-switching AF mode – the camera behaves as though in one-shot mode unless the subject moves after focus has been
achieved, in which case it switches to continuous AF mode.
For stationary subjects, use manual focus (MF) or one-shot AF mode. For moving subjects, use Continuous AF mode. For
stationary subjects that may be expected to move, such as wildlife, consider using the auto-switching AF mode if you have it.
Remember that
AF activity consumes battery power, so if this becomes an issue, MF may be a better choice.
How accurately you need to focus depends on
depth of field, which depends on focal length, aperture and subject distance.
Many cameras have a number of selectable AF sensors. You can generally choose any specific sensor or let the camera
choose for you. If you let the camera choose, it will generally select the sensor that finds the nearest subject element. This can be useful,
for example, when shooting sports action with long lenses. Usually, there will be some indication in the viewfinder of which sensor(s)
achieved focus. It is obviously important to know what the camera decided to focus on. It may (or may not) be appropriate to the image
you intended to make. For deliberate photography, you should select which sensor the camera should use yourself. This can be the centre
sensor, which is often a cross-sensor (see next section) or a specific sensor that aligns with your main point of interest. Some cameras
with a large number of sensors allow selection of a reduced number of sensors for faster, but possibly less accurate, autofocus.
If you use the centre sensor in one-shot mode, be careful not to swing the camera through a very large angle if you
then recompose, as this will move the plane of focus and could cause that detail in your image to be blurred. This issue (which also applies
to manual focusing) is mainly a concern when using fast normal and moderate wide-angle lenses. Long lenses cannot be moved through much
of an angle while still holding your point of focus in the field of view (and it would be eccentric at least to focus on something not in
the picture), and extreme wide-angle lenses, not being especially fast, have great depth of field.
Canon-specific information
This description of the AF system in many of Canon’s SLRs and DSLRs is simplified, but essentially based on several
apparently well-informed posts on the Fred Miranda forum, which in their turn have drawn from
material in (1) Canon's book "Lens Work III", (2) their US patent application and (3) comments from senior people at Canon USA.
Other camera makers may use very similar or quite different systems. If anyone can offer detailed information on these, please send us
feedback.
The AF sensors are located in the floor of the mirror box. They receive the image through the semi-silvered mirror,
which is then reflected downward by a secondary mirror hinged to the back of the main mirror. This forms a virtual focusing plane that is
supposed to be calibrated to the same plane as the film or sensor.
There are three types of sensor: vertical, horizontal and cross sensors. Different camera models have different
numbers of sensors of these types (older entry-level cameras having as few as 3 sensors; top-level professional cameras having an
array of up to 45 sensors). Each vertical and horizontal AF sensor consists of a pair of short lines of pixels forming an array. Two crossed
arrays (one vertical, one horizontal) comprise the cross sensor(s). With lenses of f/2.8 or faster, some cameras can activate additional
vertical arrays, providing the potential for more accurate focusing.
These arrays are sensitive to linear image contrast details (i.e. stripes) that are perpendicular (or roughly so)
to the alignment of the array. Horizontal arrays (shown as horizontal rectangles in the viewfinder) are sensitive to stripes with a
pronounced vertical component; vertical arrays (shown as vertical rectangles in the viewfinder) are sensitive to stripes with a pronounced
horizontal component. Cross sensors are sensitive to both vertical and horizontal stripes. When the additional arrays are activated, the
combined input increases focusing accuracy by a factor of 3.
The pixel arrays are actually about three times longer than the viewfinder markings would suggest. Therefore, the
sensors may react to image details that are outside the viewfinder markings, and may focus on them instead of on details within the sensor
markings.
When you activate AF (by half-pressing the shutter release or the * button, depending on your custom function
configuration), each line of pixels in each sensor array looks at the image from the ‘opposite direction’ (presumably this means from
the opposite side of the lens, which is at full aperture during AF operation) of the other line of pixels, allowing the system to measure
the phase difference of the light from each direction. It calculates the distance and direction the lens must be moved to cancel the phase
differences, causes the lens to move accordingly, and then stop. It does not hunt for best focus.
If the starting point is so far out of focus that the sensor can't identify a phase difference, the camera racks the
lens once forward and once backward to find a detectable difference. If it can't find a detectable phase difference during that exercise,
it stops.
Although the camera AF system does not take a "second look" to see if the intended focus has been achieved,
the lens focus mechanism does check it has moved the amount and direction commanded by the camera, and thus corrects for any play in the lens
mechanism. This can often be detected as a small "twitch" at the end of the longer initial movements.
How accurate is the AF system? When the camera determines how the lens must move to cancel the phase difference, it
does so within a tolerance of one "depth of focus" (see below) for lenses slower than f/2.8 down to f/5.6. For lenses with maximum
aperture of f/2.8 and faster, the tolerance is 1/3 of the depth of focus. Depth of focus is the range at the sensor plane within which the
image of a point will be reproduced as a blur smaller than the manufacturer's designated "circle of confusion" (CoC). Canon's
standard for the circle of confusion is 0.035mm for the 24x36mm (“full frame”) format and 0.02mm for the APS-C format (with a crop factor
of 1.6×). This value for the CoC diameter is based on maintaining the appearance of sharpness in a 6x9 inch print at about a 10 inch viewing
distance (not a very stringent criterion – check also the articles on Depth of Field and
Resolution).
 |
