Many compact digital cameras
offer a number of shooting modes. This article is intended
to clarify the use and functions of modes that are often
referred to as a group as P/A/S/M modes, shorthand for Program,
Aperture Priority, Shutter Priority and Manual mode. Unlike
the ubiquitous Auto mode which usually leaves the
decision making for most photographic settings to algorithms,
the P/A/S/M modes give the user some measure of control over
how an image is captured, and each offers control over distinct
aspects of a camera.
Program Mode (P)
Program mode is very similar to the simplest shooting mode: Auto mode.
Indeed if some manufacturers use these designations interchangeably as
both the Auto and Program modes let the camera control aperture and shutter
speed, the Program mode is supposed to provide some controls over
other photographic parameters. Most Program modes allow the user to choose
alternative settings for white balance, exposure compensation, metering
pattern, and with some cameras, focus point selection.
Some cameras also offer a souped up version of the basic Program mode.
Often called Program Shift mode, it lets the user choose from a
range of alternative combinations of aperture/shutter speeds that would
result in photos with generally similar exposure characteristics, but which
would give preference to freezing movement (faster shutter speed and wider
aperture), or to depth of field (smaller aperture and slower shutter speed).
For example,
an aperture of f8 at 1/125 sec would produce an image with
a large area in focus — a long depth of field — but
could still freeze a relatively slow moving subject. While
an aperture of f2.8 and a shutter speed of 1/750 sec. would
allow the background to fall out of the focus zone but freeze
most fast movement. (The effect has been exaggerated in the images shown
here)
Typical
uses:
While the Program mode ensures that the camera will select a combination
of aperture and shutter speed that will produce a well-exposed image, it
still allows the photographer to select other important parameters. For
instance, the camera's Auto white balance may not be ideal for the lighting
environment, and the user can make that correction.
With a Program Shift mode, the user gets the opportunity to tweak the Program
to suit the subject without having to switch to either of the Priority
modes. The process is fast and usually very efficient, so long as the consequences
of any shift are understood, and should always result in a well-exposed
image.
Aperture Priority
As the name indicates, this mode lets the user select the aperture while
the camera matches it to a shutter speed. Control over the aperture means
control over the depth of field in the image: the zone in the image that
is in focus.
The rule is the larger the aperture (small f-number values) the smaller
the in-focus zone in the image that will be; while the smaller the aperture
(bigger f-number values), the bigger the in-focus zone will be.
Of course, since
the aperture selection controls the quantity of light that
enters the camera and reaches the sensor, it has an impact
on the shutter speed. Smaller apertures will generally require
a slower shutter speed while large apertures can necessitate
higher shutter speeds.
This mode, like the Shutter Priority and Program modes, usually allows
the user to choose all other photographic parameters.
Aperture
f3.4
The large aperture — remember that low f-number values indicate
a large aperture — helps blur the background in this image, making
the subjects dominant.
Typical
uses:
Aperture priority is a desirable feature when capturing macro images, as
generally the macro mode offers a very limited depth of field. Selecting
as small an aperture as the camera allows (f8, f10 or more if available)
and stabilizing the camera to avoid camera shake will increase the depth
of field.
Similarly, Aperture Priority is perfectly suited to portraiture. Selecting
a wide aperture ensures that the subject is crisply in focus while the
background blurs softly, and becomes less distracting.
Shutter Priority
Shutter priority is the exact opposite of Aperture priority, offering the
user the opportunity to choose the shutter speed while the camera finds
a corresponding aperture. With Shutter priority the user can choose to
freeze action, or allow it to become blurred. According to the shutter
speed selected, the image can exhibit greater or lesser depth of field
depending on the aperture the camera has selected. And, just like the Program
mode, the Shutter priority mode normally offers complete control over parameters
such as white balance, metering, etc.
Although some cameras allow for long exposures — 5
seconds or more — using their shutter priority modes,
these are usually not ideal for night photography.
Since, by definition, a shutter priority mode leaves the selection of the
aperture to the camera and the camera attempts to capture a life-like exposure,
it can be nearly impossible to capture an image that is brighter than normal,
something that is usually desirable with night time scenery.
Shutter
speed 1/400 sec.
The fast shutter speed froze the skater, but required an aperture of f2.8
which produced a short depth of field.
Typical
uses:
Shutter Priority is well-suited to action or sport photography, and even
capturing clear images of active children. However, unless there is plenty
of light, a fast shutter will usually require a fairly wide aperture to
allow as much light as possible to get into the camera in a short time
.
Manual Mode
Manual mode is a full
control mode. With it the user gets to choose both
aperture and shutter speed while guided to a proper exposure
by the metering. While all other parameters can also be
specified, exposure compensation is not available since
the same effect is achieved with slight variations in either
the aperture or the shutter speed.
Manual mode is particularly useful when an external flash is used, as it
can ensure synchronization, meaning that the shutter will be open while
the flash fires.
Manual mode is ideal for capturing photos at night or to obtain special
effects such as over or under exposures.
Aperture f2.8, shutter speed 1/2 sec.
Typical
uses:
Unlike the shutter priority mode, the Manual mode is ideal for long exposures
and night time photography. Since both aperture and shutter speed are under
the control of the user, it is possible to override the metering and produce
photos that are brighter than what the camera would yield otherwise.
Similarly, the Manual mode is ideal for images that are captured under
perfectly controlled lighting conditions, such as a studio environment.
Conclusion
Understanding the function and the use of P/A/S/M modes offers the user
a flexibility that is not offered by built-in scene modes, however well-designed
these may be. In addition, getting familiar with modes that override
the defaults of a camera guarantees that with a bit of practice, a complete
understanding of how the camera works will develop, resulting in even
better photographic results.
