What Is Exposure
Even though digital cameras are more common nowadays, many of the
concepts of digital photography are inspired if not named after the film
photography of previous years. It is useful for us to have a quick
understanding of how film cameras work hence.
Film is but some
sort of plastic sheet with silver salts coated on top. However, the
silver coating is actually light-sensitive, and the individual grains of
light-sensitive salt will chemically react to any light it receives,
turning variably darker as a result as the light falls. That is
basically why your film negatives look the way they do, for those who
have handled film before. For colour images, the concept is similar, but
there are multiple layers instead, each layer chemically manipulated to
be sensitive only to a certain colour component of light e.g. red;
green; blue light. Remember how white light is actually a combination of
several colours i.e. your rainbow colours? Yes, that is where all your
colour component light comes from to react to the specific layers.
The
camera lens does the work of focusing reflected light rays (remember
now, that that’s how the human eye sees as well) in such a way that it
all falls nicely across the film, rather than having a blurred,
unfocused image. The film is safe behind a shutter which blocks out all
the light from reacting with the former when you don’t need it yet – you
do not want to over-expose your film or the image will be too bright, or under-expose
it or it will be too dim. It is only when you have framed the shot you
want, then you press the shutter button to open it. That is when the
magic of the light reaction occurs, and when the shutter door shuts
after a while, you have your image, nicely focused and therefore,
“properly exposed”!
And that is what exposure is. Photography,
whether film or digital, is at its most fundamental level about
exposure – by that we mean playing with the amount of light your film or
light sensor receives, in the case of digital cameras which have
replaced the film with the latter.
Playing With Light: Aperture, Shutter Speed, ISO
Most
of us on this article probably have no idea what is happening “behind
the scenes” when we take a shot.The digital camera is working quickly,
sometimes within milliseconds, using its combination of processors and
sensors, coupled with its understanding of human visual perception, to
correctly expose the picture – that is to say, it must determine the
right amount of light to enter the sensor.
The individual
light-sensitive photodiodes on our digital camera sensors have replaced
the single-usage silver coats of film, converting light received to
electrical voltage, then digital data, to generate your final image.
Colour filters on top of the sensor have also done away with the need
for multiple layers of silver coats for colour imaging. Yet still, the
theory and reliance on light and the need for a proper exposure is the
same. As earlier mentioned, if we have too much light, our picture will
come out too bright or “washed out”. Too little light, it will be too
dim with “shadows”. How does the camera control the amount of light
falling upon the sensor?
It comes down to these three variables it can play with: aperture; shutter speed and ISO.
Aperture: Look at the seven blades close in on this lens, leaving an approximate circular hole for light to pass through to the sensor.
Source: Wikipedia
Aperture: How Wide Is Your Lens Opening
Aperture is the most straightforward and
simple way to control the picture exposure. Inside the lens unit, there
exists a certain mechanism that closes in or expands out, subject to
the will of the operator. This mechanism, known as an iris (as I
shall later explain), is usually made out of opaque, blade-shaped parts,
closing in to leave an approximate circular pattern open on the lens
(Refer to image). The opening or closing of this iris will directly
affect how much light enters through the lens into the sensor of your
camera.
In fact, the function of this is similar to how our
eye pupils works. When you are in a dim-lit place, your pupils seem to
dilate or expand to allow in more light, while the reverse is true under
brighter lighting conditions. Internally, your iris is doing the
adjustment, and the former we name the earlier-mentioned mechanism
after.
The term aperture is used simply to refer to the ratio of the opening of the iris. Consequently, you may also hear it synonymously used with f-number or f-stop.
This is because it is expressed as a fraction of how much of the lens
is exposed to light. If the iris covers half of the area of the lens, we
say
that the aperture now is f/2 (said as “f-two”), just as we would say
“one-over-two” for a usual fraction. If
it covers three-quarters, it is f/4. If it only leaves 1/22 of the lens
area open for light to pass through, it is f/22. The wider the lens
opening is, the more light can pass through in a given instant – easy to
see? What most amateurs might be confused with is the inverse relation
of the f-number to the actual width of the opening – just remember that
it is a fraction. F-big number, narrower hole. F-small number,
wider hole. Also, the vocabulary is tricky. To avoid mix-ups because
one could say “big aperture, small hole” and eventually confuse
“aperture” with the “hole”, I usually reserve the adjectives “big” and
“small” when I am talking about the f-number, and “wider” and
“narrower” when I wish to refer to the physical opening so that even
when I say use the phrases “wide aperture” and “small aperture”
interchangeably, I can very easily know which is which, because a number
cannot be wide or narrow (Smiley face).
Interestingly,
because of the laws of physics, a side effect comes alongside with the
variable aperture. A wider aperture (see, as long as the adjective is
correct, I can use “aperture” for everything!) makes it harder for the
lens to make all the light rays converge properly on the sensor. Some
light rays, especially those reflected from regions further away from
the point of focus, cannot converge on the sensor enough to give a sharp
result. Hence, the area in focus is typically smaller – the area of focus we know as depth-of-field (DOF), and hence a wider aperture leads to a more shallow DOF. Conversely, a narrower aperture leads to a better-focused image and a less shallow, or larger
DOF. While not directly related to exposure, this bit of information
about DOF has certain implications for applied photography.
Bear all that newly learnt knowledge about aperture in mind now as we discuss the next bit of theory – the shutter speed.
Effects of “camera shake”
Source: Adapted from original at PDPhoto.org
Shutter Speed: How Long Your Shutter Stays Open
Shutter speed, luckily, is a lot easier to understand. It
refers to the amount of time the shutter stays open from the moment you
press the shutter button. Until there is a need for a picture to be
taken, the sensor sits behind the shutter, protected by the latter
against all light coming from the Universe in its dark, unlit space
within the camera just like a sheet of photographic film. The moment you
need to take a shot, however, the shutter opens and light floods into
the sensor at once. As how I described earlier on, the light sensed is
converted finally to your digital picture file from this process.
Then,
the longer the shutter remains open, the more light there is coming in
over time. A long shutter is not always desirable though, especially if
most of the time you are shooting without a tripod. While the shutter is
open, any movement of and in the scene e.g. subject movement or
photographer hand shakes (we call this “camera shake”), will give you a
blurred image because the changed light rays are re-projected onto the
same sensor photodiodes, which “merges” them with the older light and
colour information it has. In extreme cases, you get “ghosting”. Hence,
if we can have it, we mostly use a fast shutter speed for stationary
subjects to avoid the issue of camera shake.
Aesthetically
however, if there is a moving subject you are trying to capture, a
longer shutter speed could be more desirable than a fast one for the
“ghosting” could bring out the sense of movement in the picture. We will
look into that later.
You may already realize the relation
between shutter speed and aperture. The latter is akin to a “rate of
light coming in” measure, and when applied to the shutter speed, gives
you sort of a total control of the amount of light coming into your
sensor. But that is still one more trick up our sleeve to complete the
equation – the ISO.
Viewing the image at full size, can you see the random jitter, grainy bits around? The wall is otherwise supposed to be a clean, solid purple. The grainy bits are what we call camera noise, affecting image quality and making it look flat.
Source: Public Domain Photos
ISO: Sensor Sensitivity
Sure, you can now manually control how much light enters physically
to your sensor via the aperture and shutter speed. What if you are
shooting in a really dark place though? A dimly lit environment, or a
night scene for instance, where there is simply not enough light and a
longer shutter is not desirable?
Come in the ISO. ISO has to do
with how sensitive the sensor is to every bit of light that strikes its
photodiode. For film photography, the film is sold in different
packagings or “film speed”. Higher speed film rolls would have larger
grains of silver on its coating than lower speed films, allowing a
greater reaction under the same light – hence being more sensitive or higher speed
hence of reaction. They were sold in rolls of “ISO 100”; “ISO 400”;
“ISO 1600” etc. with the larger number denoting a higher speed. The
“ISO” actually does not mean anything but is just a reference to the
International Organization for Standardization which standardized the
film speed measurements across manufacturers.
In digital sensors,
the ISO reference is retained. Since there are no physical silver
grains, the ISO works by varying the amplification of the electrical
signals converted from the light that falls on the photodiode. The
greater the ISO number, the greater the light-electrical signal, is amplified, and hence the more sensitive your sensor is to light.
ISO
sounds like a good way to amplify the amount of light perceived by your
sensor from whatever little light there is, thus helping you with your
effective exposure level. To me, adjusting the ISO value is like a last
resort, it is “artificial” rather than “natural” compared to adjusting
the aperture and shutter speed. This is because there is always inherent
noise (random fluctuations, not necessarily acoustic) in any electronic
system and some of the light energy received is not perfectly
translated to electronic signals; instead it may be lost as heat, or
random signals. This is a physical law. Think of yourself turning up the
radio volume – you hear louder music, but also a greater static buzz.
Hence, when you are amplifying the signal received by the sensor, you
are not only amplifying the light-electric signals, but also the
electronic noise signals. If you look at any picture close enough, you
will see these random coloured pixels scattered all over the place as a
result of this unwanted noise. While they may not necessarily change
your picture to something else; you can probably still identify your
subject, noise will have a definite negative effect on the quality of
your shot.
The amount of noise generated depends from camera to
camera, sensor to sensor. This is why SLRs generally perform better
under lower light. They usually have larger sensors, meaning that for
the same number of photodiodes as a compact camera, the photodiodes can
be made larger, maximizing the light received per diode (being more
sensitive inherently without “artificial ISO amplification”) and have a
higher throughput of the light-electronic signal conversion.
In case you are thinking that noise is a digital-only problem, the larger silver grains on film also create a “coarser” image without the ability for finer gradations with smaller grain – it affects the finished photo in the same way as digital noise does to our pictures.
Humans 1 v AI 0
That is it for the three gateways of light – aperture; shutter speed; ISO. But first, why do we want to set all these manually?
Truth
to be told, even though most digital cameras boast an automatic mode to
set these variables without user intervention, the camera processor is
still not the human eye. It uses algorithms and guesswork to formulate a
picture it thinks is best for our visual perception.
Sometimes, it fails because humankind tend to know ourselves, and how we
like to see what we see, better. Other times, it is confused by objects
in the scene that trap its processor in some kind of loop. It goes – is
it a face? Is it a light source? It cannot tell.
Another reason
is because the processors are programmed to go for the lowest common
denominator i.e. the effect or finish that most people usually want. In
automatic mode for instance, when capturing a wide scenic shot of
mountains and forests, generally people want detail – all of the scene
should be in maximal focus. The camera might auto-detect the scene as
such, and adjust the aperture to achieve a maximal DOF accordingly
hence. However, what if for some quirky creative reason, I want to
emphasize just the mountain peak? The camera does not have a million
modes for a million scenarios – that is when knowing how to override the
camera defaults prove useful.
Now that you have learnt the theory, you may wish to proceed to the next
part of this feature on the practical aspect of exposure.
