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It's all about light ... yes, and your subject. But as you usually
get to decide about the latter one, you often have to accept,
whichever light conditions are available.
"Aperture", "exposure", "sensitivity"
sounds familiar to you? Know what "A", "T(S)" and ISO stands for on
your camera's dials? Great, then move straight on to the
second part!
Or rather not? Ok then, let's start
with the basics!
A digital camera's sensor is the pendant to a conventional camera's
film. It's sensitive to light, and that's where the similarities
end.
The sensor of most compact fixed lens cameras ist just about 9 x 6
millimetres in size. It can take picture after picture without need
for a replacement (e.g. film transport) and it's sensitivity to
light can be changed on the fly.
In front of the sensor, there's the
lens: Traditionally not interchangeable with compact cameras, but
especially well adjusted to the sensor's needs (or rather should be
... or the cam was quite cheap)
A compact camera's lens has three features:
-
the focal
range, usually given in millimetres with a "full frame equiv."
suffix, which indicates the focal lengths, the camera would
have, if it had a full frame 35x24mm sensor. Otherwise, focal
lengths would not be compareable between cameras with different
sensor sizes.
-
Aperture
- in most cases, the lowest number at each end of the focal
range is given. Note that this is actually a fraction (f/2,8 ..
f/4,0) so that a lower number actually translates into a bigger
aperture.
-
Shutter
- contrary to DSLR systems, the aperture doubles as a basic
shutter. Exposures are then controlled electronically directly on
the sensor.
Let's leave the
focal range for now, more important are the aperture and the
exposure time.
The aperture controls the amount of light, that reaches the sensor.
Given an aperture number of 2,8 an aperture set to 4,0 will halve
the amount of light, the sensor gets. And with 5,6 that will again
be cut in half. Your photo is going to get darker and darker as the
aperture closes.
Because that
isn't a pretty thing, the exposure time (colloqually "shutter
speed") comes into play: whereas the aperture controls the amount
of light, the shutter speed controls the period during which the
sensor is exposed to light.
Understandably, a photo taken at f/4,0 with a shutter speed of 1/100
second appears as bright as a photo taken at f/2,8 with a shutter
speed of 1/200 second.
"Now what's the difference?" I hear you ask.
To be honest, with our small sensor cameras, there isn't much of a
difference for normal use (there is a difference at a very close
distance to the subject or at very high focal lengths, see part 2
and 3)
I'd recommend to leave the aperture wide open all the time. The lens
is often reasonably sharp, you get the highest possible shutter
speed (smaller risk of camera shake) and there's no diffraction.
Longer shutter
speeds are good if you want to impart a sense of speed and blur
everything except your moving subject (see part 3, panning)
On the other hand, flying debris and roost trails look good at very
high shutter speeds because that freezes the motion and you can see
particles in mid air.
If the available light doesn't long for a high enough shutter speed
(usually 1/1000 sec up to 1/4000 sec) and your aperture can't open
any wider, there's a third option: sensitivity! This is usually
indicated by an ISO number, where ISO 100 often means base
sensitivity. ISO 200 makes the sensor twice as susceptible to light,
meaning cutting the exposure time in half.
There is downside
to this deal however: increasing ISO numbers increases the noise
seen on the pictures (or the sometimes really bad artifacts of noise
reduction from the camera) and decreases the sensor's dynamic range (the
gap between the darkest and the brightest areas the sensor can at
the most record in a single picture before it goes completely black
or pure white.)
Compact cameras suffer a lot from higher ISO settings, because their
small sensors (about 1/3rd to 1/5th the size of a DSLR) need high
amplificiaton which degrades the original signal.
Better keep your ISO-setting to 100 or 200. With some cameras, you
can go up to 400 and perhaps 800.
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