Although separate functions, I'm writing about metering and autofocus together because
cameras tend to perform the two jobs together and this affects how you take your
pictures.
What the Camera Does
Most cameras allow the shutter release button (the one you press to take a picture)
to be pressed halfway down. This is usually the point at which the camera
makes its metering and auto-focus desicions. It's important to think about
where you are pointing the camera when you do this. This is why...
Auto-Focus
The auto-focus will usually be set to make whatever is at the centre of the view
finder sharp when the shutter release is pressed half way. On most SLR cameras
it is also possible to select other 'spots' for the camera to focus on. These
usually light up in the view finder when selected so you can tell what the camera
is going to focus on.
It's important to make sure that you are focusing on the correct thing which sounds
obvious, but read on. Remember
if you are taking a photo of a person and there are some objects between them and
the camera make sure the focus is on the person. This is especially important when
thinking about how you picure is going to be composed (see composion for more details). Often you won't want the main subject of the picture in the middle so you'll
need to press the shutter release half way to focus whilst pointing the camera directly
at the subject and then move the camera to place the subject where you want it in
the frame (while still holding that button half way down). Once you've recomposed
the shot you click it the rest of the way to take the picture.
Some more recent cameras (compact as well as SLR) have facial recognition built
in to them. These look for faces in the picture and try to make sure that
they are in focus even if you don't follow the method I've just described.
Metering
Metering is the process where the camera measures the light from the scene and works
out what settings to use to get the correct exposure. The camera normally
does this when you press the shutter release down half way (at the same time as
auto-focusing). Most compact cameras will meter over the whole scene.
This means that they look at the entire picutre, work out what
aperture and shutter settings to use and
what you get is a picture which is on average correctly exposed. This works
well a lot of the time, but not always. Usually cameras use what's called a weighted
average. This means that they assume one area of the image is more important
than the rest (usually the centre). Most SLR cameras also allow 'spot metering'.
This means that the camera looks at one small area and adjusts the settings to suit
that 'spot'. As with the auto-focus, it's usually possible to select which
point you want to use and have it displayed in the view finder. Usually the
same spots will be used for both auto-focus and metering.
Why do you want to meter over different areas or spots? Well it all comes
down to trying to make the photograph look the way you see it. When you look
at a scene your eyes move back and forth to take the whole view in and they adjust
themselves as they do it. The combination of your eye and brain can take in
a scene with bright areas and shadowy areas and allow you to see it all with good
detail. The camera has to try to campture the whole scene with just one selection
of settings. If the camera is set to get the correct exposure on a bright
object (say a summer sky) then the darker areas (perhaps some rocks on a beach)
will be badly underexposed and may even appear black with no detail. If you go to the other extreme and meter on the
dark areas then you'll be able to see all of the texture on the rocks, but the sky
might be badly over exposed. If too over exposed the sky will be pure white
and there will be no detail there at all. This is a common way to ruin otherwise
really nice pictures.
What you need to do is try to get the camera to meter on something that is not at
either of the extremes of bright or dark. A good way to do this is to meter
on grass or trees that are reasonably lit. This gives a good compromise compromise
and with any luck you'll get a detailed sky and foreground.
With a digital camera you can also look at the picture you've just taken and check
if the sky has been 'bleached' or if the foreground is all in shadow and try metering
on a different area if it's not quite right. Practice makes perfect.
Separating the Two
So both auto-focus and metering occur when you press the shutter release half way.
This is a bit of a pain because you may well not want to meter and focus on the
same thing. With compact cameras there's often not a great deal you can do
about this. Just try to pick something to meter on that is the same distance
from the camera as your subject.
SLR cameras often have an 'exposure lock' fascility. This works by keeping
the aperture and shutter
settings fixed while you sort out the focus and recompose your shot. To do
this point the camera at the area you want to meter from and press the shutter release
half way down. Now press and hold the exposure lock button down. The
aperture and shutter settings are held as long as you keep that exposure lock button
pressed down. You can now let the shutter release button back up, recompose
your shot and press the release half way to focus and then take your picture.
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