One of the most misunderstood areas
to do with all types of cameras, film or digital, seems to be the
one concerning how a camera meters and exposes a scene.
Many seem to confuse metering and exposure, some even believing they
are one and the same, because the act of metering produces an
exposure value [Ev]. So we'll take a look at how metering works,
the various types used, and the various program and picture exposure
modes most cameras are equipped with.
Basic Metering.
The first light meters used were
separate from the camera and were incident light meters. These
measure the light falling on the subject or scene to be taken. You
may well recognize this type if you have at some time seen a
photographer hold a light meter in front of someone whose portrait
they were about to take. Many experienced professional
photographers, especially when engaged on studio or wedding shots
still prefer to meter in this way. The disadvantage is that whilst
in a studio the amount of light can be carefully controlled and
adjusted to suit, whether using natural or artificial light [flash
units], outside, except on 'flat' [dull overcast] days, the light
can vary quite a bit from shot to shot, and sometimes from metering
the shot to taking it.
Using an incident light meter to take
account of these factors comes with experience, and the advantage is
that the measurement taken is not affected by the differing amounts
of light reflected off the scene or subject being photographed. The
difficulty with this method of metering is that it is necessarily
slow, and you really need to be near the subject being taken.
The other method of measuring light
uses the amount of light reflected by a subject or scene. This can
be a more accurate measurement for individual subjects, but as
different subjects reflect varying amounts of light this brings the
problem of balancing the exposure in a shot to suit all the subjects
in it. Snow and sand for example are highly reflective, whilst earth
normally is not. Including large areas of sky in meter readings can
also cause exposure problems to occur depending on the type of sky
involved i.e. a deep clear blue sky, a grey overcast, or a
bright diffused one with thin grey cloud.
Most meters take their reading from a
wide angle covering a large area, but there are special meters known
as spot meters that can take reflected light readings from very
small areas, the user using an optical viewfinder to choose the spot
to meter from. Originally individual meters measured only incidental
light, whilst nowadays most are now capable of taking not only
incident and reflected light readings but flash readings as well.
Some are even able to take spot readings as well.
The one drawback of these meters is
that it is another individual piece of equipment to carry around and
camera makers eventually started to fit meters into their cameras.
At first these were just reflected light meters on the outside
of the camera body, but eventually these became internal
devices which took readings of the light entering the lens [TTL - Through
The Lens] - reflected light
readings - and advised the user of the exposure required, exactly as
individual meters do.
As cameras advanced and became more electronic
and less mechanical, so cameras began not only to meter a scene but
to set the required exposure automatically. It is this that has
led to the belief that metering and exposure are one and the
same. Today there are a
large number of different 'auto-exposure' modes fitted into cameras,
Av [Aperture priority], Tv [Time value or Shutter priority], Program
[ both with, and without, 'program shift'], and 'Picture Modes'
[Program modes specially written to suit specific situations].
Determining the correct exposure.
When meters were first introduced
into cameras it became apparent that using a metered exposure based on all
the light from the scene resulted in incorrect exposure, usually
under-exposure, since many shots used landscape format and included
a large portion of sky. So a metering method known as
centre-weighted average was developed, that took it's reading from
the lower central portion of the shot, and generally gave more
accurate exposure.
This is okay whilst the camera is used in
landscape format, but can give un-accurate reading when the camera
is used in portrait orientation. Because of this and the problems
associated with large areas of sky and the resultant under-exposure,
photographers have long been advised to 'take a reading off the
ground', by pointing their cameras downward, and locking in the
resultant exposure. Grass or grey tarmacadam [roads/path] are good
to 'meter off'. It's really the equivalent of taking an incident
light reading.
Meters are set to a base measurement of 18% reflected
light, as this figure has been found to be the mean average of light
reflectance. You may well encounter ' 18% grey cards' which are sold
on the basis that they reflect 18% of the light falling on them, and a
reading can be taken off them in difficult situations to ensure
correct exposure in difficult lighting conditions.
In more recent years a new metering
method has evolved splitting the scene into a number of zones,
anything from three upwards, and then comparing the readings from
each zone to come up with an exposure to suit the scene as a whole.
These have developed to such an extent that some forms of this
metering have anything up to 256 zones and take into account the
colour from each zone. However, no matter how sophisticated they may
be, in the end all they do is 'average' the exposure across the
image frame, with perhaps a bias towards certain zones.
When you have a very high contrast
scene, where there are subjects with both very high and low
reflectance, as say in high summer with very bright areas and deep dark shadows,
no camera, whether film or digital, can meter to expose all areas
correctly, because bright areas will require a very short exposure
and dark areas a long one. A meter which 'averages ' the scene will
produce an exposure that will over-expose the light areas and
under-expose the dark. In this situation a choice has to be made as
to which area it is important to expose correctly, and then produce
a correct exposure by the spot metering of this area.
Although many users feel that
centre-weighted has been replaced by Multi-zone and is no longer
needed it is still the preferred choice of many in the many
situations where sky is involved. It is also better when using
flash. Flash, by its very nature, does not illuminate a scene
evenly, but concentrates most of it's output to the centre. As
centre-weighted meters the scene in a similar way it is better than
multi-zone which tries to meter for the periphery of a shot and
can end up producing an exposure that over-exposes, or burns out,
the detail at the centre.
As no one metering method is capable
of being 100% accurate all of the time, in order to help the camera
user cope with anything they might face, many cameras incorporate
all three metering methods commonly used, centre-weighted,
multi-zone and spot. It is up to the knowledge and experience of the
user to understand and decide when to use each type.
Exposure
Values.
Just like hand held light meters, a
camera's meter produces an Exposure Value [Ev], whichever metering
method is used, which is then translated into a shutter and aperture
combination that will produce the correct exposure for that Ev. Many
coupled combinations are available for each Ev, and are dependant on
the ISO speed being used.
All matters to do with exposure use a
simple premise. This is that each value either up or down a scale,
ISO rating, Shutter speed [Tv], Aperture [Av], or Exposure Value are
either doubled or halved, in other words the amount of light let in
is either doubled or halved. These progressions are commonly called
'full stops'. 'Half stops' or 'third stops', also know as half or
third Ev's, are also employed, particularly in lenses and exposure
compensation.
For example Aperture values run
in the scale [full stops] :-
These are a geometric
progression involving the area of a circle, in this case a cameras
optical lens. As far as we are aware, no optical company has ever
made a F1.0 lens. The front optic would be just too big to be
viable. The 'fastest' lenses we know of are some 50mm f1.2's.
Generally the fastest 50mm lenses are F1.4's, with some 'zoom'
lenses being F2.8's. The vast majority of lenses however are usually
'slower' than this, F4.0 is common.
F4.0 lets in twice the amount of
light that F5.6 does, but only half the amount that F2.8 does.
As do
ISO ratings:-
25, 50,
100, 200, 400, 800, 1600, 3200.
Ev
values usually run in the range 0-23. 0 is the
combination:- 1sec / F1.0. at ISO100. It represents
almost total darkness.
The Ev range that a cameras
meter is able to cover varies widely and revolves around the type of
camera used. In the case of most SLR cameras, a range of 1-21 Ev at
ISO100 using a F1.4 lens is common. The ISO range may also be wide.
Digicams invariably have much narrower ranges.
All of these ranges are
inter-connected, in that changing the ISO rating by one stop means
the Ev alters one stop also, which means the Tv/Av combination also
alters by one stop, but note, only one half of the
combination alters, either Tv or Av. Not both.
For example; say at
ISO100 you have
the Ev combination 1/60th at f4.0. Increasing the ISO rating to 200
will alter this to 1/125th at f4.0, and at ISO400, 1/250th at f4.0.
Alternatively they become 1/60th at f5.6 and 1/60th at f8.0
respectively.
Exposure Modes
There are several different exposure
modes that a camera user may choose to use. The basic ones are
Auto-exposure [usually known as 'Program' mode], Aperture Priority
[Av], Shutter Priority [Tv], and 'Manual'.
Exposure
is the application of the metering Ev. Originally this was
always set manually, by reading the values and setting the
correct combination of shutter speed and aperture. Then
cameras were equipped with indicators to show when the correct
Ev had been set. These then developed into Aperture priority
and Shutter priority, and finally Auto-exposure.
Program Mode.
When in Program mode, the camera will
set a Tv/Av combination according to the Ev that it's meter has
produced for the ISO rating it is set to. Many cameras now set this
combination in conjunction with the focal length and aperture values
of the lens that is
being used on it, in order to help offset camera shake. This usually
involves using the
old photography 'law' that the shutter speed should be at least the
equivalent of the focal length of the lens. i.e. for a 50mm lens
1/60th, for a 100mm 1/125th etc. The actual combination will of
course depend on the amount of light. As light levels increase, the
mode will alter the combination to suit the program 'line', which
will have been set to close the aperture down progressively, once
high enough shutter speeds have been reached, in line with the Ev
combinations available for the correct exposure. If a high
enough shutter speed to suit the focal length of the lens in use
cannot be set many cameras will issue a warning by indicating that
flash needs to be used. Some will automatically use the flash, but
of course this is only of use if the subject is within flash range.
The problem with using this mode is
that the user has no control over the Tv/Av combinations set. If the
camera is mounted on a tripod you may want to have a slower shutter
speed and a smaller aperture for greater depth of field, or if the
camera is being hand held and it is windy, you may want to increase
the shutter speed to offset any camera shake. So many cameras now
provide Program Mode with the means to 'shift' the Ev combination to
one that suits, within the Ev range available, usually by turning a
dial.
Hyper Program.
This mode is unique to some Pentax
SLR's, but is worth mentioning nonetheless. For this is a 'program
shift' mode with a difference. It uses two dials on the camera's
body, one each front and back. In this mode if the Ev combination is
not to your liking moving the front dial immediately engages Tv
mode, and moving the back one Av mode. Pressing a button returns you
to the program line. So it's like the three modes in one, and you
can switch between the three almost instantly.
Aperture
Priority. [Av].
In this mode the user sets the
aperture that they want to use and the camera sets the correctly
matching shutter speed in line with the available Ev combinations.
If it can't do this, because the aperture selected is outside the
range available, a warning will be given in some way, perhaps by the
flashing of the selected aperture, until an aperture is selected
that it is within range. The advantage of this mode is that as and
when the light levels change the aperture selected will remain the
same, so long as it remains in range, and the camera simply changes
the shutter speed to match.
This mode is useful if say you want a
large aperture i.e. 'wide open', to ensure a de-focused background
or a small one for a greater depth of field. Another use, which
confuses some, is to set the aperture to its maximum, wide open, to
ensure the highest possible shutter speed, in changing, and lower,
light levels, where time cannot be wasted constantly checking
settings.
Shutter Priority. [Tv].
This mode is the reverse of Av, and
here the user sets the shutter speed required and the camera sets
the matching aperture. Again the shutter speed selected must lie
within the Ev range produced by the camera's meter.
With this mode the advantage is that
a shutter speed high enough to freeze action can be set, or that say
when using flash, a slower than normal speed can be used, slow sync,
to freeze action with some movement blur visible, 1/60, 1/30 and
1/15th are quite useful for this, depending on the speed of
the motion being captured. Another use is setting the reciprocal of
the Lens focal length, [1/60th for a 50mm, 1/250th for a 200mm etc],
to ensure that in light levels past this requirement all extra light
is used in reducing the aperture size for the greatest possible
depth of field.
Manual Mode.
As its name suggests, with this the
user sets both the aperture and the shutter speed. This finds
several uses, when long exposures are required, for night-time
shooting, or when non-dedicated flash units, ring or studio-flash
etc, are employed, i.e. whenever the user wants to use an exposure
combination different to that which the camera suggests. Some
cameras do not offer a meter reading when set in this mode, but many
do, advising the user when a combination that matches the Ev reading
has been set.
With this mode a Tv/Av combination
can be set that doesn't match the Ev readings from the camera's
meter but the shutter will still fire. In the other modes used most
cameras shutters will not fire if an incorrect combination has been
set, i.e. one that is out of the camera meters Ev range. This is to
prevent the user from wasting shots unwillingly or unknowingly, and
is probably of more importance to users of film cameras, where it
was developed, than users of digital, since shots can soon be
deleted and no wastage occurs, except time and effort of course!
Bulb
Mode
In
bulb mode an aperture value is set manually and when the
shutter is fired it stays open for as long as the shutter
button remains pressed. This mode enables shutter speeds of
any duration, and longer than can be set in the other exposure
modes. This was a mode often used in the past when timed
camera shutter speeds rarely exceeded 1sec in duration. Now
that speeds as long as 30 sec's are common it is used less
often but there are times when this is not long enough.
It's
favourite use is mainly in connection with low light and night
time exposures. Firework displays are a good example. Or
capturing trailing car lights at night and stars in the night
sky. 'Painting with light', using either flash guns or torches
is another use to which it is put.
Subject or Picture
Modes.
Some cameras are
equipped with additional or alternative exposure modes to those
commonly found, and these are known as subject or picture modes.
These are program modes which have been developed to suit novice
users who are unsure as to the best exposure and metering methods to
use in a particular circumstance, and offer a more tailored approach
than the basic program mode. In addition to setting a certain
program line for the exposure to suit the subject being shot, most
also set the metering method as well. Some even set the AF mode and
automatically use flash when necessary. Common modes are; Portrait,
Landscape, Sport [Action], Night-time, Flash, etc.
To give some examples;
In Landscape a
camera would probably set Multi-zone metering, use Aperture
priority set to give the smallest aperture possible, with single
shot AF.
In Portrait it would
be Centre-weighted or Spot metering, again Aperture Priority set
to be 'wide open', with single shot AF.
Sport mode would
combine Shutter priority set to give the fastest speed, with
Multi-zone metering and continuous AF.
All these modes are, as
you can see, combinations of the metering, exposure and drive modes
the camera has been equipped with. Different cameras will use
variations of the different combinations, but how well any work
depends as much on the level of light at the time of taking the
shot, as of the ability of the camera, or user, to set the mode it
uses.
As a result many digital
cameras using Subject modes have an Auto-ISO mode where the camera
will automatically increase the ISO rating if the current ISO rating
cannot give the required Ev combinations. Circumstances where this
would occur would be for example, when sport mode was engaged in low
light levels and a fast enough shutter speed could not be set.
The metering and
exposure modes a camera may have vary greatly depending on the type
of camera it is, and it's intended user. Basic Digicams may only
have multi-zone metering and subject modes, with no manual override
for the user, and no indication of the Ev settings used [ISO/Tv/Av],
whilst those intended for experienced users, such as top end D-SLR's
may have all the metering, exposure and ISO modes, but not auto-ISO
or subject modes. Middle ranking cameras, called prosumer models,
may have various combinations of all the features.
