Through-the-lens metering
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Through-The-Lens (TTL) metering is a photographic term describing a feature of
cameras capable of measuring light levels in a scene through their lens. This
information can then be used to select a proper exposure (average luminance), and/or
control the amount of light emitted by a flash connected to the camera.
Through-the-lens metering is most often associated with single-lens reflex (SLR)
cameras. A light sensor can be incorporated into the pentaprism or pentamirror, the
mechanism by which an SLR allows the viewfinder to see directly out the lens.
However, TTL metering systems have been incorporated into other types of cameras.
All digital "point & shoot" cameras use TTL metering, performed by the imaging sensor
itself. With digital SLR cameras, the metering is typically carried out by a
metering/autofocus module located at the bottom of the mirror chamber, underneath the
mirror.
In more advanced modern cameras multiple 'segments' are used to acquire the amount
of light in different places of the picture. Depending on the mode the photographer has
selected, this information is then used to correctly set the exposure. With a simple spot
meter, a single spot on the picture is selected. The camera sets the exposure in order to
get that particular spot properly exposed. With multiple segment metering (also known
as matrix metering), the values of the different segments are combined and weighted to
set the correct exposure. Implementations of these metering modes vary between
cameras and manufacturers, making it difficult to predict how a scene will be exposed
when switching cameras.
[edit] Through the lens flash metering
The process of calculating the correct amount of flash light can also be done 'through
the lens'. This is being done in a significantly different way than non-flash 'through the
lens' metering. The actual metering itself happens in two different ways, depending on
the medium. Digital TTL works differently than analog TTL.
The analog version of TTL works as follows: when the incoming light hits the film, a
part of it is reflected towards a sensor. This sensor controls the flash. If enough light is
captured, the flash is stopped. The biggest problem here is the film; not all brands and
types reflect the light to the same amount. Nevertheless, this method of flash metering is
more advanced than those previously used.
With digital, this way of metering is not possible any more since a CMOS or CCD chip,
used to collect the light, is not reflective enough. There are a few older digital cameras
which still use the analog technique, but these are getting rare. The Fujifilm S1, S1 and
S3 are the most known to use this technique.
Digital TTL works as follows: Before the actual exposure one or more small flashes,
called "pre-flashes", are emitted. The light returning through the lens is measured and
this value is used to calculate the amount of light necessary for the actual exposure.
Multiple pre-flashes can be used to improve the flash output. Canon refers to this
technique as "E-TTL" and has later improved the system with "E-TTL II". The first
form of digital TTL by Nikon was called "D-TTL" and the improved variant nowadays
is called "i-TTL".
When using first-curtain flash (which means the flash fires directly after the sensor is
exposed), the preflashes and main flash appear as one to the human eye, as there is very
little time between them. When using second curtain flash (the flash fires at the end of
the exposure) and a slow shutterspeed, the distinction between the main flash and the
preflashes is more obvious.
Some cameras and flash units take more information into account when calculating the
necessary flash output, including the distance of the subject to the lens. This improves
the lighting when a subject is placed in front of a background. If the lens is focused on
the subject, the flash will be controlled to allow for proper exposure on the subject, thus
leaving the background underexposed. Alternatively, if the lens is focused on the
background, the background will be properly exposed, leaving the subject in the
foreground typically overexposed. This technique requires both a camera capable of
calculating the distance information, as well as the lens being capable of communicating
the focal distance to the body. Nikon refers to this technique as "3D matrix metering",
although different camera manufacturers use different terms for this technique. Canon
incorporated this technique in E-TTL II.
More advanced TTL-flash techniques include off-shoe-flashing. This is the case when
one or more flash units are located at different locations around the subject. In this case
a 'commander' unit is used to control all of the units in an advanced way. The
commander unit usually transmits these signals as lightflashes, although some TTL
capable radio transmitters are now available. The photographer can usually vary the
light-ratio's between the different flashes on the commander unit.
[edit] See also
• Canon EOS flash system
[edit] External links
• How TTL metering works, from "Flash Photography with Canon EOS Cameras"
by N. K. Guy
• The TTL Flash System, by Moose Petersen
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