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, 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) used to form an upright image of the view out of the lens. However, TTL metering systems have been incorporated into other types of cameras. All digital "point & shoot" cameras use TTL metering, with the metering being done by the imaging sensor itself. With digital SLR cameras, the metering is in most situations done 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, the values of the different segments are combined in a pretty advanced way to set the correct exposure. This way of metering is also called matrix metering. Every manufacturer implements this metering mode slightly different. This makes it difficult to predict how a scene will be exposed by the camera when switching from one camera to another.
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 to make this kind of metering possible. 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".
Some cameras and flash units take more information into account when calculating the necessary flash output. This includes the distance of the subject to the lens and 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 focus distance to the body. Nikon refers to this technique as "3D matrix metering", although different camera manufacturers use different terms for this technique.
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. This way, the camera itself does not even need to know how many or the location of the flash units.
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