This information can then be used to set the optimal film or image sensor exposure (average luminance), it can also be used to control the amount of light emitted by a flash unit connected to the camera.
In most film and digital SLRs, the light sensor(s) for exposure metering are incorporated into the pentaprism or pentamirror, the mechanism by which a SLR allows the viewfinder to see directly through the lens.
One of the advantages of this approach is that the measuring result requires no adjustments when changing focusing screens or viewfinders.
Also, some of the cameras using this configuration (e.g. the Minolta 9000) are virtually immune against measurement errors caused by light reaching the metering cells at larger angles, for example with tilt–shift lenses.
Some early Pentax DSLRs could use this same configuration for TTL flash metering as well, but since the reflectance properties of image sensors differ significantly from those of film, this method proved to be unreliable in practice.
Digital SLRs supporting live view or video will use the read out of the image sensor itself for exposure metering in these modes.
This also applies to Sony's SLT digital cameras, which use the image sensor for exposure metering all the time.
As of 2012,[update] no digital SLR or SLT camera on the market supported any form of real-time TTL flash metering using the image sensor.
However, it can be expected that such methods will be introduced as image sensor technology progresses, given the advantages of metering with real-time feedback and without preflash.
In many advanced modern cameras multiple 'segments' are used to acquire the amount of light in different places of the picture.
[2] In the OM-2's Auto Dynamic Metering (ADM) system the first shutter curtain had the lens-facing side coated with a computer generated pattern of white blocks to emulate an average scene.
As the shutter speed increased, the actual light reflecting off the film surface was measured and the timing of the second curtain's release adjusted accordingly.
This gave cameras equipped with this system the ability to adjust to changes in lighting during the actual exposure which was useful for specialist applications such as photomicrography and astronomical photography.
Leica later used a variation of this system, as did Pentax with their Integrated Direct Metering (IDM) in the LX camera.
The analog version of TTL works as follows: when the incoming light hits the film, a part of it is reflected towards a sensor.
The one exception was Polaroid's instant slide film which had a black surface and could not be used in TTL flash mode.