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How does through-the-lens metering measure the light in a scene?
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technology of photography
Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).
Through-the-lens (TTL) metering measures light as it passes through the camera lens, ensuring the reading accounts for the lens's angle of view, close-up corrections, and other optical factors.12
In single-lens reflex (SLR) cameras, this process typically occurs in one of the following ways:
- Pentaprism Measurement: Photocells located in the pentaprism housing measure the brightness of the image projected onto the focusing screen.1
- Beam-Splitting: Some systems divert light from the lens to a photocell before it reaches the screen using beam-splitting arrangements or partially reflecting mirrors.1
- Off-the-Film (OTF) Measurement: Advanced systems measure light reflected directly from the film surface or from a specially structured first shutter blind during the exposure.1
By reading the light after it has been focused by the lens, TTL metering provides a more accurate assessment of the actual light reaching the film or image sensor compared to external meters. This data is then processed by the camera's circuitry to automatically or semiautomatically adjust the aperture and shutter speed for optimal exposure.21