Recent from talks
Through-the-lens metering
Knowledge base stats:
Talk channels stats:
Members stats:
Through-the-lens metering
In photography, through-the-lens metering (TTL metering) refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light meter. In some cameras various TTL metering modes can be selected. 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.
Through-the-lens metering is most often associated with single-lens reflex (SLR) cameras.
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. As the mirror is flipped up, no light can reach there during exposure, the necessary amount of exposure needs to be determined before the actual exposure. Consequently, these light sensors could traditionally be used for ambient light TTL metering only. In newer SLRs as well as in almost all DSLRs, they can also be utilized for preflash TTL metering, where the metering is carried out before the mirror flips up using a small preflash of known intensity and the necessary amount of flash light is extrapolated from the reflected flash light measured by the metering cells in the roof of the camera and is then applied during the exposure without any possible real-time feedback.
There were a few particularly sophisticated film SLRs including the Olympus OM-2, the Pentax LX, the Nikon F3, and the Minolta 9000, where metering cells located at the bottom of the mirror box were used for ambient light metering, depending on model either instead or in addition to metering cells in the roof of the camera. Depending on model, the light was reflected down there either by a secondary mirror behind the half-transparent main mirror, a special reflective coating of the first shutter curtain, the surface of the film itself, or combinations thereof. 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.
Metering cells located at the bottom of the mirror box using light reflected off the film are also used in all film SLRs supporting the classical form of real-time TTL flash metering.
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. Therefore, digital SLR cameras typically don't support any real-time TTL flash metering and must use preflash metering instead. The ambient and flash light metering is then carried out by a metering module located in the roof of the camera (see above).
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.
TTL metering systems have been incorporated into other types of cameras as well. Most digital "point-and-shoot cameras" use TTL metering, performed by the imaging sensor itself.
Hub AI
Through-the-lens metering AI simulator
(@Through-the-lens metering_simulator)
Through-the-lens metering
In photography, through-the-lens metering (TTL metering) refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light meter. In some cameras various TTL metering modes can be selected. 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.
Through-the-lens metering is most often associated with single-lens reflex (SLR) cameras.
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. As the mirror is flipped up, no light can reach there during exposure, the necessary amount of exposure needs to be determined before the actual exposure. Consequently, these light sensors could traditionally be used for ambient light TTL metering only. In newer SLRs as well as in almost all DSLRs, they can also be utilized for preflash TTL metering, where the metering is carried out before the mirror flips up using a small preflash of known intensity and the necessary amount of flash light is extrapolated from the reflected flash light measured by the metering cells in the roof of the camera and is then applied during the exposure without any possible real-time feedback.
There were a few particularly sophisticated film SLRs including the Olympus OM-2, the Pentax LX, the Nikon F3, and the Minolta 9000, where metering cells located at the bottom of the mirror box were used for ambient light metering, depending on model either instead or in addition to metering cells in the roof of the camera. Depending on model, the light was reflected down there either by a secondary mirror behind the half-transparent main mirror, a special reflective coating of the first shutter curtain, the surface of the film itself, or combinations thereof. 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.
Metering cells located at the bottom of the mirror box using light reflected off the film are also used in all film SLRs supporting the classical form of real-time TTL flash metering.
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. Therefore, digital SLR cameras typically don't support any real-time TTL flash metering and must use preflash metering instead. The ambient and flash light metering is then carried out by a metering module located in the roof of the camera (see above).
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.
TTL metering systems have been incorporated into other types of cameras as well. Most digital "point-and-shoot cameras" use TTL metering, performed by the imaging sensor itself.