Flash exposure metering

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The measurement of flash light in photography is called flash exposure measurement . The light intensity of the flash is measured. The flash exposure is measured externally with a special flash exposure meter or an exposure meter with a dual function. Inside the camera is measured through the lens (TTL flash measurement). There is also measurement via a photo cell on the flash unit. Some devices also measure the amount of ambient light .

functionality

Because of the very short flash times (at least 1/800 second to 1/2000 second), camera shutter speeds for controlling the exposure with the central shutter rarely play a role (only above 1/500 s shutter speed). With the central shutter, the entire picture is always exposed to light for a certain period of time. With the focal plane shutter , only a relatively long shutter speed is possible, as this is the only way to ensure that the entire film surface is reached by the flash (usually from around 1/200 second and faster, the image is no longer fully exposed to light, as there is only an open strip across the film moves, with older cameras often from 1/50 of a second). This time is known as the "synchronization time". The exposure of the flash light component is controlled with the aperture in both cases.

The intensity of the flash depends not only on the power of the flash unit but also on the distance to the object. This means that nearby objects are brightened by the flash more than the background. The exposure time as well as the aperture affect the brightness of the background, which is only slightly brightened by the flash. If the flash light component dominates the ambient light, e.g. B. with close objects in the foreground, then the exposure is essentially determined by the flash light component. The near subject is thus mainly correctly exposed via the aperture, since the flash is usually much shorter than the shutter speed. With this effect, with a constant aperture, the background can be depicted dark with a short shutter speed or light with a long shutter speed. This can be used in different ways in terms of design. It is thus possible, for example, to expose the subject exclusively using the flash, even when there is ambient light. Disturbing backgrounds can also be drawn very dark and thus “cut out” the motif.

Flash exposure meter

Flash exposure meters otherwise work like normal exposure meters with both light metering and object metering . When measuring the light, a cap (opaque hemisphere or a blind) must be pushed in front of the measuring cell. During the measurement, reference is made to a standardized remission of 18% light emission (value on the gray card or reduction in light due to the dome). Light and object measurement should produce the same results if the subject has a remission of 18%. Subjects that are too bright (high key) result in underexposure when measuring objects, and subjects that are too dark (low key) result in overexposure if the measured value is not corrected beforehand (see exposure measurement ).

Flash system

The flash system can usually be triggered in two ways:

  • Triggered by the flash exposure meter (sync cable) or
  • Triggered by the flash itself during an activation time of the measuring device (without cable).

TTL flash metering

With TTL flash metering , exposure metering takes place through the lens used (TTL = Through the Lens ). The TTL flash measurement has largely replaced flash units with their own sensor ( computer flash ) or external flash exposure meters in modern SLR cameras .

The TTL measurement has the advantage that the light that falls through the lens onto the film or, in the case of digital cameras, onto the sensor , is measured exactly when flashing .

Further advantages are:

  • The measurement accurately captures the captured image. Light sources just outside the image that the sensor of the computer flash could still detect do not interfere.
  • The measurement automatically takes into account different focal lengths of the lenses used, photographic filters , the extension length for close-ups and deviations of the actual light intensity of the lens from the geometric light intensity.
  • The measurement can be continued and corrected during the exposure . However, this is only supported by a few manufacturers.
  • Depending on the camera used, measurement techniques such as spot metering or multiple field metering can also be used for the flash.
  • Depending on the camera used and the functions of the flash unit, it differs from the normal "synchronization on the first curtain", i.e. H. Triggering of the flash at the beginning of the exposure, also the "synchronization to the second curtain" possible, d. H. at the end of the exposure time. With moving objects in the subject, wiping effects can be better controlled in connection with longer exposure times. In the strict sense, this is not a property of TTL measurement, but results from the close connection between flash and camera.
  • With computer flash, the parameters film speed and aperture usually have to be transferred manually from the camera to the flash unit. Operating errors are possible here that cannot occur with TTL flash measurement.

The TTL flash measurement is not standardized independently of the manufacturer and in some cases has been modified several times within the manufacturer over the years. Flash units for TTL flash measurement must therefore always be matched to the camera model used and cannot be used universally. With the SCA system , however, the same flash unit can be used with different camera systems by exchanging an adapter . Furthermore, the exposure control by the camera requires additional electrical contacts in addition to the synchronous contact. This requires special, expensive connection cables for unleashed flashing . The combination of several flash units to achieve certain lighting effects is also not easily possible, so that alternative measurement options have by no means become unnecessary.

functionality

Older TTL flash systems regulate the amount of flash light required for the subject situation during the exposure, sometimes also taking into account the measured subject distance. The light reflected from the film is captured by one or more photodiodes in the camera base. This method also allows ambient light (e.g. background lighting) to flow into the exposure measurement. If the emitted flash is sufficient, the camera transmits a switch-off signal to the flash unit.

Newer systems send out a weak measuring flash shortly before the picture is taken (pre-flash) and use this to calculate the required output of the flash unit. Both the previously described photocells for classic flash measurement and the measurement system for ambient light are used for the measurement. Digital system cameras mostly work exclusively with the measuring system for ambient light and no longer support older TTL flash units.