Neutral-density (ND) filters have very high filter factors (2 … 1000) for exposure. Especially where very dense filters are used, long, application-oriented exposure times are achieved which exceed than the camera’s maximum exposure time of 30 seconds. The cameras must be operated in the time exposure mode in this case (bulb setting), for which exposure time is controlled manually by means of remote shutter release or via remote shutter release with integrated timer. Exposure measurement must be conducted without filter and the filter factor has to be added subsequently by the photographer, which is not only time-consuming but rather prone to error as well.
In the case of external exposure meters, the filter factor can be adjusted as a correction value and is taken into consideration during exposure measurement. The user can directly accept the f-stop / shutter speed combination which is displayed after measurement, and obtains a precise exposure with these values.
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