Open aperture measurement
Single lens reflex cameras (SLR, single lens reflex) equipped with open aperture measurement measure the exposure with the aperture open , i. H. the largest possible opening of the lens .
The value of the aperture set on the lens for the recording (i.e. the reduced incidence of light during the recording) is transmitted either mechanically or electronically to the camera electronics relative to the largest aperture and is calculated or simulated by this in advance ( aperture simulation ), provided that the open aperture measurement is combined with an automatic exposure ; otherwise it is not necessary to transfer the aperture value desired for the recording to the "camera electronics". The exposure metering then determines the required exposure time on this basis and displays it as a default in the viewfinder (for manual time setting) or automatically assigns it to the shutter ( aperture priority ), often combined with a display that warns of excessively long times that are prone to blurring .
At the moment of release, the oscillating mirror is folded up, the spring shutter is closed to the preset value and then the focal plane shutter is opened for exposure.
The open aperture measurement has the advantage that the viewfinder image always remains as bright as possible when the exposure is measured through the lens ( see TTL ). Together with the fact that the image in the viewfinder by having the minimum depth of focus facilitate this focusing, both directly on the shim as well as the use of focusing aids such sectional image indicators and microprism that at smaller F-numbers than about 1: not 5,6 virtually are more useful. When measuring the working aperture with very small aperture values such as 16 or 22, on the other hand, almost nothing can usually be seen in the viewfinder. In addition, a disadvantage of the CdS photoresistors that were often used in the past is largely compensated for, which sometimes react extremely slowly at low brightness. If the lens is very dimmed, such cells can take several seconds to display the final measured value.
The disadvantage of the open aperture measurement is that the depth of field cannot be assessed in the viewfinder image. Therefore, some cameras have a dimming button .
With certain accessories attached to single-lens SLRs, an open aperture measurement is not possible, for example when using T-2 adapters , or with extension devices ( bellows devices ) if aperture transmission cannot be used, e.g. B. with special micro and macro lenses or when mounting a lens in retro position . With some newer camera systems that rely entirely on electrical actuation and transmission of the aperture settings, an open aperture measurement is also possible in such special cases; also with some older camera systems that rely on the electrical transmission of the aperture setting, but without its electrical operation.
In the case of mirror lens lenses, measurement is always made with the lens fully open due to the lack of an adjustable aperture.
The Japanese manufacturer Topcon launched the first 35mm single lens reflex camera with open aperture measurement in 1964 . The first German SLR with TTL open aperture measurement was the Contaflex Super BC from Zeiss-Ikon / Voigtländer from 1965 . Leitz has offered them with the Leicaflex SL since 1968 . Many current cameras, but for the first time the Minolta XD series and the Olympus OM-2 , only set the final exposure time at the moment of shutter release immediately after the shutter is closed. This absorbs the tolerances of the shutter mechanism, and in the case of the OM 2 it is even possible to take lighting changes into account for long-term exposures.
Due to the system, the open aperture measurement does not play a role in viewfinder cameras, although these also often measure the exposure through the lens.
See also: aperture coupling , dimming button