Schneider Optics PC-SUPER-ANGULON 28 mm f/2.8 - Manual (Page 13)

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If the measurement is not carried out using a separate hand exposure me-
ter, but using the TTL exposure meter integrated in the camera, it is neces-
sary to measure in the centered position of the parallel shift while stop-
ping down to the working aperture because the arrangement and align-
ment of the measuring cell of almost all SLR cameras requires a direction
of light incidence axially symmetrically to the lens bayonet . With an oblique
incidence through the lens displaced from the center, some of the light
bundle can be shadowed so that then less light is incident onto the mea-
suring cell . A higher exposure would therefore be displayed or set with an
automatic exposure system so that overexposure would result .
If the exposure is measured with a non-shifted lens, the camera must
equally be aligned with the motif as if the motif would later also thus be
photographed with a non-shifted lens .
The darkening of the image visible in the viewfinder with a lens shifted a
large distance does not carry over to the image and so also does not re-
quire any correction of the exposure . This is due to a larger light fall-off at
the open aperture and due to the viewfinder system being aligned to an
exactly centered lens and not being adjustable to the shifted lens as well as
to the mirror not going down far enough for the low position of the lens in
the event of a downward shift . No vignetting occurs at the actual taking .
Measuring the Exposure