Nikon PC NIKKOR 35mm f-2.8 - Manual (Page 11)

Stopped-Down Measurement
The PC-Nikkor lens must be stopped down to deter­
mine the exposure when used on TTL cameras.
Because the finder image progressively becomes darker
as the lens is stopped down, you should focus first
before taking a meter reading.
It is important to note that exposure measurement
must be performed before the lens is shifted; should
measurement be performed after shifting, erroneous
meter readings may occur. With automatic exposure
cameras, such as the Nikon F3, FE2 or FE use the
camera’s memory lock to retain the correct shutter
speed before shifting the lens for the final picture
composition. Since the Nikon EM doesn’t have a
memory lock, the correct exposure cannot be obtain­
ed while shifting.