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When shooting a DSLR with ancient screw-mount lenses, you actually have to operate the aperture by yourself. The lenses have a lever or switch with two positions; "A" (aperture stays wide open, useful for focusing) and "M" (aperture stops down to selected setting). So you'd first have to focus in the A position, then select your aperture, then meter and shoot in the M position. It's just as inconvenient as it sounds.
