Canon T 90 - Manual (Page 133)

Stopped-down metering is necessary whenever
you insert accessories, such as Extension
Tubes M, between the camera and the FD lens
for close-up photography.
1) Insert the slot of the accessory manual diaph­
ragm adapter (optionally available) over the
tip of the automatic aperture lever at the rear
of the lehs.
2) Push the lever counterclockwise and lower
the adapter into the groove,
3) Mount the lens as it is onto the accessory
Do not mount the lens with the manual diaphragm
adapter directly onto the camera or the Auto Bellows
etc
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