Canon 512XL - Manual (Page 28)

Fading
I When EE/manual aperture switch ring is rotated
past f/45, the F.O. mark appears in the aperture
indicator in the viewfinder. The aperture is completely
closed then. Therefore, a fade-out effect in which the
film gradually darkens or a fade-in of the opposite effect
may be performed with this ring.
• In bright outdoors, the aperture closes down to the
very small opening of f/16 or 22. Since this aperture
opening is almost similar to that of a completely closed
aperture, fade-out or fade-in would be performed too
quickly. Use the Canon ND filter for smoother fading
effects.