Vivitar POWER ZOOM DF-383 - Manual (Page 16)

Manual Motor Zoom Control
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The "Zoom" key permits you to change the zoom reflector's
position independently of the focal length of the lens. The LCD
display then indicates "M" and adjusts the zoom position of the
reflector.
By slightly tapping the "Zoom" key you can select the zoom
position in the following sequence: 24-28-35-50-70-85mm.
Auto Zoom position M24 M28 M35 M50 M70 M85 Auto Zoom
position