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The diopter goes in front of the existing eyepiece glass.
There should be a thin, black metal ring with two small notches on the outside of the existing eyepiece. Simply unscrew this ring, place the diopter on top (in front of) the existing eyepiece, and screw the ring back in.
David S. Odess
Factory trained Hasselblad technician
28 South Main Street #104
Randolph, MA 02368
(781) 963-1166 firstname.lastname@example.org
Got it David! Apparently this is the wrong diopter as it still is clearer without it. Because the diopters are not labeled it would be easy for someone to put it in the wrong box. I bought it used from Ebay. Thanks for your help,