Set it on T or B. I can't remember if that lens has T. For T, use the cable release to start the exposure and then use the cable release to end the exposure. For B, use the cable release to start the exposure and hold the cable release until end the exposure.
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Warning!! Handling a Hasselblad can be harmful to your financial well being!
Nothing beats a great piece of glass!
I leave the digital work for the urologists and proctologists.