The present participle of diaphragm, referring to the act of equipping something with a diaphragm or adjusting a diaphragm's opening.
From diaphragm (noun) + the present participle suffix -ing, used as a verb in technical contexts to describe the action of fitting or adjusting diaphragm components.
In photography, 'diaphragming down' (making the opening smaller) increases the depth of field so more of your image is in focus—this is why landscape photographers use small apertures while portrait photographers use large ones to blur backgrounds.
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