There are simple T-shirt bras and lacy lingerie. Silky slips and nursing bras. Shapewear, strapless, corsets, bathing suits. The sizes run from the smallest smalls all the way up to a 46M. For items that are mostly meant to be hidden by clothing, underwear comes in a wide selection. But at Iris Lingerie in Boerum Hill, Brooklyn, customers rarely paw through the racks of unmentionables. Instead, women put themselves in the capable hands of Iris Clarke.

Ms. Clarke has been in the bra business for 30 years, working in both retail and design before opening her store in 2008. “I dream bras,” she said. Whether she’s at work or going for a walk, Ms. Clarke said, she can’t help sizing up the fit of bras passing by.

The store is on the basement floor of a bridal boutique. Its location — down a set of stairs and around several corners — makes Iris Lingerie feel like a small, well-lit speakeasy. But no one is trying to keep the store a secret. Ms. Clarke has been referred to as the “bra whisperer,” and if online reviews are to be believed, there are many women whose lives have been altered at Iris Lingerie. (As L.V. from Brooklyn wrote in her 2013 review on Yelp: “Life changing!”)

Such devotion comes from the shop’s old-fashioned approach to fitting bras. Bodeline Dautruche, 21, a second-time customer who was stocking up on undergarments before going back to college, said Ms. Clarke did not even measure her before finding the perfect fit. After whisking women to one of three dressing rooms, Ms. Clarke will ask them to remove their tops, and with a brief look, she will know everything she needs to come back with the right size.