Okay, I'm kind of cheating on this one. I found the top photo of the Flatiron Building through fog via This Isn't Happiness, who in turn found it on The New York Times' site. The photo was taken by one Jessie Tarbox Beals circa 1920. I spotted it on that Tumblr site and was blown away. I walk basically right through that strip twice a day. In any case, this morning, I tried to replicate the shot and find the exact spot Beals must have been standing in (now right in front of the mysterious Commodore Criterion building, where it's always Christmas). My picture really isn't quite as dramatic as Beals', but it's an interesting contrast just the same.
Looking South on Fifth Avenue from the northwest corner of 25th Street, 1920:
Looking South on Fifth Avenue from the northwest corner of 25th Street, 2009: