A few months ago, I encountered a news article that referred to “laser-sharp focus.” I was amused by what I thought was a recombinant idiom, but then when I Googled the phrase, I was surprised to discover that it's in very widespread use, including in plenty of established publications.
The original phrase, “razor-sharp focus,” makes more literal sense (though I know that's an odd thing to say about a metaphor): razors are literally sharp. But then again, when we say that focus is “sharp,” we don't actually mean sharp like a razor.
So I went to the OED for a history of “sharp.” Originally (going back to Old English), it had to do with having a good edge or point for cutting or piercing. Not too much later, it developed a metaphorical meaning: “Acute or penetrating in intellect or perception.” And then a while later it started to mean having acute vision or hearing. And then: “Keen-witted and alert in practical matters, businesslike.”
So by 1697 (the earliest cite listed for that last meaning, though I wouldn't be surprised if it had been used that way earlier), “sharp” has metaphorical connections to both vision and business. So I can imagine that could easily lead to the idea of someone's eyes having a sharp focus, literally and/or metaphorically.
Meanwhile, it also began to refer to an image or object having clearly delineated edges, and by 1883 there's a reference to a photographic image being sharp. So that's an image with a sharp focus.
And then somewhere along the way those uses of “sharp” relating to vision and/or images got combined with the metaphorical comparison to a razor (Thackeray referred to “Epigrams that were as sharp as razors” in Vanity Fair in 1848, though presumably that's not the first use of that phrase). Seems like a natural combination, a way of saying that the focus is not just sharp but very sharp—but still, it's a bit of a mixed metaphor.
And then along came lasers.
And lasers are sharply focused light.
Well, okay, lasers are actually coherent light. But it's easy to think of them—not in a scientific sense, just looking at them from a lay perspective—as being a very sharply focused beam of light.
Also, lasers can be used to cut, and an edge produced by a cutting laser seems (at least in the popular imagination) like it ought to be even sharper than the edge of a razor.
So it makes perfect sense to start with the idea of a “razor-sharp focus” and then update it to the modern world and an idea of even greater sharpness, to creat “laser-sharp focus.”
And it actually makes the metaphor more coherent (if you'll pardon the pun).
So although I initially thought the phrase was a little goofy, taking an already somewhat over-the-top metaphor and magnifying it, I'm now really pleased with it. It takes a longstanding phrase that consisted of two somewhat incompatible metaphors, and it intensifies (heh) the metaphor while also making it more consistent.
(Originally wrote this in February 2012, but didn't post it 'til now.)