For those of you who might be a little confused, let's unpack the Push Twice Method a little bit. When you push your camera shutter release button half way, your camera's autofocus system snaps into action and adjusts the lens so that your shot will be sharp. That we know. The idea is that if you take your finger off the shutter and push the shutter half way again, the AF system will fine-tune the focus.
Turns out, that's some photographer hogwash. The camera buffs at LensRentals.com ran a battery of tests and discovered that it makes no difference whether you autofocus once or twice. Using a 50mm lens, on both a Canon 5D Mark II and a Canon 5D Mark III, they tested the single-focus method versus the multiple focus method. The testers found that the sharpness at the centre of an image — as well as the average sharpness of a photo — did not change using the multiple AF method.
That might sound insignificant, but tapping that focus button more than once is a common technique. Not every photographer in the world focuses this way, but a lot of people — incuding your writer, here — focus over and over and over compulsively. In fact, whenever possible, I take the AF off the shutter completely and map it to a button on the back of a DSLR — just so I can hit it trillions of times.
Looks like we can all give it a rest.