Quote:
Originally Posted by SandeeWig
Is this true for practically any lens? For example, an 18-55mm IS kit lens?
If so, what you're saying is to select the proper focus point (i.e. if you have 9 points, select the one closest to what you want in focus), vs. focusing at center then repositioning camera to the composition you want. Yes??
Second question is...if you manually focus from 2 ft. away, how can you tell if the eyes are the sharpest focus vs. the nose, for example? It's not necessarily easy to tell in the viewfinder. What do you do then?
Thanks and good thread.  |
No, it's not, as I covered in your other thread.
It's only true when the difference in the hypotenuse to the long side of the triangle is larger than the depth of field. I take it back, I don't think it's right at all, the way it's stated.
The focal length is completely unchanged when pivoting the sensor (assuming you could perfectly). It does not get longer or shorter as you move it, think of it as an imaginary broom handle extending from your lens. If you could, in this painful and somewhat disturbing example, place one end of the broom handle on the eyeball and the other against your lens, and then rotate it to point the broom handle somewhere else, the broom handle doesn't magically get longer or shorter unless you change it. Similarly, the only real change to the focal distance is the tiny amount caused by the light being diffracted through the end of the lens, so the real distance you're calculating is the one between the eyes and the end of the lens in both positions. In real world terms, you'd have to be so ridiculously close for this to make any bit of difference as to be meaningless. Macro photographers MAY care, and even then I'm not entirely sure. The real worry is that the error in your ability to perfectly rotate the camera about the sensor will introduce error larger than your depth of field, and that'll only matter for extremely narrow ranges.