Good morning ...
My cats woke me a little while ago, and since I can't get back
to sleep right away I thought I'd annoy the list with more
questions and discussions about focusing.
Ed Mathews, in an earlier post, said:
> Even if your subject and camera
> remain in perfect position, the
> differences in the lens field of
> flatness can often make the
> focus point you achieved in the
> center of the viewfinder wrong to
> begin with, especially when close
> to the subject an/or using very
> limited DOF.
> I've often been very careful to
> AF focus on an eyeball, and
> carefully recompose, only to
> have it slightly out on the
> resulting negative and print.
> Lately I've discovered the problem
> to be mostly lens field curvature
> as mentioned [...] above.
> It's amazing how different the true
> plane of focus can be at close
> distances on axis vs. slightly off
> axis for all but macro lenses.
Well, Ed's comment intrigued me, and, coincidentally, I came
across this comment from someone who seems to be quite
knowledgeable about these things. He works for Zeiss:
"Why do you think focusing aids are always positioned in the
center of the screen? Because industry wants you to compose your
images center-oriented? Not at all! Let me give some important
information from inside a lens manufacturer:
Sharpness of a well-centrated lens is always highest in the
center of the image. You knew that one already. Did you also
know that depth of field is smallest in the center? Yes, Dof is
not the same everywhere in the frame! Now, combine maximum
sharpness and lowest Dof: This combination produces the clearest
possible, highly reactive focusing criterion. Since this optimum
is available in the very center of the frame only, any focusing
aid should logically be positioned there.
What I am also saying is: Dof grows as you move from the center
to the corner. And sharpness drops. So, when you focus first
with the central focusing aid, then reframe and keep the focus
previously set, your image should come out sharp."
So, what this fellow says seems to run counter to what Ed says
he's experienced. I wonder if Ed's experiencing something
different. Ed, could it be that by moving the camera when
recomposing you're moving the plane of focus enough to induce an
error? Have I missed something here?
--
Shel Belinkoff
mailto:[EMAIL PROTECTED]
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