Don't take my word for it - try it yourself. Take a large aperture lens of
normal or moderate telephoto focal length, like a 50mm F1.4 or the 77mm F1.8
or something. To exaggerate the condition, focus EXTREMELY carefully at
the len's closest focusing point in the very center. Then simply turn the
camera slightly so that what you focused on is now about midway between the
center and the edge of the frame. At that point, I bet you can slightly
adjust the focusing to improve it a little. This is what happens when you
focus on an eyelash or eyeball using the center of the frame, and then
recompose.
Thanks,
Ed
----- Original Message -----
From: "Shel Belinkoff" <[EMAIL PROTECTED]>
To: "Pentax List" <[EMAIL PROTECTED]>
Sent: Tuesday, February 20, 2001 4:35 AM
Subject: Focusing ... Again!
> 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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>
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