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Re: [OM] More on DOF and focal length (by Ctein)

Subject: Re: [OM] More on DOF and focal length (by Ctein)
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Mon, 29 Jun 2009 14:59:20 -0500
>
> Ummm, Ken, a "zone of in-focus" vs a "plane of in-focus" is called depth
> of field.  :-)  Want to try again?  I don't think you said what you meant.
>

Actually, Chuck it is exactly what I meant to say.  Every "normal" lens has
a specific "plane of in-focus".  As you stop the lens down, you increase DoF
by keeping the "circle of confusion" below a certain diameter. But there is
only one plane of critical focus, no matter what aperture you are using. The
farther from this plane of critical focus (or plane of in-focus as I
originally called it), there is blurring going on, but as long as the
diameter of the blur is smaller than the user-defined "circle of confusion"
it is in focus or within the DoF.

What I am referring to with "zone of in-focus" is a characteristic of many
modern lens designs which use aspherical-shaped lens elements is that the
plane of critical focus doesn't actually exist. What is happening is that
there is no one specific plane that defines this point. Aspherical-shaped
elements actually end up folding the light paths into themselves and prevent
the linear spread of the out-of-focus image area. This is why some old
classic lens designs have a very smooth bokeh which balloons out in a smooth
and unimpeded manner, whereas some new lens designs (especially zooms) limit
how wide the bokeh will spread and whether or not the edges of the bokeh
folds back into itself.  When something is severely out of focus, you'll see
the bokeh balloons turn into donuts. But when it is only slightly out of
focus, the edges have folded inward just enough to artifically sharpen the
slightly out of focus subjects.

AG
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