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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 08:35:09 -0500
>
> Here is a crop of 28mm at F5.6, a really tiny crop (100% pixel size of 21MP
> sensor).
>
> http://www.accura.com.hk/temp/IMG_5203.JPG
>
> and 300mm F5.6 full image resized:
>
> http://www.accura.com.hk/temp/IMG_5202.JPG
>
> Both were taken at same camera position.
>
> I might did it wrong, is that what you mean?
>


We must not confuse "bokeh" with DoF.  The 300mm image is exhibiting the
bokeh characteristics of a specific lens design (I'm assuming Zuiko 300/4.5
as it has this distinct look), but the cropped image shows the bokeh
characteristics of a modern wide-zoom.  The backgrounds in each shot are
out-of-focus, so there is no comparing DoF, just bokeh.

Ctein alludes to something in one of his last posts in that topic when he
states:

"Something I hadn't mentioned, and possibly should have sooner, is that
trying to nuance this too far is pointless, because real-world lenses NEVER
have the same DoF that the equations predict. The precise shape of the light
'cone' near the point of focus has a big effect on where the diameter of the
envelope hits the CoC limit, and the shape of that envelope is very
sensitive to lens designs and residual aberrations. There have even been
lenses with a floating element that tweaked the mix of aberrations so as to
let the photographer alter the balance between near and far DoFs!"

This is something that I've been harping about in the diffraction-limits and
DoF discussions for years.  Many modern telecentric lens designs with
multiple asperical-shaped lenses have the ability to direct the "cone" in
such a way that you get very little spread and in fact will actually invert
the cone for out-of-focus light paths forming ring-shaped bokeh artifacts.
I have noticed over and over again how some modern lenses do not have a
"plane of in-focus" but more of a "zone of in-focus"

AG
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