What you describe makes it sound as though the cone of light reaching
and exiting the plane of focus is not really a cone but rather a
cylinder (or at least a cone with very shallow angles). Do you have
reference to any technical papers that state how this comes about
optically? My view of an aspherical element is that it has no magic
powers of reshaping a light beam other than correcting those abberations
relevant to a spherical optic. Show me how its done.
Chuck Norcutt
Ken Norton wrote:
>> 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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