aflundi wrote:
Really? Floyd, I thought the point of the defocus control was to control the over/under correction for spherical aberration. That if set positive, it would yield smooth background boken, smooth foreground bokeh when set negative, and neutral bokeh when the DC is set at 0 with perhaps a little sharper in-plane focus.
I don't have one, so I haven't had a chance to play first hand, but I thought I had read enough Nikon papers to have fooled myself into thinking I understood it.
It does in fact control correction for spherical aberration, and one effect is essentially what you describe. But moving the DOF is the most significant and most visible change, and is what the adjustment ring is calibrated to match.
For example if it is set to affect the background and focused at 15 feet, you get about 21 inches of DOF at f/8. If the intent is to have a subject's face sharp and everything from the ears back increasingly out of focus it is possible to focus about 5 inches in front of the subject. That required accurate measurements and a DOF table. Twenty-five years ago, using film, a mistake was a disaster. But with the DC lens the focus can be on the eyes and the DC control might be set to f/8 or f/11 (for less out of focus ears). It would be easy and accurate every time. Today with immediate preview we can sort of blindly move the focus out in front and just snap a shot to see the effect.
By the same token people often expect setting the DC control to f/4 or even f/2 should cause even smoother bokeh, and instead they get a very blurry image. When the actual aperture is the same value as the DC setting the edge of the DOF is just past the plane of focus. Of course knowing exactly how that works can be used for odd effects...