Re: [Leica] Re: DOF -Optical vs Apparent

[Rob McElroy <idag@pce.net>]

John Collier wrote:

> If this is your definition of optical depth of field versus apparent depth
> of field, you are incorrect.

John -I stand by my definition of optical depth of field (used to differentiate it from apparent depth of field). I was referring to the calculated
distance, using optical formulas, which are easily found in books and on the websites already given by other members.  Here is Arthur Cox's very apt
definition of depth of field:  "The region in front of and behind the focused distance, within which object points still produce an image of a
required standard of sharpness".

> There is only a plane of focus, it has no
> width.

This is absolutely correct and I have not claimed otherwise.  My reference to the 6" DOF in my example was a random figure that would have come off of
a published DOF chart using accepted COC parameters.  Now whether the COC standards should be adjusted because of new lens designs is another subject
that I was not referring to.

> Any observable width is an optical illusion involving varying sizes
> of COC and the image size you choose to observe; that is, in photographic
> terms, contact, reduction or enlargement.
>
> If we are discussing this "optical depth of field" then I agree, negative
> size, lens focal length and focused distance has no effect. It is always a
> plane having no width. Naturally you do not need a DOF table to calculate
> this. You use a DOF table to calculate what the depth of focus our brains
> are fooled into seeing. A DOF table has to calculated using a fixed COC, a
> fixed output (whether that involves enlargement, reduction or contacting the
> negative) and a fixed viewing distance. This DOF table calculates the extent
> of the optical illusion of being in focus. The only points in actual focus
> are those that bisect the plane of focus.

You are quite correct, but I think we were referring to two different things using the same term.  I was using "optical depth of field" to describe
the plane of focus PLUS the acceptable distance in front of and behind that plane - according to the published COC formulas.  That is why I used the
term DOF instead of just plane of focus.

At a given magnification, if the resolution of your image is reduced (by whatever factors), the sharpest areas of your image (namely at the plane of
focus), will result in the visual impression that the plane of focus (along with its calculated DOF) has become closer (in visual sharpness) to the
rendition of the OOF areas.  This results in an APPARENT gain in depth of field.  This is a real phenomena, not imagined, and my use of the word
"apparent" doesn't mean that it isn't quite visible.  On the contrary, it is a very useful tool when trying to make the best print or enlargement.  If
you make a portrait of someone, shot fairly wide open at say f2.8, and one of the eyes of your subject looks a bit blurry because it fell beyond the
accepted COC standard (in other words it was out of focus) you can often make the print look much better by slightly softening it (reducing the
resolution) and bringing the sharpest parts closer to "the look" of the OOF eye.  Photoshop's sharpening tool does just the opposite by sharpening
(increasing the contrast) at the edges of objects, to make them appear like there was a smaller COC when those edges hit the film in the first place.

Ok - that's enough DOF, COC, POF talk for me today.

Time to go shoot some pictures.

Regards,
Rob McElroy
Buffalo, NY


> > From: Rob McElroy <idag@pce.net>
> >
> > If we assume, for this example, that the optical depth of field was say 6", if
> > you look critically (using a loupe) at all three images, you will see
> > that even with the wide-angle lens - anything falling outside that 6" DOF, IS
> > technically out of focus (not critically sharp).  Here's the big BUT
> > though.  With the wide-angle lens, our brain recognizes many OOF objects and
> > doesn't think that those out of focus areas look too bad (not too fuzzy),
> > but with the telephoto lens the OOF objects are almost unrecognizable (way too
> > fuzzy) and our brain tells us that they are definitely OOF.  And no,
> > this isn't fuzzy logic.