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

[Rob McElroy <idag@pce.net>]

"austin@darkroom.com" wrote:

> > In response to Jim Brick, austin@darkroom.com wrote "I believe you
> > are 'confusing' DOF on the film with DOF on the printed image at
> > viewed distance. They are different.  Even on the print, then again at
> > viewed distance have different 'DOF' too."
>
> > This theory confuses the issue even more.  "DOF on the printed image at
> > viewed distance" - there is no such thing!
>
> Of course there is such a thing.

Austin- I respectfully disagree with you.  You are confusing sharpness/resolution/mtf values with depth of field.

>  You can make a print that has less resolution than is on the film, and thefore, it make things that aren't in critical focus on the film, be in as sharp a focus as the print can produce.

This concept is of course true, but it is not OPTICAL depth of field, it is only APPARENT depth of field (the subjective interpretation of how the image appears to a given observer).  The only thing that has happened in your example is that either the enlarging lens, or the paper you used to make your print, or both -  have degraded the RESOLUTION of the original negative, giving the false impression
that there is more DOF.  It's a decrease in resolution resulting in an optical illusion, not an increase in DOF.  The plane of sharpest focus is still present, it just doesn't have as much detail in it.  The OOF areas are still OOF.

>  > The enlargement process itself introduces an overall flattening of fine
> > detail when large prints are made.  This flattening helps to make
> > the sharpest (critically in-focus) areas look more like the slightly out
> > of focus areas, thus giving the whole image the FALSE APPEARANCE of
> > greater DOF.
>
> Er, film has the same property.  No matter how sharp the lense is, it can only render, on the film, as sharp as the film is.  Anything sharper than the film will record is 'flattened' as you call it.  This is hardly apparent, it is real.

Your theory is true but it does NOT relate to depth of field.  It is a resolution issue.  No matter how much resolution or how little resolution a particular system of lens, film, enlarging lens, and enlarging paper ends up with in the final print, the DOF has not changed from the initial pushing of the shutter release. Period!  You can put a lousy lens on your enlarger and turn an extremely sharp
negative into a medium resolution print, but all you have done is softened/flattened the detail-contrast of the image, resulting in less of a visual difference between the critically sharp areas and the less sharp areas. Again, not a DOF issue.  Maybe someone should make up a new word for this phenomena.

> > Another example is:  if you put a slight diffusing filter over your lens
> > when you expose a scene, the DOF isn't changed by the filter - but
> > visually, the scene takes on a more uniform level of sharpness
> > (or lack of it) and our brain THINKS there's more depth of field.
>
> Oh yes the DOF does change if the diffusing filter is diffuse enough to reduce the resolution of the lense lower than that of the film.

No it doesn't.  Here's where we need the new word.  If someone can come up with "bokeh" to describe the look of OOF images, then surely we ought to be able to find a common word to describe this phenomena of the effect of lower resolution on apparent depth of field.

Respectfully,
Rob McElroy
Buffalo, NY