Archived posting to the Leica Users Group, 2003/08/20
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
At 2:00 AM -0400 8/20/03, Afterswift@aol.com wrote:
>In a message dated 8/19/03 9:49:38 PM, henningw@archiphoto.com writes:
>
><< Depth of field takes on a new meaning with digital cameras, and
>becomes especially significant with their short focal lengths. Zone
>focussing actually means now that a _ZONE_ is _IN FOCUS_, not just a
>plane with some additional space before and behind in near focus.
>
>It is a consequence of digitisation. >>
>-------------------------------------------
>Henning,
>
>I don't know enough about digital to dispute what you say. I reckon when we
>get down to basic physics, we can't say that absolute focus is a knife edge
>across an image mediated by the aperture and distance from the subject. We can
>only define focus in relative terms. I believe that view is what you have in
>mind. And I think it's correct.
No, that's not it. 'In focus' in a purely analogue description with a
perfect lens would produce a 'knife edge', or infinitely shallow
plane of focus for any object plane. Aperture and object distance do
no change this.
Since even film is not completely analogue (film grain), lenses are
not perfect and output media have their own limitations on
definition, and we view the output with our own optical limitations,
we start defining a circle of confusion that quantifies what we
gather into a 'zone of focus' under certain shooting conditions.
Digital takes that one step further; at one certain distance, the
'true focus distance', a point object is imaged as a single pixel.
Then, you have to be a definite distance off the 'true focus
distance' before the point is no longer imaged as a single pixel, but
as four pixels. That is digitisation, and it means that you have to
be a certain distance off the true focus before you can dectect _any_
loss of focus.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
- --
To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html