P3D Re: 2519 - focus

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Fri, 23 Jan 1998 09:58:27 -0700
>From: Duncan Waldron <J.Waldron@xxxxxxxxx>
>Subject: P3D Re: 2519 - focus

>Need more words... well, I can always add more, but whether that would
>really help, is another matter! IMO, the terms in current (and widely
>accepted) usage - at least among those of us who were trained according
>to, and from time to time refer to, the standard texts - are both adequate
>and understandable. 

Apologies. As I'm sure you're aware, it may be a worthy accomplishment to
gain a piece of knowledge, but sometimes it's even more difficult to convey
that knowledge to another person. I have referred to photography texts in
the past, and probably will do so again in the near future, but I doubt that
most of the readership of P3D has immediate plans to look up these texts.
Since this issue can have a direct impact on how well certain 3D photographs
turn out, perhaps it would be worthwhile to try to define these terms a little
more precisely.

>Let me try another tack (more words...); you use the lens to focus, 
>therefore depth of focus is near the lens, ie at the focal plane. 

So for a typical 35mm camera in typical use, depth of focus would be a
matter of millimeters? And it would be inside the camera?

>In my example, the depth of field (not focus) is 8 metres - the distance
>from 2 metres away to 10 metres away. 

The example from your previous post (depth of field under the following
conditions is 2 meters to 10 meters, with focus set at 5 meters) appears
to be much more useful than the restatement in this post (depth of field
under these conditions is 8 meters). The (2 meters to 10 meters) gives both
a near distance and a far distance, while the (8 meters) gives neither, and
for any situation where the focus is set beyond the hyperfocal distance,
the single-parameter value would be "infinity", without giving any information
about where the near distance is.

Both you and John B gave what are undoubtedly correct explanations, but they
were also highly abstract, with the exception of the partial example you gave.
I think what may be most useful at this point is a set of concrete examples
of how exactly depth of field and depth of focus would be expressed, with
nothing left out:

"Under [the conditions that matter], and with focus set to 5 meters,
   the depth of field is [one parameter? two parameters? what units?]."

"Under [the conditions that matter], and with focus set to 5 meters,
   the depth of focus is [one parameter? two parameters? what units?]."
   (And are those distances inside the camera?)

I'm not too concerned with the exact math at the moment - that can be filled
in later - just with what *kinds* of things we're talking about.

Analogy: in SI ("the metric system"), every unit can be expressed in terms
of ~6 fundamental units, for example energy (Joules) can be expressed
as kg m^2/s^2. In some cases (other than energy) there are constants that 
must be figured in, but at least the ability to clearly describe the parameters
is more useful than saying "energy = 5" (especially since people sometimes
use the term "energy" when they mean *power*).

John R


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