P3D Re: 2519 - focus

[Duncan Waldron <J.Waldron@xxxxxxxxx>]

On Tue, 20 Jan 1998, John W Roberts wrote:

> 
> I think maybe you need more words to describe the distinction between these
> two terms. From what's written above, "depth of focus" sounds like a pretty
> useless term - why not discard the current definition and use the term for
> something else? In the example you give, is the "depth of focus" 3 meters
> (the distance from five meters away to two meters away), or is it 5 meters
> (the distance from five meters away to ten meters away)? By "point of focus",
> do you mean the same thing as "focal plane"?
> 

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. 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. You photograph what's in front of you, which is within the field of
view your picture encompasses, so that's where the depth of field is - out
there in the field. I guess I am trying to say you'll have to get used to
the terminology, because it is what it is; maybe the terminology wasn't
defined well enough in the early days, but there you go.

In my example, the depth of field (not focus) is 8 metres - the distance
from 2 metres away to 10 metres away. Depth of field and depth of focus
extend in front of *and* behind the subject and the film (focal) plane
respectively. 

"Point of focus" I used in this example to mean "the subject". An
imprecise term - I'm sorry; smackum wrist. %~{

Duncan W



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