Just some thoughts that might make it easier to visualize.
> Imagine a perfect lens . . . The lens gathers the light from this point
and brings it to a
> focus. . . . Now imagine those rays of light coming to focus behind the
> lens. They form a "cone" whose tip is the point of sharpest focus. If an
object
> is closer to the lens than the object we are focused on, _its_ cones of
light
> will come to focus _behind_ the film plane. . . . Similarly, objects
farther away than the
> principal object come into focus in _front_ of the film plane. . . . .
<snip>
Summarizing, you've got these little cones of light coming from objects at
different distances, and the film plane only hits the tip of one of them.
The rest have roughly circular images of varying size, depending on the
distance of these objects from the object in focus. On the final print, your
eye can't see any difference below some minimum size circle that defines
your depth of field.
Now, visualize the cone formed by the aperture stop and the focal point and
you'll see why lens design doesn't matter. Long, thin cones, i.e. large/slow
f/#s, have greater depth of field because the width of the cone changes
slowly as the tip of the cone moves. Short squat cones (low/fast f/#) have a
much faster change in cone diameter as you move from the tip of the cone, so
depth of field is less.
Depth of focus changes with focussed object distance result from the inverse
relationship between image and object distances; the infamous 1/f = 1/i +1/o
equation where f is focal length, i is image distance and o is object
distance. When the object is really far away, 1/o is near zero and you have
great depth of field. When o = i, the depth of field collapses to almost
nothing because it's now equal to the depth of focus on the film plane.
Here, again though, the f/# is an overriding factor, since it still
determines the width of the cones of light at the film plane.
Of course other things do matter with real-world lenses and full-spectrum
light, but avoiding that discussion is what "perfect lenses" are all about.
Hope this helps!
Frank, a newbie and OM-1 owner-to-be just because of this list (I bought
Mike Stoesz's)
PS
> The warning on an OM1 would says "Warning: use can cause Zuikoholism.
Truer words were never spoken. In fact, the effect may actually precede use.
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