Re: Distribution of DoF

On Apr 21, 5:24 am, Monica Schulz <monica.sch...@xxxxxxx> wrote:
It sounds like the OP is asking about the depth of field, not the
depth of focus. The depth of field is distributed around the objects
in the field, not around the image plane: over what distance (in the
field of view) are objects in focus for a fixed image plane.

Depth of field is definitely not "symetrical" about the distance where
objects are perfectly in focus, since it is possible for this range to
extend to infinity.

The depth of field is exactly it. As I understand the case dof and
depth of focus are related in the following way: The depth of focus
covers some fraction of the focal length and the depth of field covers
that same fraction of the object distance. As depth of focus is always
symetrically distributed around the image plane but depth of field is
not I would like to understand where this asymmetricity does come
from.

Regards!
Monica

I tried to Google for a good figure to go along with my explanation
(which follows), but didn't find one within 5 minutes of searching.
So I'll assume you're familiar with figures that show "cones of light"
between the object and lens, and from the lens to the image plane,
like the figures here:

http://dspace.dial.pipex.com/town/pipexdsl/p/apuo76/digifotoinfo/articles/DOF/DOF%20how.htm

The two cones of light (object-to-lens, and lens-to-image) are
determined by the lens and the object's location. Moving the image
plane will not change the size of the light cones. So the image plane
must move the same distance, whether to farther or closer to the lens,
before the circle of confusion (diameter of the light cone at the
image plane) is large enough to be considered out of focus.

On the other hand, if the object distance changes (to determine depth
of field), this also changes the angular size of the light cones.
Moving the object toward the lens will lessen the light cone's angle
on the image side, and moving the object farther away increases the
angle of the light cone at the image plane. Moreover, the distance
that the point of "exact focus" moves is not a linear function of
object distance, being given by the well known relation

1/i + 1/o = 1/f

Since the angles of the light cones change as the object distance
moves, and also the "exact focus" distance changes nonlinearly, the
object will have to move different distances (closer or farther)
before the circle of confusion at the image plane is of the
appropriate size.

Hope I've made it clear enough ... I'm still just on my first cup of
coffee today :-)

Mark

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