Depth Of Field

Is it true that when photographs are taken with a camera (using manual
focus - set before taking the photos and not changed until after taking the
photos) at a wide aperture (lower number - about 3.6) - the depth of field
should affect the same parts of the image regardless of the orientation of
the camera (assuming the photos were taken in the same spot at the same
angle to the subject)?

What I mean is that if I took one shot then rotated the camera 180 degrees
and took another shot - should they both have the same parts of the image
out of focus? Or could the out of focus part of the image rotate 180
degrees too?


Thanks for any replies.

"Matalog" wrote in message
...

What I mean is that if I took one shot then rotated the camera 180 degrees
and took another shot - should they both have the same parts of the image
out of focus?

Yes.

Or could the out of focus part of the image rotate 180 degrees too?

I think of depth of field as a spherical region around the camera. The
edge of this sphere is the same as the focal point, and its thickness
is is calculable by focal length and aperture.

May sound silly, but I envision it as an invisible ball around the
camera, with a varying thickness and diameter.

Regardless - it's spherical, so rotation of the camera will have little
effect on it.

BD wrote:
Or could the out of focus part of the image rotate 180 degrees too?

I think of depth of field as a spherical region around the camera. The
edge of this sphere is the same as the focal point, and its thickness
is is calculable by focal length and aperture.

snip

Regardless - it's spherical, so rotation of the camera will have little
effect on it.


Assuming a subject of a perpendicular plane, is the focus different at
the left & right edges of the sensor compared to the center of the
sensor (or focal plane) [horizontal] because of their increased
distance from the center of the lens?

I would say that it depends on your depth of field, and thus your
aperture size. I don't have any 'evidence' to back myself up, but let
me kind of think out loud for a sec:

I did a portrait shoot not long ago, and mucked around with aperture
size; I put the aperture wide open (which for that lens was f/1.8), and
at 7 feet away, I got a DOF of maybe a few inches.In that shot, the
subject's eye was in perfect focus, but at her ear was a little soft.
Makes a nice effect, actually. So if the subject were holding up a
perfectly flat piece of cardboard which filled my entire field of view,
I expect the center would be in focus but the edges would not, given
that the edges are further away from my lens than the center is.
Obviously, the smaller the subject or the further away, the less of a
difference this will make.

BUT - if I increase my f-stop, thereby shrinking the aperture, the
depth of field will become wider.... this is the 'thickness' of that
sphere in my silly analogy. Where it had been a couple of inches thick
at f/1.8, it might be closer to a foot thick at f/4. The depth of field
would still be spherical in shape, but its thickness would allow for
both the center of the cardboard and the edges to be within the range
of the depth of field, and therefore the entire piece of cardboard
would still be in focus.

One other interesting topic, while we're on the subject: hyperfocal
distance. Basically, as I said, the depth of field can vary in
thickness depending on your aperture - the smaller the aperture (or
higher the f-stop), the thicker the DOF. Now - say you're in a biiig
room, taking pictures of stuff at the back wall. If the 'center' of
your DOF is right at your focal point, then there will be things that
are closer to you than that focal point which will remain in focus at a
given f-stop, and there will be things *BEHIND* the focal point that
will still remain in focus (this extra 'room in your depth of field' is
wasted, because it's behind the wall). What a person can do, to take
advantage of the full range of your DOF, is to actually focus on a
point *closer to you* than the subject. So say you have a room that's
100 feet end-to-end: if you focus on a point that's 30 feet closer to
you than the very back, your subject at the back can still be in focus
because of your DOF. Why would someone do this? to allow _more_ of your
scene to be in focus at the same time. The closer to your your focus
point is, the closer that objects can be while still remaining in
focus. If your DOF is thick enough, that is.

Kind of a convoluted ramble, I know - but there are actually charts
available that can tell you what your hyperfocal distance is, given a
certain focal length and f-stop.

Clear as mud? GOOD! For me as well!! ;-)

Ryan wrote:

Assuming a subject of a perpendicular plane, is the focus different at
the left & right edges of the sensor compared to the center of the
sensor (or focal plane) [horizontal] because of their increased
distance from the center of the lens?

In general, yes. The effect is called "field curvature". It is not
related to depth of field per se. For further discussion:

http://www.vanwalree.com/optics/astigmatism.html

wrote:

Ryan wrote:

Assuming a subject of a perpendicular plane, is the focus different at
the left & right edges of the sensor compared to the center of the
sensor (or focal plane) [horizontal] because of their increased
distance from the center of the lens?

In general, yes. The effect is called "field curvature". It is not
related to depth of field per se. For further discussion:

http://www.vanwalree.com/optics/astigmatism.html

Oh, excuse me, yes, there would be a DOF effect in the scenario you
describe because of different subject distances. Still, read the
referenced URL anyways; all of the optical goo at
http://www.vanwalree.com/optics .htmlis worth digesting.

Except that its not.

Then what say you on the topic of 'field curvature'?

"BD" wrote in message
ups.com...
I think of depth of field as a spherical region around the camera. The
This would be good except the depth of field is more like a plane tangential
to a sphere. In your view, if you had a very tall creature, and focussed on
its midsection you would expect its feet and head to be out of focus. This
is not the case.

Regardless - it's spherical
Except that its not.

so rotation of the camera will have little effect on it.
This we agree on.

-Andrew

BD wrote:
Or could the out of focus part of the image rotate 180 degrees too?

I think of depth of field as a spherical region around the camera. The
edge of this sphere is the same as the focal point, and its thickness
is is calculable by focal length and aperture.

May sound silly, but I envision it as an invisible ball around the
camera, with a varying thickness and diameter.

Regardless - it's spherical, so rotation of the camera will have
little effect on it.

I love the explanation. I will say that there should be an allowance
for the fact that with most lenses it is not exactly spherical. As an
extreme example is the flat field of most macro lenses.

--
Joseph Meehan

Dia duit