depth of field and apparture, basic question..

I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?

one would expect that when the aparture is high, that mean that the
iris is closing the lens almost completely , and by this not letting
light from wide angle points to get into the camera, so logic say that
the depth of field (mean the focus of elements far from the center )
should be blurred.

can someone explain this ?

thanks

"Over G" schrieb im Newsbeitrag
oups.com...
I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?

one would expect that when the aparture is high, that mean that the
iris is closing the lens almost completely , and by this not letting
light from wide angle points to get into the camera, so logic say that
the depth of field (mean the focus of elements far from the center )
should be blurred.

can someone explain this ?

thanks

What logic?

http://en.wikipedia.org/wiki/Depth_of_field

u are welcome

On Apr 12, 5:31 pm, "Over G" wrote:
I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?

one would expect that when the aparture is high, that mean that the
iris is closing the lens almost completely , and by this not letting
light from wide angle points to get into the camera, so logic say that
the depth of field (mean the focus of elements far from the center )
should be blurred.

can someone explain this ?

Yes. You are lost in a circle of confusion.

http://en.wikipedia.org/wiki/Circle_of_confusion

HTH.

--
YOP...

On 12 Apr 2007 15:31:35 -0700, "Over G" wrote:

I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?

one would expect that when the aparture is high, that mean that the
iris is closing the lens almost completely , and by this not letting
light from wide angle points to get into the camera, so logic say that
the depth of field (mean the focus of elements far from the center )
should be blurred.

can someone explain this ?

When the aperture is small, then the bundle of light rays from the
aperture to the sensor is narrower. There are many of these light
bundles (corresponding to different places in the subject) and they
are in the shape of cones. The base can be considered to be at the
aperture and the apex is near the sensor. The cones all have the same
size base (the aperture) but some are longer or shorter than others,
depending on where they come from in the subject.

The plane of the sensor intersects those cones. Some of those bundles
will be intersected exactly at the apex. Those bundles are the ones
that are in focus. The intersections form points.

The other cones will be cut in places that are not the apex, and the
shape of the intersection will be a circle. Those are the ones that
are not in focus. The size of the circle depends on simple
geometry---the length of the cone (which corresponds to subject
distance), the distance from the aperture to the sensor, and the size
of the aperture.

Smaller apertures give smaller circles on the sensor. That makes those
image points seem sharper, and that corresponds to better depth of
field.

If the aperture was infinitely small (and neglecting the effects of
diffraction), then everything would be in focus.

The size of the aperture does not affect the ability to see objects
that are at wide angles.

KS

Yes. You are lost in a circle of confusion.

Good response ;)

On Apr 13, 2:13 am, King Sardon wrote:
On 12 Apr 2007 15:31:35 -0700, "Over G" wrote:

I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?

one would expect that when the aparture is high, that mean that the
iris is closing the lens almost completely , and by this not letting
light from wide angle points to get into the camera, so logic say that
the depth of field (mean the focus of elements far from the center )
should be blurred.

can someone explain this ?

When the aperture is small, then the bundle of light rays from the
aperture to the sensor is narrower. There are many of these light
bundles (corresponding to different places in the subject) and they
are in the shape of cones. The base can be considered to be at the
aperture and the apex is near the sensor. The cones all have the same
size base (the aperture) but some are longer or shorter than others,
depending on where they come from in the subject.

The plane of the sensor intersects those cones. Some of those bundles
will be intersected exactly at the apex. Those bundles are the ones
that are in focus. The intersections form points.

The other cones will be cut in places that are not the apex, and the
shape of the intersection will be a circle. Those are the ones that
are not in focus. The size of the circle depends on simple
geometry---the length of the cone (which corresponds to subject
distance), the distance from the aperture to the sensor, and the size
of the aperture.

Smaller apertures give smaller circles on the sensor. That makes those
image points seem sharper, and that corresponds to better depth of
field.

If the aperture was infinitely small (and neglecting the effects of
diffraction), then everything would be in focus.

The size of the aperture does not affect the ability to see objects
that are at wide angles.

KS

Very VEry good answer
thanks .

Hi King,

Thank you for your thoughtful answer to G's question.

Best,

Conrad

On Apr 12, 4:13 pm, King Sardon wrote:
On 12 Apr 2007 15:31:35 -0700, "Over G" wrote:

I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?



When the aperture is small, then the bundle of light rays from the
aperture to the sensor is narrower.

On Apr 12, 5:31 pm, "Over G" wrote:
I Woud like to know why physically when the apparture is very high (8
and so..) there is a better depth of field, why this effect occur ?

one would expect that when the aparture is high, that mean that the
iris is closing the lens almost completely , and by this not letting
light from wide angle points to get into the camera, so logic say that
the depth of field (mean the focus of elements far from the center )
should be blurred.

can someone explain this ?

thanks



You seem to be confusing field angle (wideness of the field of view)
with focus. The light from each point in a tightly focused image
comes from a SINGLE point on the object. Draw a diagram (look up so-
called paraxial ray trace) of a lens focusing a point source. With a
large aperture, light leaving the point travels a large angle to the
lens, then leaves the lens at a steep angle to the focal plane. Thus
it moves rapidly away from the true focus point when the ray is at a
slighty different distance from the lens than the true focal point.

A ray passing through the lens near the center is only slightly
inclined, and hence is never far from the line from the object point
through the CENTER of the lens to the true focus point.

Look up "pinhole" lenses, and draw the diagram of a pinhole lens
focusing a point source. That should help.