Lens focus adjustments

On 1/8/2015 10:44 PM, Eric Stevens wrote:
On Thu, 08 Jan 2015 17:38:32 -0500, PeterN wrote:

On 1/8/2015 12:23 PM, Wally wrote:
On Wed, 07 Jan 2015 21:35:03 -0500, Mort wrote:

Hi,

Hi

In the old days of no money, we used to tape a newspaper page to a flat
wall, then photograph it from a 45 degree angle, with the focus on an X
drawn onto the page. By looking at the negative or slide, you could
readily see if the focus was correct or off, and if off in which
direction and by how much. What is cheaper as a test than a newspaper
page?

For focus accuracy, you should focus on a high contrast object that is
PARALLEL to the sensor. Those AF points are bigger than depicted. If
the target is at 45 deg, then the AF point may focus on the part of
the target that is closest to the camera, thus front-focusing a bit.

W

With the exception of certain types of pohotgraphy, such as forensic &
scientific, it is rare that the focus plane is exactly parallel to the
focal plane.

Having spent 30+ years doing forensic photography I can say that I
cannot recall a single occasion when I have had to set up the camera
so that the image plane is parallel to the object plane. There have
been occasions when it might have been useful but, when needed, I have
always had software to correct errors in parallelism.


OK I shoyuld have said "possible exception."
I added the exceptions to try to head off divisionary comments.

--
PeterN

On Fri, 09 Jan 2015 10:53:35 -0500, PeterN wrote:

On 1/8/2015 10:44 PM, Eric Stevens wrote:
On Thu, 08 Jan 2015 17:38:32 -0500, PeterN wrote:

On 1/8/2015 12:23 PM, Wally wrote:
On Wed, 07 Jan 2015 21:35:03 -0500, Mort wrote:

Hi,

Hi

In the old days of no money, we used to tape a newspaper page to a flat
wall, then photograph it from a 45 degree angle, with the focus on an X
drawn onto the page. By looking at the negative or slide, you could
readily see if the focus was correct or off, and if off in which
direction and by how much. What is cheaper as a test than a newspaper
page?

For focus accuracy, you should focus on a high contrast object that is
PARALLEL to the sensor. Those AF points are bigger than depicted. If
the target is at 45 deg, then the AF point may focus on the part of
the target that is closest to the camera, thus front-focusing a bit.

W

With the exception of certain types of pohotgraphy, such as forensic &
scientific, it is rare that the focus plane is exactly parallel to the
focal plane.

Having spent 30+ years doing forensic photography I can say that I
cannot recall a single occasion when I have had to set up the camera
so that the image plane is parallel to the object plane. There have
been occasions when it might have been useful but, when needed, I have
always had software to correct errors in parallelism.


OK I shoyuld have said "possible exception."
I added the exceptions to try to head off divisionary comments.

Just getting my pennyworth in.

Mind you, mine was rather specialised subsection of forensic
photography: mechanical failures, fires, motor vehicle (usually heavy)
accidents etc. I used a camera to record the scene and important
details, not directly as an analytic tool.
--

Regards,

Eric Stevens

On 1/9/2015 5:30 PM, Eric Stevens wrote:
On Fri, 09 Jan 2015 10:53:35 -0500, PeterN wrote:

On 1/8/2015 10:44 PM, Eric Stevens wrote:
On Thu, 08 Jan 2015 17:38:32 -0500, PeterN wrote:

On 1/8/2015 12:23 PM, Wally wrote:
On Wed, 07 Jan 2015 21:35:03 -0500, Mort wrote:

Hi,

Hi

In the old days of no money, we used to tape a newspaper page to a flat
wall, then photograph it from a 45 degree angle, with the focus on an X
drawn onto the page. By looking at the negative or slide, you could
readily see if the focus was correct or off, and if off in which
direction and by how much. What is cheaper as a test than a newspaper
page?

For focus accuracy, you should focus on a high contrast object that is
PARALLEL to the sensor. Those AF points are bigger than depicted. If
the target is at 45 deg, then the AF point may focus on the part of
the target that is closest to the camera, thus front-focusing a bit.

W

With the exception of certain types of pohotgraphy, such as forensic &
scientific, it is rare that the focus plane is exactly parallel to the
focal plane.

Having spent 30+ years doing forensic photography I can say that I
cannot recall a single occasion when I have had to set up the camera
so that the image plane is parallel to the object plane. There have
been occasions when it might have been useful but, when needed, I have
always had software to correct errors in parallelism.


OK I shoyuld have said "possible exception."
I added the exceptions to try to head off divisionary comments.

Just getting my pennyworth in.

Mind you, mine was rather specialised subsection of forensic
photography: mechanical failures, fires, motor vehicle (usually heavy)
accidents etc. I used a camera to record the scene and important
details, not directly as an analytic tool.


No offense taken. To me as long as the photographer can truthfully
testify that the image taken fairly and accurately
represents____________, it is fine with me.

--
PeterN

On Fri, 09 Jan 2015 20:50:24 -0500, PeterN wrote:

On 1/9/2015 5:30 PM, Eric Stevens wrote:
On Fri, 09 Jan 2015 10:53:35 -0500, PeterN wrote:

On 1/8/2015 10:44 PM, Eric Stevens wrote:
On Thu, 08 Jan 2015 17:38:32 -0500, PeterN wrote:

On 1/8/2015 12:23 PM, Wally wrote:
On Wed, 07 Jan 2015 21:35:03 -0500, Mort wrote:

Hi,

Hi

In the old days of no money, we used to tape a newspaper page to a flat
wall, then photograph it from a 45 degree angle, with the focus on an X
drawn onto the page. By looking at the negative or slide, you could
readily see if the focus was correct or off, and if off in which
direction and by how much. What is cheaper as a test than a newspaper
page?

For focus accuracy, you should focus on a high contrast object that is
PARALLEL to the sensor. Those AF points are bigger than depicted. If
the target is at 45 deg, then the AF point may focus on the part of
the target that is closest to the camera, thus front-focusing a bit.

W

With the exception of certain types of pohotgraphy, such as forensic &
scientific, it is rare that the focus plane is exactly parallel to the
focal plane.

Having spent 30+ years doing forensic photography I can say that I
cannot recall a single occasion when I have had to set up the camera
so that the image plane is parallel to the object plane. There have
been occasions when it might have been useful but, when needed, I have
always had software to correct errors in parallelism.


OK I shoyuld have said "possible exception."
I added the exceptions to try to head off divisionary comments.

Just getting my pennyworth in.

Mind you, mine was rather specialised subsection of forensic
photography: mechanical failures, fires, motor vehicle (usually heavy)
accidents etc. I used a camera to record the scene and important
details, not directly as an analytic tool.


No offense taken. To me as long as the photographer can truthfully
testify that the image taken fairly and accurately
represents____________, it is fine with me.

That was 'I'.
--

Regards,

Eric Stevens