20D LOSES FOCUS (300 F/4L IS Update)

My previous two tests of the 300 f/4L IS showed some peculiarities
that warranted further investigation. Either the lens was very soft
or else it wasn't focusing properly.
So today I did a quickie test, this time with a brick wall at about a
45-degree angle from the camera. This way I could see if perhaps it
was a focusing (or a backfocus) problem. Once again, I used the
tripod and mirror lockup to go for the sharpest image. The IS was
turned off since the lens was mounted on a tripod.

Here is the result:
http://www.pbase.com/bret/image/75308794/original

The red square is the center focal point used by the AF system. That
is where proper focus should be attained. However, as you can see,
this image has a backfocus problem. The area to the left of the
square (the part of the wall that is farther away) is in focus while
the area to the right is not. Further testing when I got home from
closer distances didn't show this problem.

I suspect that the IS is somehow throwing the image off. As I
mentioned, I turned the IS off for this shot, but somehow I think the
IS system is the culprit here. I mounted the lens to the tripod and
chose my focal point. Then if I switch the IS on and try to re-focus
I can see the image shifting in the viewfinder. Now when I turn it
off the original focal point may not be where it was. So I would
guess the problem probably has something to do with how you turn the
IS off and on. Just a guess.

Incidentally, my latest tests also included a few shots with the 400 f/
5.6L which shows none of the backfocusing problems. And even when the
300 f/4L IS focused properly it still wasn't as sharp as the 400 f/
5.6L.
So if you must have a 300mm lens, start saving for the 300 f/2.8L
which is highly acclaimed. Otherwise, the 400 f/5.6L still rules it
in this price range.

Annika1980 wrote:

My previous two tests of the 300 f/4L IS showed some peculiarities
that warranted further investigation. Either the lens was very soft
or else it wasn't focusing properly.
So today I did a quickie test, this time with a brick wall at about a
45-degree angle from the camera. This way I could see if perhaps it
was a focusing (or a backfocus) problem. Once again, I used the
tripod and mirror lockup to go for the sharpest image. The IS was
turned off since the lens was mounted on a tripod.

Here is the result:
http://www.pbase.com/bret/image/75308794/original

The red square is the center focal point used by the AF system. That
is where proper focus should be attained. However, as you can see,
this image has a backfocus problem. The area to the left of the
square (the part of the wall that is farther away) is in focus while
the area to the right is not. Further testing when I got home from
closer distances didn't show this problem.

I suspect that the IS is somehow throwing the image off.

So does this backfocus issue happen with all lenses? I seem to recall
that it's something that can be factory adjusted in the camera body but
maybe the lens can effect it too. Thanks for sharing the test results
and summarizing your conclusions. It's tedious work but can be a great
way to learn!

As I
mentioned, I turned the IS off for this shot, but somehow I think the
IS system is the culprit here. I mounted the lens to the tripod and
chose my focal point. Then if I switch the IS on and try to re-focus
I can see the image shifting in the viewfinder. Now when I turn it
off the original focal point may not be where it was. So I would
guess the problem probably has something to do with how you turn the
IS off and on. Just a guess.

Incidentally, my latest tests also included a few shots with the 400 f/
5.6L which shows none of the backfocusing problems. And even when the
300 f/4L IS focused properly it still wasn't as sharp as the 400 f/
5.6L.
So if you must have a 300mm lens, start saving for the 300 f/2.8L
which is highly acclaimed. Otherwise, the 400 f/5.6L still rules it
in this price range.

On Mar 7, 12:04 am, Paul Furman wrote:

I suspect that the IS is somehow throwing the image off.

So does this backfocus issue happen with all lenses?

No, and it only appears sporadically with the 300 f/4L. That's why I
suspect another problem such as complications caused by the IS
mechanism.

On Mar 7, 12:16 am, "Annika1980" wrote:
On Mar 7, 12:04 am, Paul Furman wrote:



I suspect that the IS is somehow throwing the image off.

So does this backfocus issue happen with all lenses?

No, and it only appears sporadically with the 300 f/4L. That's why I
suspect another problem such as complications caused by the IS
mechanism.

It's a good possibility that the 300 f/4L needs to be calibrated by a
technician.
Helen

Paul Furman wrote:
Annika1980 wrote:

My previous two tests of the 300 f/4L IS showed some peculiarities
that warranted further investigation. Either the lens was very soft
or else it wasn't focusing properly.
So today I did a quickie test, this time with a brick wall at about a
45-degree angle from the camera. This way I could see if perhaps it
was a focusing (or a backfocus) problem. Once again, I used the
tripod and mirror lockup to go for the sharpest image. The IS was
turned off since the lens was mounted on a tripod.

Here is the result:
http://www.pbase.com/bret/image/75308794/original

The red square is the center focal point used by the AF system. That
is where proper focus should be attained. However, as you can see,
this image has a backfocus problem. The area to the left of the
square (the part of the wall that is farther away) is in focus while
the area to the right is not. Further testing when I got home from
closer distances didn't show this problem.

I suspect that the IS is somehow throwing the image off.

So does this backfocus issue happen with all lenses? I seem to recall
that it's something that can be factory adjusted in the camera body but
maybe the lens can effect it too.
snip
There is an adjustment in the body, but it affects all lenses fitted to
the camera. If it's only one lens giving trouble, then it's the lens,
not the body. Bret is on the right track I think.

Colin D.

--
Posted via a free Usenet account from http://www.teranews.com

"Colin_D" wrote in message
.. .
: There is an adjustment in the body, but it affects all lenses fitted to
: the camera. If it's only one lens giving trouble, then it's the lens,
: not the body. Bret is on the right track I think.
:
: Colin D.
:
: --
:
Maybe when he dropped the camera something went astray?

On Mar 7, 3:08 am, "Douglas" wrote:
"Colin_D" wrote in message

.. .
: There is an adjustment in the body, but it affects all lenses fitted to
: the camera. If it's only one lens giving trouble, then it's the lens,
: not the body. Bret is on the right track I think.
:
: Colin D.
:
: --
:
Maybe when he dropped the camera something went astray?

Douglas,
Maybe you need to read the all posts and not just one. Your sniping
at Bret is proving that you still have sourgrapes with Bret. He
responded to Paul with information on the 400 f/5.6 and how sharp it
was with the same test at the same time.

Time to end the bickering and mud slinging gentlemen.


Draco



Getting even isn't good enough.

On Mar 7, 3:08 am, "Douglas" wrote:
:
Maybe when he dropped the camera something went astray?

And oddly, it only goes astray with that lens in certain conditions.

Which begs the famous question, "How do it know?"

Annika1980 wrote:
...
So today I did a quickie test, this time with a brick wall at about a
45-degree angle from the camera.
...

Focusing on a plane "at about a 45-degree angle from the camera" does not
produce a meaningful test of the AF system, as it has already been explained
repeatedly to all those "testers" who were trying to test their AF by focusing
on rulers at an angle. The target has to be parallel to the camera's film plane.

I see you focused on a chipped area in the wall. Did you do that just to give
the AF something better to grab onto, or were you actually trying to find a spot
that's parallel to the camera's film plane? In any case, that chipped spot is
too small.

Get a good piece of cardboard and make some contrasty markings on it. Attach it
to your wall so that it is parallel to the film plane of the camera. Focus on it
and make sure it covers the entire AF sensor area (which is somewhat larger than
the red indicator area in the viewfinder). Then take a shot and examine the
focus on wall.

"Draco" wrote in message
ups.com...
: On Mar 7, 3:08 am, "Douglas" wrote:
: "Colin_D" wrote in message
:
: .. .
: : There is an adjustment in the body, but it affects all lenses fitted
to
: : the camera. If it's only one lens giving trouble, then it's the lens,
: : not the body. Bret is on the right track I think.
: :
: : Colin D.
: :
: : --
: :
: Maybe when he dropped the camera something went astray?
:
: Douglas,
: Maybe you need to read the all posts and not just one. Your sniping
: at Bret is proving that you still have sourgrapes with Bret. He
: responded to Paul with information on the 400 f/5.6 and how sharp it
: was with the same test at the same time.
:
: Time to end the bickering and mud slinging gentlemen.
:
:
: Draco
:
:
:
: Getting even isn't good enough.
:
-----------------
In case you don't understand a 20D's functions Draco... The on-board
computers react to lenses attached to them and make adjustments to many
normally automatically done settings. One of these is to alter the 'scene'
functions of speed and aperture bias based on lens information alone.

Sure, shooting in RAW mode bypasses much of this but not all of it.
Rebooting the camera after attaching a lens with it's own on-board computer
generally resets both the camera and the lens. My assumption that all might
not be well with his particular 20D after it had been dropped is gained from
my own experiences with those cameras.

Time to end the bickering? Surely Draco, you need to look at more than your
navel before making inflammatory statement like that... Or would you rather
see a situation where neither of us replier to the other's posts with
relevant information? That would be narrow minded, wouldn't it?

Douglas