Focus discrepancies in Rebel XT and XTi, manual and auto focus

I have noticed the most puzzling and disturbing focus discrepancies in
both my Rebel XT and my XTi.

Please do the following tests with yours (and please report back!):

Set your camera on a tripod
Set up a scene with some sharp black-on-white writing in the center
Set your lens to auto-focus and press the shutter button half way down
until the lens focuses
Adjust your eyepiece dioptric to your eyesight
Shoot a picture at wide aperture (f2.8 will do)

Now, without disturbing anything, set your lens to manual focus
Move the focusing ring to dislodge the previous focus setting
Re-focus carefully on your viewing screen (do not re-adjust your
diopters!)
Shoot a picture at the same f-stop

Now upload your pictures to your computer and compare the two
side-by-side by enlarging the center to the maximum (try "actual
pixels" with Photoshop)

I've repeated these tests with every conceivable variation of lenses,
cameras, with glasses and without, with additional supplementary
diopter correction lenses, etcetera.

INVARIABLY, the picture done in auto-focus is right on, but the one
done manually shows a pronounced focus shift! You can readily see the
difference in sharpness between the two.

I sent the XTi to Canon and it was returned with the same problem,
after thorough "inspection and adjusting".

What is going on?

Do you see the same discrepancies in your own tests?

Rita Ä Berkowitz wrote:
Bob G wrote:

I sent the XTi to Canon and it was returned with the same problem,
after thorough "inspection and adjusting".

They didn't find a problem since this is normal.


If that's the case it's a shame. I can't really believe it. Do other
brands show the same problem?
And why is not this a problem with film auto-focus Canon SLRs? Or is
it?
And why isn't there a warning anywhere in the instruction manual?
I personally like to set my lenses to manual focus, I hate to press a
"hold" button and refocus if I don't agree with the camera's chosen
point.
That renders the Canon Rebel XT and XTi near useless to me.

BTW, I spent two hours on the phone with various Canon technicians this
afternoon.

They seem to think the problem is with my eyes, that I need to adjust
the eyepiece diopter.

Never a word that this baffling camera behavior is "normal".

That's why I'd like to know if other users are getting the same results
from the tests.

"Bob G" wrote in message
oups.com...
:I have noticed the most puzzling and disturbing focus discrepancies in
: both my Rebel XT and my XTi.
:
: Please do the following tests with yours (and please report back!):
:
: Set your camera on a tripod
: Set up a scene with some sharp black-on-white writing in the center
: Set your lens to auto-focus and press the shutter button half way down
: until the lens focuses
: Adjust your eyepiece dioptric to your eyesight
: Shoot a picture at wide aperture (f2.8 will do)
:
: Now, without disturbing anything, set your lens to manual focus
: Move the focusing ring to dislodge the previous focus setting
: Re-focus carefully on your viewing screen (do not re-adjust your
: diopters!)
: Shoot a picture at the same f-stop
:
: Now upload your pictures to your computer and compare the two
: side-by-side by enlarging the center to the maximum (try "actual
: pixels" with Photoshop)
:
: I've repeated these tests with every conceivable variation of lenses,
: cameras, with glasses and without, with additional supplementary
: diopter correction lenses, etcetera.
:
: INVARIABLY, the picture done in auto-focus is right on, but the one
: done manually shows a pronounced focus shift! You can readily see the
: difference in sharpness between the two.
:
: I sent the XTi to Canon and it was returned with the same problem,
: after thorough "inspection and adjusting".
:
: What is going on?
:
: Do you see the same discrepancies in your own tests?
:
Listen to what Rita says here Bob. Your camera is an Auto Focus camera. If
you can't get a sharp pic on AF, it has a back focus error. This is not a
Professional level camera and you can't expect it to perform like one. If
you truly do want to manually focus this camera, get a manual focus screen
for it. You may have a problem then with detecting correct AF but it's all a
compromise on these cheap SLRs.

--

Australian Wedding Photography between Kempsy, NSW and Sunshine Coast.
http://www.photosbydouglas.com
Digital photos enlarged and printed on Canvas
http://canvas.photosbydouglas.com

"Bob G" wrote in message
oups.com...
BTW, I spent two hours on the phone with various Canon technicians this
afternoon.

They seem to think the problem is with my eyes, that I need to adjust
the eyepiece diopter.

Never a word that this baffling camera behavior is "normal".

That's why I'd like to know if other users are getting the same results
from the tests.

My problem was DEFINITELY fixed with the diopter. Shooting through a
viewfinder only, I can never trust my own judgement as to whether I've
manually focused correctly or not.

I would not be a good subject for your proposed test...

dwight

"dwight" wrote in message
...
"Bob G" wrote in message
oups.com...
BTW, I spent two hours on the phone with various Canon technicians this
afternoon.

They seem to think the problem is with my eyes, that I need to adjust
the eyepiece diopter.

Never a word that this baffling camera behavior is "normal".

That's why I'd like to know if other users are getting the same results
from the tests.

My problem was DEFINITELY fixed with the diopter.

Which brings up a question I've been meaning to ask for some time. How
accurate is the built-in diopter adjustment on a low end Canon? I have a
Rebel XT and I'm -.5 in one eye and -.75 in the other.

And if I start doing more manual focus I will definitely get a Katz Eye,
I've been thinking about it for quite some time.

Greg

dwight wrote:

My problem was DEFINITELY fixed with the diopter. Shooting through a
viewfinder only, I can never trust my own judgement as to whether I've
manually focused correctly or not.

I would not be a good subject for your proposed test...

My test removes the possibility of a diopter problem.
Notice that in my test the diopter is calibrated after the camera has
autofocussed
Using manual focus then should give the exact same point of focus,
since the eyepiece has been corrected for your eyesight already.

And if I start doing more manual focus I will definitely get a Katz Eye,
I've been thinking about it for quite some time.

Greg


A Katz Eye will not correct an inherent focus discrepancy, should you
have one.

These are screens whose only virtue is that they're brighter and more
contrasty and thereby facilitate focusing, PROVIDED you have a
perfectly aligned system to begin with.

On Wed, 17 Jan 2007 18:18:25 -0800, "G.T."
wrote:


"dwight" wrote in message
...
"Bob G" wrote in message
oups.com...
BTW, I spent two hours on the phone with various Canon technicians this
afternoon.

They seem to think the problem is with my eyes, that I need to adjust
the eyepiece diopter.

Never a word that this baffling camera behavior is "normal".

That's why I'd like to know if other users are getting the same results
from the tests.

My problem was DEFINITELY fixed with the diopter.

Which brings up a question I've been meaning to ask for some time. How
accurate is the built-in diopter adjustment on a low end Canon? I have a
Rebel XT and I'm -.5 in one eye and -.75 in the other.

And if I start doing more manual focus I will definitely get a Katz Eye,
I've been thinking about it for quite some time.

Greg



with the camera turned off, can you see the focusing squared clearly?
Watch them while you adjust the diopter correction.

"Bob G" wrote in message
ups.com...


And if I start doing more manual focus I will definitely get a Katz Eye,
I've been thinking about it for quite some time.

Greg


A Katz Eye will not correct an inherent focus discrepancy, should you
have one.

These are screens whose only virtue is that they're brighter and more
contrasty and thereby facilitate focusing, PROVIDED you have a
perfectly aligned system to begin with.


Yes, but as others have pointed out the lack of a decent focusing screen on
current autofocus DSLRs is probably your problem with manual focus, not an
inherent problem with your Rebels.

Greg