Beware: is your zoom parfocal?

[jean bernier]

--
Jean Bernier

Just discovered to my dismay that my 70-200 VR is not parfocal. Been using it for months almost always on AF and got sharp results al the time even wide open.

Recently, I chose manual focus for a job as the subject was stationary. Went full zoom to get a better and easier focus, then zoomed back to 70 mm to take the shots (f:8). Focus was off. It focused BEHIND the subject, not by a great amount, but enough to almost ruin the shots, even with a rather small aperture. Is this correctly called "back focusing"?

I dug out the manual (who would study a lens manual?) and found this phrase on page 18: "First turn the zoom ring until the desired composition is framed in the viewfinder before focusing" Yeah sure, great advice. For idiots like me, there should have be something like: Warning! Focusing at a given focal lenth does not garantee correct focus at another focal lenth or plainly: "this zoom lens is not parfocal". In the form of a yellow sticker stuck on the front element.
Sorry for the whining.

Bad habits from using my AF 35-70 2:8 for so many years without ever seeing it behaving this nasty.
Oh well, you have been warned.
Anybody been observing this? Search did not come up with anything yet.
JB

 

[DougJones]

Hi Jean,

Yeah, since the advent of autofocus cameras many years ago most zoom lenses are no longer parfocal so the old zoom in to focus method is useless. Back in my old days of Pentax Spotmatics, film movie cameras and even early video cameras with manual focus modes, I used the technique all the time. Welcome to today's cheaper lenses.
--
Doug Jones

 

[wing88]

lately I use more primes ) so no worries there )
I just love my 200/1.8 but thanks for the tip although
I do not intend to buy the 70-200 ..

what are your MF plans doing ? I get a 1DSII in a few weeks
and have ordered a ZD under certain restrictions .. the
pricing on the ZD looks agressive .. from what I have been
told they want to go below the 1DSII at least here in oz ..
but then again I believe it when I see the invoice ... I will
try the 1DSII but very likely will end up using the ZD for
the studio ... I have another 2 people now working with me
and I get to the stage where I need to decide on more backup
equipment ... which way are you going ? MF ? or S3 ..

Id like to look at the S3 but I just can not see myself waiting
on the camera all the time .. just have to see how fast it writes
the raw files ... 1.7 f/s is ok as long as it is continuous for
certain tasks ... with the buffer of 3 RAWS and write times of
10 seconds it will surely not make it in my bag .. whats you take ?

anyway .. good to see ya around .. and try that 200/2 someday ..

I am waiting for my test sample but am not really too keen .. its a bit of a waste on an 1.5 crop camera ... wish Nikon did a decent 1.3..
may be a Kodak afterall ...

I am really into high ISO available light at the moment ... the MKII
has boosted that interest a great deal .. especially with portraits ...

did you do anything else on the Valeo ?

regards
gmd

--
Jean Bernier

Just discovered to my dismay that my 70-200 VR is not parfocal.
Been using it for months almost always on AF and got sharp results
al the time even wide open.
Recently, I chose manual focus for a job as the subject was
stationary. Went full zoom to get a better and easier focus, then
zoomed back to 70 mm to take the shots (f:8). Focus was off. It
focused BEHIND the subject, not by a great amount, but enough to
almost ruin the shots, even with a rather small aperture. Is this
correctly called "back focusing"?
I dug out the manual (who would study a lens manual?) and found
this phrase on page 18: "First turn the zoom ring until the
desired composition is framed in the viewfinder before focusing"
Yeah sure, great advice. For idiots like me, there should have be
something like: Warning! Focusing at a given focal lenth does not
garantee correct focus at another focal lenth or plainly: "this
zoom lens is not parfocal". In the form of a yellow sticker stuck
on the front element.
Sorry for the whining.
Bad habits from using my AF 35-70 2:8 for so many years without
ever seeing it behaving this nasty.
Oh well, you have been warned.
Anybody been observing this? Search did not come up with anything
yet.
JB

--
People who quote others have nothing to say - GMD

 

[Paulo Abreu]

Hi Jean,

This is a big surprise for me. My 80-200f2.8 ED doesn't change focus with zoom, actually Nikon even recomends on the instruction sheet to zoom in to 200mm, focus and then zoom out for a more precise focus (mostly on manual but can be done also with focus lock in AF mode).

Regards

--
Jean Bernier

Just discovered to my dismay that my 70-200 VR is not parfocal.
Been using it for months almost always on AF and got sharp results
al the time even wide open.
Recently, I chose manual focus for a job as the subject was
stationary. Went full zoom to get a better and easier focus, then
zoomed back to 70 mm to take the shots (f:8). Focus was off. It
focused BEHIND the subject, not by a great amount, but enough to
almost ruin the shots, even with a rather small aperture. Is this
correctly called "back focusing"?
I dug out the manual (who would study a lens manual?) and found
this phrase on page 18: "First turn the zoom ring until the
desired composition is framed in the viewfinder before focusing"
Yeah sure, great advice. For idiots like me, there should have be
something like: Warning! Focusing at a given focal lenth does not
garantee correct focus at another focal lenth or plainly: "this
zoom lens is not parfocal". In the form of a yellow sticker stuck
on the front element.
Sorry for the whining.
Bad habits from using my AF 35-70 2:8 for so many years without
ever seeing it behaving this nasty.
Oh well, you have been warned.
Anybody been observing this? Search did not come up with anything
yet.
JB

--
Paulo Abreu,

'It is not worthy to make a video of your life - just keep the best moments in pictures!'

 

[wing88]

have learned something ... never tried it with mine ..
it's one of my favourite portrait lenses ... have to give
it a try ..
thanks for that
gmd

This is a big surprise for me. My 80-200f2.8 ED doesn't change
focus with zoom, actually Nikon even recomends on the instruction
sheet to zoom in to 200mm, focus and then zoom out for a more
precise focus (mostly on manual but can be done also with focus
lock in AF mode).

Regards

--
Jean Bernier

Just discovered to my dismay that my 70-200 VR is not parfocal.
Been using it for months almost always on AF and got sharp results
al the time even wide open.
Recently, I chose manual focus for a job as the subject was
stationary. Went full zoom to get a better and easier focus, then
zoomed back to 70 mm to take the shots (f:8). Focus was off. It
focused BEHIND the subject, not by a great amount, but enough to
almost ruin the shots, even with a rather small aperture. Is this
correctly called "back focusing"?
I dug out the manual (who would study a lens manual?) and found
this phrase on page 18: "First turn the zoom ring until the
desired composition is framed in the viewfinder before focusing"
Yeah sure, great advice. For idiots like me, there should have be
something like: Warning! Focusing at a given focal lenth does not
garantee correct focus at another focal lenth or plainly: "this
zoom lens is not parfocal". In the form of a yellow sticker stuck
on the front element.
Sorry for the whining.
Bad habits from using my AF 35-70 2:8 for so many years without
ever seeing it behaving this nasty.
Oh well, you have been warned.
Anybody been observing this? Search did not come up with anything
yet.
JB

--
Paulo Abreu,

'It is not worthy to make a video of your life - just keep the best
moments in pictures!'

--
People who quote others have nothing to say - GMD

 

[jean bernier]

lately I use more primes ) so no worries there )
I just love my 200/1.8 but thanks for the tip although
I do not intend to buy the 70-200 ..

Thought you had it, like every other lens nikon made. I put it on the test bench in the studio: at ten feet/three meters distance, at 70mm the zoom focuses 3 inches/8mm behind the 200mm focus point. This may not seem like a lot especially at f:8, but it definately kills the sharpness we use to get from the S2. Two bodies were used for the test...confirmed. With AE and AF, zoom sins are usually pardoned by the camera: variable aperture, variable focus or "un-parfocality" are corrected in auto modes. Beware when shooting in manual.

The shot was printed on canvas for a presentation. Size 4x6 feet.This medium happily is blurry in itself so I saved my a&% this time again.

what are your MF plans doing ? I get a 1DSII in a few weeks
and have ordered a ZD under certain restrictions .. the
pricing on the ZD looks agressive .. from what I have been
told they want to go below the 1DSII at least here in oz ..
but then again I believe it when I see the invoice ... I will
try the 1DSII but very likely will end up using the ZD for
the studio ... I have another 2 people now working with me
and I get to the stage where I need to decide on more backup
equipment ... which way are you going ? MF ? or S3 ..

ZD: cheaper than the 1dsmkII? Hope you're right. This would be a big surprise to me, but one is allowed to dream. Just offer me a MF with 22mPix, good color and three lenses and I'm set...at mkII price, I'm buying. The mkII, well I would truly like to see samples done with top-performing primes. I'm arranging for a hands-on test as soon as the camera arrives. But I would prefer a ZD if it's worth anything.

Id like to look at the S3 but I just can not see myself waiting
on the camera all the time .. just have to see how fast it writes
the raw files ... 1.7 f/s is ok as long as it is continuous for
certain tasks ... with the buffer of 3 RAWS and write times of
10 seconds it will surely not make it in my bag .. whats you take ?

The S3 is a big deception as far as my expectations are met: I want a big chip and I want it now (but can't bite the bullet). DR is controlled when shooting by using fill-flash or reflectors when out of the studio. Others do landscape, architecture where fixing the lighting is impossible. They could benefit from more DR but could probably EX-process two versions of their files at different ISO's , mix them and get mostly the same results. So who needs the S3? I guess those who do the very least processing, that is if the JPEG's benefit truly from the additional DR. Humm...... sincerily hope reviews will be raving about the image quality... could be the nicest back-up camera to have around.

anyway .. good to see ya around .. and try that 200/2 someday ..
I am waiting for my test sample but am not really too keen .. its a
bit of a waste on an 1.5 crop camera ... wish Nikon did a decent
1.3..
may be a Kodak afterall ...

I am really into high ISO available light at the moment ... the MKII
has boosted that interest a great deal .. especially with portraits

Is that you I read about making 7k exposures on a polo outing? My, you will have to hire someone just to pick the best shots out of the lot. Going through 200 exposures seems like a lot to me...

...

did you do anything else on the Valeo ?

That's been a one-time hands-on opportunity. Too much money, a bit clumsy arrangement. But the definition is there.

regards

gmd

same to ya mate...

 

[wing88]

7k files yes with around 3000 RAW + JPG 500 S2 RAWS
and 500 MKI JPG only .. so all in all around 4K images
which will melt down to around 2500-3000 ..
They are all pretty good .. no touching necessary ..
am loading them .. the problem is I am on dial up
and it takes 4-5 days to load to my server in Canada ...
faster to send a disk ) .. which I will in future because
I am loading hires files ... we are working on a viewer
which allows zoomed viewing of hi res images without being
able to pinch them .. they are not loaded as a whole so its
hard to get them out of the system .. impossible for the
average person anyway .. and then I will sell hires stuff
online with onliune payment ... currently I sell the disk
with Mid res JPG and keep the HIRES for print for myself ..

did put a preview up because quite a few people have emailed
me already for the pics ...

http://www.perthdigital.com/album/polox/preview.html

have also a new marketing strategy .. for each print I will
give away a T-shirt and for the larger ones a Polo shirt ..
will stir the local pot of event photogs here ...

I have decided to go commercial now ... not just as a hobby
and will use a few unusual marketing concepts for that ...
the tshirt thing is one ...

I just love the sports stuff .. and I love the sequences which
I get from the MKII ... Let me know what you think of the
1DSMKII .. I will test it also with my 200/1.8 .. it's my
reference lens now ...

have fun
gmd

lately I use more primes ) so no worries there )
I just love my 200/1.8 but thanks for the tip although
I do not intend to buy the 70-200 ..

Thought you had it, like every other lens nikon made. I put it on
the test bench in the studio: at ten feet/three meters distance, at
70mm the zoom focuses 3 inches/8mm behind the 200mm focus point.
This may not seem like a lot especially at f:8, but it definately
kills the sharpness we use to get from the S2. Two bodies were
used for the test...confirmed. With AE and AF, zoom sins are
usually pardoned by the camera: variable aperture, variable focus
or "un-parfocality" are corrected in auto modes. Beware when
shooting in manual.
The shot was printed on canvas for a presentation. Size 4x6
feet.This medium happily is blurry in itself so I saved my a&% this
time again.

what are your MF plans doing ? I get a 1DSII in a few weeks
and have ordered a ZD under certain restrictions .. the
pricing on the ZD looks agressive .. from what I have been
told they want to go below the 1DSII at least here in oz ..
but then again I believe it when I see the invoice ... I will
try the 1DSII but very likely will end up using the ZD for
the studio ... I have another 2 people now working with me
and I get to the stage where I need to decide on more backup
equipment ... which way are you going ? MF ? or S3 ..

ZD: cheaper than the 1dsmkII? Hope you're right. This would be a
big surprise to me, but one is allowed to dream. Just offer me a
MF with 22mPix, good color and three lenses and I'm set...at mkII
price, I'm buying. The mkII, well I would truly like to see
samples done with top-performing primes. I'm arranging for a
hands-on test as soon as the camera arrives. But I would prefer a
ZD if it's worth anything.

Id like to look at the S3 but I just can not see myself waiting
on the camera all the time .. just have to see how fast it writes
the raw files ... 1.7 f/s is ok as long as it is continuous for
certain tasks ... with the buffer of 3 RAWS and write times of
10 seconds it will surely not make it in my bag .. whats you take ?

The S3 is a big deception as far as my expectations are met: I want a big chip and I want it now (but can't bite the bullet). DR is controlled when shooting by using fill-flash or reflectors when out of the studio. Others do landscape, architecture where fixing the lighting is impossible. They could benefit from more DR but could probably EX-process two versions of their files at different ISO's , mix them and get mostly the same results. So who needs the S3? I guess those who do the very least processing, that is if the JPEG's benefit truly from the additional DR. Humm...... sincerily hope reviews will be raving about the image quality... could be the nicest back-up camera to have around.

anyway .. good to see ya around .. and try that 200/2 someday ..
I am waiting for my test sample but am not really too keen .. its a
bit of a waste on an 1.5 crop camera ... wish Nikon did a decent
1.3..
may be a Kodak afterall ...

I am really into high ISO available light at the moment ... the MKII
has boosted that interest a great deal .. especially with portraits

Is that you I read about making 7k exposures on a polo outing? My,
you will have to hire someone just to pick the best shots out of
the lot. Going through 200 exposures seems like a lot to me...

...

did you do anything else on the Valeo ?

That's been a one-time hands-on opportunity. Too much money, a bit
clumsy arrangement. But the definition is there.

regards

gmd

same to ya mate...

--
People who quote others have nothing to say - GMD

 

[Skipper494]

--
Jean Bernier

Just discovered to my dismay that my 70-200 VR is not parfocal.
Been using it for months almost always on AF and got sharp results
al the time even wide open.

You've just discovered that things rarely get better! The build quality of many of today's zooms would not have been tolerated twenty years ago, when most were parfocal and manual was the norm.

On a trip to the UK in 1981, we visited a model farm and ordered a cream tea in their restaurant. They asked if we would mind waiting a few minutes, as they had only just milked the cows. Try that in your supermarket!
Skipper494.

 

[Chris Copeland]

The 12-24 DX is certainly not "parfocal", which is a new word for me. The focus change when zooming is very noticable, I thought they were all like that.

Always learning something, occasionally for the first time!

Chris

 

[Classic Man]

Hi Jean,
Yeah, since the advent of autofocus cameras many years ago most
zoom lenses are no longer parfocal so the old zoom in to focus
method is useless. Back in my old days of Pentax Spotmatics, film
movie cameras and even early video cameras with manual focus modes,
I used the technique all the time. Welcome to today's cheaper
lenses.

well, the 70-200VR is not "cheap", I had expected better.

My tests indicate the AF-S 18-70DX is indeed parfocal; ie doesn't lose focus as you zoom.

This is a deal breaker for me. Are you sure you don't own a defective copy?

 

[Derrel87035]

This is an interesting topic. One thing to always remember--when you shoot photos through glass, always,always,always stop the lens down with the depth of field plunger and observe the focus thru the finder,and also take the focusing ring and move it back and forth and visually inspect the focus you get when the lens is stopped down to the actual shooting aperture,and when the effect of distorting glass is factored in.

Focus shift can occur when the changing from wide-open aperture or what used to be called the "viewing aperture" to stopped down or "taking" aperture. So, focus may shift with aperture.

Focus may shift with focal length as well. Focus is usally more-accurately ascertained by the human eye and brain at longer focal lengths,like at the 200 end of an 80-200 zoom, or at 105 on a 28-105mm zoom. BUT, if the lens has significant focus shift as it is zoomed, focusing at the longer end and zooming might cause a big,big focusing problem when focusing by eye.With modern AF systems, I think they make it a lot less neccessary to design lenses that are totally parfocal--IF users allow AF to focus the lenses!

My 70-200VR seems to be good....I have no focusing problems with it. But the 28-200G Nikkor, which I actually LIKE a lot, has significant focus shift as it is moved through its focusing range. It's significant enough that I can actually SEE the focus shifting as I zoom....but I still like the lens.

 

[jean bernier]

Hi Jean,
Yeah, since the advent of autofocus cameras many years ago most
zoom lenses are no longer parfocal so the old zoom in to focus
method is useless. Back in my old days of Pentax Spotmatics, film
movie cameras and even early video cameras with manual focus modes,
I used the technique all the time. Welcome to today's cheaper
lenses.

well, the 70-200VR is not "cheap", I had expected better.

My tests indicate the AF-S 18-70DX is indeed parfocal; ie doesn't
lose focus as you zoom.

This is a deal breaker for me. Are you sure you don't own a
defective copy?

--
Jean Bernier

Why would it be a defective copy? The instructions are "clear" about this: "frame the picture first and THEN focus" BTW the phenomenon cannot be observed when using AF lock: green focus- confirm light will stay on forever as long as you keep the shutter half-depressed (single advance) no matter how out-of-focus what you're aiming at is. Focus manually at 200mm and zoom back then you will see the green light turn off around 105 mm to 70mm.

But your question is worth investigating further.

I presume designing a lens is the art of balancing compromises, and parfocality was sacrificed in order to make the lens shine in other areas. I do not think it's a cheap lens. It's very sharp, VR works great, aperture is constant, focus fast and silent, great feel (and weight!) But it has this imperfection. I must remember to refocus each time I touch the zoom ring. Should be easy as I kicked myself enough I can hardly sit.

 

[jean bernier]

This is an interesting topic. One thing to always remember--when
you shoot photos through glass, always,always,always stop the lens
down with the depth of field plunger and observe the focus thru the
finder,and also take the focusing ring and move it back and forth
and visually inspect the focus you get when the lens is stopped
down to the actual shooting aperture,and when the effect of
distorting glass is factored in.
Focus shift can occur when the changing from wide-open aperture or
what used to be called the "viewing aperture" to stopped down or
"taking" aperture. So, focus may shift with aperture.

Read a long time ago about this one. Never observed it though.

Focus may shift with focal length as well. Focus is usally
more-accurately ascertained by the human eye and brain at longer
focal lengths,like at the 200 end of an 80-200 zoom, or at 105 on a
28-105mm zoom. BUT, if the lens has significant focus shift as it
is zoomed, focusing at the longer end and zooming might cause a
big,big focusing problem when focusing by eye.With modern AF
systems, I think they make it a lot less neccessary to design
lenses that are totally parfocal--IF users allow AF to focus the
lenses!

And correct these small discrepancies in aperture and focus, that's right.
Actually, it's kind of neat. The full manual shooter is however disadvantaged.

My 70-200VR seems to be good....I have no focusing problems with
it. But the 28-200G Nikkor, which I actually LIKE a lot, has
significant focus shift as it is moved through its focusing range.
It's significant enough that I can actually SEE the focus shifting
as I zoom....but I still like the lens.

Thanks for replying. Have you checked your 70-200 VR lately on this issue?
If it's free from shifting then I have a bad copy.
JB

--
Jean Bernier

 

[Steve Bingham]

Try this. Focus at 200mm. I would bet the rest of the range will be in focus. Focus at 70m and you run the risk of 200mm being out of focus. Focusing first at full zoom has been standard operating procedure since zooms were invented.

Steve Bingham
http://www.dustylens.com

 

[DougJones]

I knew some of my zooms were not parfocal because I'd noticed the focus shift but I rarely use manual focus with the zooms. So I tested them all and as expected, the (very) old ones were fine - my old Nikkor 80-200 f2.8 one touch and the Tamron 80-210 which I use when travelling light. The one touch is generally considered the best optics of all Nikon's 80,70-200 lenses and was made for easy manual focus and zoom - a dream to use for the zoom in, focus, and zoom to reframe quickly with one hand touch. It surprised me that the cheap Tamron was spot on parfocally but I guess it comes from the same manual focus era.

However the 2 Sigma recent design EX zooms I have are definitely not parfocal over the whole zoom range but start to shift off from about halfway back towards the wide end. But the extra depth of field by going wide may compensate anyway if you are not wide open.

Hey Wing - thats Gold Wing I presume? Old CX500 man myself.
I mean hey GMD
or should it now be - Hey Gerhard
You finally let the cat out of the bag with the Polo preview page

Doug Jones
(now northern) Perth

People who quote others have nothing to say - GMD

Can I quote you on that?

have learned something ... never tried it with mine ..
it's one of my favourite portrait lenses ... have to give
it a try ..
thanks for that
gmd

This is a big surprise for me. My 80-200f2.8 ED doesn't change
focus with zoom, actually Nikon even recomends on the instruction
sheet to zoom in to 200mm, focus and then zoom out for a more
precise focus (mostly on manual but can be done also with focus
lock in AF mode).

Regards

 

[wing88]

was never hiding .. gmd is used everywhere ..
the polo people have a different business card
so I use only mi first name and not my initials ..
everyone refers to me as gmd .. as long as I can think
since Phil banned me a couple of years back as you might
remember I use my old numberplate from my bike ...
yes its a lead wing ..

have you heard OZ pricing of the S3 ... my shop says
that they have been hinted 3.300 .. but that could be
a discounted price .. not 100% yet ..

c ya around
gmd

Hey Wing - thats Gold Wing I presume? Old CX500 man myself.
I mean hey GMD
or should it now be - Hey Gerhard
You finally let the cat out of the bag with the Polo preview page

Doug Jones
(now northern) Perth

People who quote others have nothing to say - GMD

Can I quote you on that?

have learned something ... never tried it with mine ..
it's one of my favourite portrait lenses ... have to give
it a try ..
thanks for that
gmd

This is a big surprise for me. My 80-200f2.8 ED doesn't change
focus with zoom, actually Nikon even recomends on the instruction
sheet to zoom in to 200mm, focus and then zoom out for a more
precise focus (mostly on manual but can be done also with focus
lock in AF mode).

Regards

--
People who quote others have nothing to say - GMD

 

[DougJones]

was never hiding .. gmd is used everywhere ..
the polo people have a different business card
so I use only mi first name and not my initials ..
everyone refers to me as gmd .. as long as I can think
since Phil banned me a couple of years back as you might
remember I use my old numberplate from my bike ...
yes its a lead wing ..

The 88 reminds me very much of the flat 4 configuration. Was this intentional or does it have a more deeply hidden meaning?

have you heard OZ pricing of the S3 ... my shop says
that they have been hinted 3.300 .. but that could be
a discounted price .. not 100% yet ..

With moving to Perth, etc, etc, not enough time to scratch myself let alone looking for local prices for S3s. Must admit that the increased DR concept interests me as one passion is landscapes, especially harsh light contrast ones. Lately I seem to be doing the old 2 level EX conversion mix a lot to pull in extra DR in "difficult" shots.
--
Doug Jones

 

[jean bernier]

Try this. Focus at 200mm. I would bet the rest of the range will be
in focus. Focus at 70m and you run the risk of 200mm being out of
focus. Focusing first at full zoom has been standard operating
procedure since zooms were invented.

Steve Bingham
http://www.dustylens.com

--
Jean Bernier

That's exactly what got me out-of-focus images: focusing at 200mm in manual and shooting at 70mm. The green light turns itself off around 105mm all the way to 70 mm when zooming back. Did not notice (or care) you know how inconvenient is the green light: hard to lock on and far from the viewfinder image.

 

[Steve Bingham]

As I typically use the lens/camera in continuous focus mode with action and prefocus every shot otherwise I hadn't noticed. I will give it a try. I am a little surprised.

Steve Bingham
http://www.dustylens.com

 

[Derrel87035]

jean,

I just tried your suggested test five times,indoors. I focused the 70-200 on the S2 in manual,using the green electronic rangefiner dot to give me the "green dot". I zoomed back using the ring and noticed that the dot stayed lighted,and the viewfinder's image appeared to stay in good,sharp focus.

I mention that I did this test indoors because I focused on a few different things, but all were flat-plane objects: china cabinet, ceiling-mounted light unit,softbox edge,TV set edge,coffee maker. Indoors, with basivally flat objects, as the field of view of the focus bracket increased as I zoomed shorter from 200 to 135 to 105 to 70mm, the focus bracket's ever-widening area still fell on "flat" objects. It's possible,especially outdoors or in less-controlled aiming, that the shorter focal lengths "see" enough of a wider angle that the focus indicator (the green dot) could be giving a signal off of a slightly closer or farther target.

I often focus manually by zooming to 200, then zooming back and re-composing at 90,125,145 mm,whatever....I think I would have seen focus shift. But then again, I shoot a lot at f/8 and around there,so I do have some DOF to cover boo-boo. I do not shoot a "lot" at f/2.8 to f/3.3 with the 70-200, but I have done some lower-light stuff at such apertures and have not noticed a problem with focus shift. My sample seems pretty much right on the money....but the little 28-200G that I like so much as a knockaround lens--WOW! it has almost as much focus shift as my old beater 35-70 AF Nikkor, the cheap,cheesy f/3.3~4.5 model from the mid-1980's!
--
Happy Shooting!
Derrel