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Backfocus - different lenses give different focal planes?

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Anonymous
March 14, 2005 6:17:18 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

I tested the backfocus of the Maxxum 7D. Links below.

For the 100mm lens, all seems well.

For the 50mm lens, it seems to focus camera side of the target.

Any ideas?

Cheers,
Alan



Tested:

100 f/2.8 macro against two targets on an oblique

50 f/1.7 against one target on an oblique.

--Max7D + lenses above
--Tripiod
--cable release
--mirror lockup
--angle finder at 2X for manual focus
--A/S off
--ambient light

100mm results: perfect

50mm results: appears to focus slightly close to the camera v. target
(2 samples of each from bunch follow).

100mm f/2.8:
http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg

50mm f/1.7:
http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg

note: on the paper target, the focus point is marked with bars with
ambient light from window.

ruler target, the focus point is the 15 cm line with
ambient light from softbox on farside of ruler.

Comments on why the 100mm shows in-focus at the selected focus point and
the 50mm does not?

Cheers,
Alan
Anonymous
March 14, 2005 10:30:07 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:
> I understand that the 1/3 - 2/3 rule doesn't apply at very close
> distances.
>
> Colin

The 1/3 2/3 rule is in fact pretty much useless. When focusing far
enough away infinity come into focus and so clearly the ratio of front
to back is not 1/3 2/3. On the other hand when the DOF is very small
you will get close to the same amount in focus in front and behind,
this happens when the DOF is small compared to the distance to the
object, which does tend to be when you are focusing in close with the
lens wide open. It should be noted that the ratio changes just with a
change in the f-number.

Scott
Anonymous
March 14, 2005 11:56:33 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

>100mm f/2.8:
>http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG

Although the target is in focus, it's not centrally focused. There is
more behind it in focus than there is in front of it. At first glance,
this appears to be very slight back focus.

>http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg

Same with this one, although less obvious. The third mm marking in
front of the focus point is more blurry than the third mm marking
behind the focus point.

Having said all that, it's what you'd expect. As you get closer to the
hyper focal distance, it becomes apparent that the share of DOF that
appears in front of the focal point vs behind gets smaller. What I'm
not certain of, (and I'd need a DOF calculator that works in mm)
should this be visible at such a short distance from the lens?

>50mm f/1.7:
>http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
>http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg
>
>note: on the paper target, the focus point is marked with bars with
>ambient light from window.
>
> ruler target, the focus point is the 15 cm line with
>ambient light from softbox on farside of ruler.
>
>Comments on why the 100mm shows in-focus at the selected focus point and
>the 50mm does not?

You cocked it up somehow?

Dunno, they look bad.

--
Owamanga!
http://www.pbase.com/owamanga
Related resources
Anonymous
March 14, 2005 11:56:34 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Owamanga wrote:
>>100mm f/2.8:
>>http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
>
>
> Although the target is in focus, it's not centrally focused. There is
> more behind it in focus than there is in front of it. At first glance,
> this appears to be very slight back focus.

eh? In this one, there is more in focus further away. This follows the
usualt 2/3 in focus beyond the plane, 1/3 focus camera side of the plane.

>
>
>>http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg
>
>
> Same with this one, although less obvious. The third mm marking in
> front of the focus point is more blurry than the third mm marking
> behind the focus point.

Same as above (3 of the marks beyond, and almost the 4th; 2 of the marks
this side) are in focus, again the 1/3 // 2/3 property.

>
> Having said all that, it's what you'd expect. As you get closer to the
> hyper focal distance, it becomes apparent that the share of DOF that
> appears in front of the focal point vs behind gets smaller. What I'm
> not certain of, (and I'd need a DOF calculator that works in mm)
> should this be visible at such a short distance from the lens?

Sure... as I said, approx. 2/3 beyond, and 1/3 on the camera side is the
usual DOF property.


>
>
>>50mm f/1.7:
>>http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
>>http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg
>
> You cocked it up somehow?

No. At least I don't think so. I repeated the tests on the 50mm over a
dozen times. On the 100 it only took about 6 frames to get 6 consistent
results (refocusing each time).

On the 50, the focus was always on the nearside (by about 2cm per the
ruler (ignoring the oblique)). I was extremently careful with the focus
of the VF diopter (0), then the angle finder focus (on the VF markings),
then on the the target ... plus cable release, plus mirror lockup. Now
I have to do these on my 20, 300 and 28-70 and 80-200 ...

Cheers,
Alan.


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 15, 2005 6:46:00 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

"Alan Browne" <alan.browne@FreeLunchVideotron.ca> wrote in message
news:ijmZd.54117$WM1.885437@wagner.videotron.net...
>
> I tested the backfocus of the Maxxum 7D. Links below.
>
> For the 100mm lens, all seems well.
>
> For the 50mm lens, it seems to focus camera side of the target.
>
> Any ideas?
>
> Cheers,
> Alan
>
>
>
> Tested:
>
> 100 f/2.8 macro against two targets on an oblique
>
> 50 f/1.7 against one target on an oblique.
>
> --Max7D + lenses above
> --Tripiod
> --cable release
> --mirror lockup
> --angle finder at 2X for manual focus
> --A/S off
> --ambient light
>
> 100mm results: perfect
>
> 50mm results: appears to focus slightly close to the camera v. target
> (2 samples of each from bunch follow).
>
> 100mm f/2.8:
> http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
> http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg
>
> 50mm f/1.7:
> http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
> http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg
>
> note: on the paper target, the focus point is marked with bars with
> ambient light from window.
>
> ruler target, the focus point is the 15 cm line with
> ambient light from softbox on farside of ruler.
>
> Comments on why the 100mm shows in-focus at the selected focus point and
> the 50mm does not?
>
> Cheers,
> Alan
How's this? Lenses, except macro, have curved fields and therefore curved
DOFs. You tested one in the center and one on the far left edge. I'd try
scribing an arc and shooting that to see if it's in, edge to edge. Or maybe
that's not it at all. Bob Hickey
Anonymous
March 15, 2005 6:46:01 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Bob Hickey wrote:

> How's this? Lenses, except macro, have curved fields and therefore curved
> DOFs. You tested one in the center and one on the far left edge. I'd try
> scribing an arc and shooting that to see if it's in, edge to edge. Or maybe
> that's not it at all. Bob Hickey

All focus was at dead center of the lens. The crops I provided were
hastilly done, not symetrical top-bottom or right left. Cropped in the
interest of your bandwidth.

But keep throwing ideas.

Cheers,
Alan


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
March 15, 2005 11:33:02 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

"
>>http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
>
> Although the target is in focus, it's not centrally focused. There is
> more behind it in focus than there is in front of it. At first glance,
> this appears to be very slight back focus.
>
------------------------
Discovering that KM's idea of "specifications" and autofocus accuracy is a
little different to what the rest of the world expect, are we? This comment
from KM Australia (the horses mouth) might shed some light on the matter:
"These are not really professional cameras so you can't expect them to have
the same focus tolerance as a pro camera". The fact Konica-Minolta don't
make a Professional DSLR probably allows them to use such comments to slide
out from under a real concern a potential customer had.

The discussion was about the God awful autofocus of their flagship DSLR no
less. It seems that a "non professional" autofocus camera (according to KM)
can have a considerable tolerance in how and on what it will focus. Another
Gem from the horse's mouth: "Anywhere inside the depth of field should be
considered accurate focus".

If KM Canada's attitude to fixing your camera's focus problem (presuming
they can - which I doubt) is anything like KM Australia's attitude to having
had 7 attempts in 5 months to fix some of their gear in my shop and, when it
was finally decided they couldn't fix it, refund my $7500 purchase price...

You could save yourself a lot future disappointment by taking it back to the
store you bought it from and swapping it for a Canon DSLR. Trade your
Minolta lenses in on a 24 ~ 70 f2.8 and get on with your photography. If you
can't afford the glass, put up with not too shabby kit lenses until you can.
You might complain about the cost but you'll never complain about the
quality of your pictures.

At the end of the line is the overwhelming fact that fewer and fewer
Professional Photographers are choosing anything else but Canon. Sure the
'D' series has had some adverse publicity about lock ups and focus issues
but considering the number sold, they are few and far between. I've looked
at every option I can to avoid Canon cameras in the past 6 months. There are
none, Canon rules.

Konica Minolta cameras are in the same class as Sigma cameras. I have a
Sigma and it gets me out of a bind now and then but I'd no sooner use it as
my working camera than I would fly to the moon.

I started my Photographic career with Minolta cameras. I shot thousands of
frames with SRT 101s. They got dropped, abused, wet and still took good
pictures. After the fiasco I had with Minolta over their gear and the
bullshit they fed me about their camera, I will never own another Minolta
product as long as my bum points to the ground. Am I biased? Yes and with
good reason. You never took my advise last time, do yourself a favour and
take it now.

Douglas
Anonymous
March 15, 2005 11:33:03 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

On Tue, 15 Mar 2005 08:33:02 +1000, Douglas <decipleofeos@yahoo.com> wrote:
>
> The discussion was about the God awful autofocus of their flagship DSLR no
> less. It seems that a "non professional" autofocus camera (according to KM)
> can have a considerable tolerance in how and on what it will focus. Another
> Gem from the horse's mouth: "Anywhere inside the depth of field should be
> considered accurate focus".

Now I'm picturing a dial on the back with which you can tell the
camera how large you intend to print the image you're about to
capture -- and from how far a distance you intend it to be viewed.

--
Ben Rosengart (212) 741-4400 x215
Sometimes it only makes sense to focus our attention on those
questions that are equal parts trivial and intriguing.
--Josh Micah Marshall
Anonymous
March 15, 2005 11:33:03 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Douglas wrote:

> "
>
>>>http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
>>
>>Although the target is in focus, it's not centrally focused. There is
>>more behind it in focus than there is in front of it. At first glance,
>>this appears to be very slight back focus.
>>
>
> ------------------------
> Discovering that KM's idea of "specifications" and autofocus accuracy is a
> little different to what the rest of the world expect, are we? This comment
> from KM Australia (the horses mouth) might shed some light on the matter:
> "These are not really professional cameras so you can't expect them to have
> the same focus tolerance as a pro camera". The fact Konica-Minolta don't
> make a Professional DSLR probably allows them to use such comments to slide
> out from under a real concern a potential customer had.

1.. I thought I was plonked Dougie?

2.. Just about every camera out there has had some mention of back focus
issues, including the Canon 10D and 20D.

"With the 10D and even with the 20D there are reports of people having
problems with "back focus" (i.e. the camera focusing behind the
subject)." http://www.photo.net/equipment/canon/20D/

3.. the 100 mm f/2.8 focuses just fine... as this image (and many others
show).
http://www.aliasimages.com/images/KM7D/ColBill_FD.U.jpg
(3000 x 2000 pixels, 2.5 MB).

4. Owamanga's comment above, is in fact wrong. The image is correctly
focussed with the right about of focuse on each side of the focus plane
(1/3 on near side, 2/3 on far side).

>
> The discussion was about the God awful autofocus of their flagship DSLR no
> less. It seems that a "non professional" autofocus camera (according to KM)

I can assure you that not only is Maxxum 7D AF accurate, but it is fast
as hell (with the 100 f/2.8 and 28-70 f/2.8). OTOH, I don't use MF very
much in any case.

>
> If KM Canada's attitude to fixing your camera's focus problem (presuming
> they can - which I doubt) is anything like KM Australia's attitude to having
> had 7 attempts in 5 months to fix some of their gear in my shop and, when it
> was finally decided they couldn't fix it, refund my $7500 purchase price...

The only Minolta item I've ever had break, was repaired for no charge
.... despite the warranty having expired. (Replaced tube and other
components in a high end flash (5400HS)).

>
> You could save yourself a lot future disappointment by taking it back to the
> store you bought it from and swapping it for a Canon DSLR. Trade your
> Minolta lenses in on a 24 ~ 70 f2.8 and get on with your photography. If you
> can't afford the glass, put up with not too shabby kit lenses until you can.
> You might complain about the cost but you'll never complain about the
> quality of your pictures.

I have a 300 f/2.8, 100 f/2.8, 50 f/1.7, 20 f/2.8, 28-70 f/2.8 and
80-200 f/2.8. All Minolta. Three of those lenses are sharper or as
sharp as their Canon counterpart.

> At the end of the line is the overwhelming fact that fewer and fewer
> Professional Photographers are choosing anything else but Canon. Sure the
> 'D' series has had some adverse publicity about lock ups and focus issues
> but considering the number sold, they are few and far between. I've looked
> at every option I can to avoid Canon cameras in the past 6 months. There are
> none, Canon rules.

If I were starting from scratch, it would most likely be Canon. But
that's not the case...

> Konica Minolta cameras are in the same class as Sigma cameras. I have a

Bullshit.

> Sigma and it gets me out of a bind now and then but I'd no sooner use it as
> my working camera than I would fly to the moon.

http://www.aliasimages.com/images/KM7D/ColBill_FD.U.jpg
Maxxum 7D with 100 f/2.8 macro. Be sure to zoom in, this is 3000 x 2000
pixels.

Sigma? Forget it. 10D or 20D with a Canon 100 f/2.8, almost.
10D or 20D with a Tamron 90 f/2.8, certainly.

>
> I started my Photographic career with Minolta cameras. I shot thousands of
> frames with SRT 101s. They got dropped, abused, wet and still took good
> pictures. After the fiasco I had with Minolta over their gear and the
> bullshit they fed me about their camera, I will never own another Minolta
> product as long as my bum points to the ground. Am I biased? Yes and with
> good reason. You never took my advise last time, do yourself a favour and
> take it now.

What advice? I've had Minolta's since the ealry 90's. I've had
consumer lenses, of which 1 out of 3 was actually very, very good, and
one that is quite good. Now, all of my lenses except one are pro, and
the only one that is not *great* is the 300 f/2.8. It is weak in flare,
but reasonably sharp.

I'm not sure who pisses you off more, Dougie, me or K-M, but you should
just keep posting your rants, they show you to be exactly what you are.

Cheers,
Alan


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 15, 2005 11:33:04 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:

> I can assure you that not only is Maxxum 7D AF accurate, but it is fast
> as hell (with the 100 f/2.8 and 28-70 f/2.8). OTOH, I don't use MF very
> much in any case.

Should have read: "OTOH, I don't use _AF_ very much in any case."

Cheers,
Alan
--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 15, 2005 2:43:32 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
>
> I tested the backfocus of the Maxxum 7D. Links below.
>
> For the 100mm lens, all seems well.
>
> For the 50mm lens, it seems to focus camera side of the target.
>
> Any ideas?
>
> Cheers,
> Alan
>
> Tested:
>
> 100 f/2.8 macro against two targets on an oblique
>
> 50 f/1.7 against one target on an oblique.
>
> --Max7D + lenses above
> --Tripiod
> --cable release
> --mirror lockup
> --angle finder at 2X for manual focus
> --A/S off
> --ambient light
>
> 100mm results: perfect
>
> 50mm results: appears to focus slightly close to the camera v. target
> (2 samples of each from bunch follow).
>
> 100mm f/2.8:
> http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
> http://www.aliasimages.com/images/KM7D/BF/PICT0721a.jpg
>
> 50mm f/1.7:
> http://www.aliasimages.com/images/KM7D/BF/PICT0717b.JPG
> http://www.aliasimages.com/images/KM7D/BF/PICT0718b.jpg
>
> note: on the paper target, the focus point is marked with bars with
> ambient light from window.
>
> ruler target, the focus point is the 15 cm line with
> ambient light from softbox on farside of ruler.
>
> Comments on why the 100mm shows in-focus at the selected focus point and
> the 50mm does not?
>
> Cheers,
> Alan

An interesting test, Alan. Looking at the 100mm lens results, I think
the focus is actually slightly behind the 15cm mark - about 15.1, and
the paper target shows a similar result - but pretty close.

The 50mm is definitely short-focused, but there may be reasons for the
discrepancy between the lenses. First, you say you focused manually
with a 2x angle finder. How did you focus the angle finder? Depending
somewhat on the type of viewfinder screen design, it's possible to have
the angle finder slightly misfocused, and to compensate by mis-focusing
the camera lens. Question: did you refocus the angle finder between the
two lens tests? If so, I think that would explain the discrepancy
between the lenses.

I have 'calibrated' my angle finder for my eye by removing the camera
lens from the camera and pointing the camera at a bright light source -
an incandescent bulb, or a well-lit white target - to get a really
bright diffuse light on the VF screen, and then critically focusing the
angle finder on the screen pattern. Then I marked the angle finder
focus ring so I can set it up any time I need it without further ado.

The second point is to be constantly aware of the VF screen pattern
while focusing the camera. Some VF screens are partially transparent to
improve image brightness, and it's possible to focus on an aerial image
that is not at the plane of the VF screen, since even after critically
setting up the angle finder, your eye can shift its focus. Personally,
I think this semi-transparent screen business is what's behind the
complaints about auto-focus cameras being difficult to focus manually.
Manual-focus cameras had/have VF screens that are less transparent, and
make it easier for the eye to see the on-screen image rather than an
aerial image.

Compare focusing a camera with a VF screen to focusing a telescope with
no screen, just an aerial image. There is no fixed point of focus, it
varies with one's eyesight. In a camera we need a fixed screen to focus
on, so the focus plane coincides with the film/sensor plane.

Colin
Anonymous
March 15, 2005 2:43:33 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

> An interesting test, Alan. Looking at the 100mm lens results, I think
> the focus is actually slightly behind the 15cm mark - about 15.1, and
> the paper target shows a similar result - but pretty close.

I dropped the paper target as, as you can see, it is not sufficiently
flat, nor the contrast sufficiently high for this test, esp. with the
100mm lens.

It was so easy to focus the 100 that I may have been careless in
accuracy. The 50 was difficult to focus, but over a dozen+ tests, the
result was consistently on the camera side of the reference.

> The 50mm is definitely short-focused, but there may be reasons for the
> discrepancy between the lenses. First, you say you focused manually
> with a 2x angle finder. How did you focus the angle finder? Depending

Very carefully using the viewfinder marks as the focus points. I
refocussed it a few times as I sometimes removed the angle finder for
other reasons. So it was always first to be checked before focussing on
the target.

> somewhat on the type of viewfinder screen design, it's possible to have
> the angle finder slightly misfocused, and to compensate by mis-focusing
> the camera lens. Question: did you refocus the angle finder between the
> two lens tests? If so, I think that would explain the discrepancy
> between the lenses.
>
> I have 'calibrated' my angle finder for my eye by removing the camera
> lens from the camera and pointing the camera at a bright light source -
> an incandescent bulb, or a well-lit white target - to get a really
> bright diffuse light on the VF screen, and then critically focusing the
> angle finder on the screen pattern. Then I marked the angle finder
> focus ring so I can set it up any time I need it without further ado.

Similar technique, but you don't need to remove the lens, just defocus
it against a white backround and the VF marks are very clear.

> The second point is to be constantly aware of the VF screen pattern
> while focusing the camera. Some VF screens are partially transparent to
> improve image brightness, and it's possible to focus on an aerial image
> that is not at the plane of the VF screen, since even after critically
> setting up the angle finder, your eye can shift its focus. Personally,
> I think this semi-transparent screen business is what's behind the
> complaints about auto-focus cameras being difficult to focus manually.
> Manual-focus cameras had/have VF screens that are less transparent, and
> make it easier for the eye to see the on-screen image rather than an
> aerial image.

AF cameras ARE difficult to focus manually for the reason you state
above plus the lack of drag in the focus mechanism when it is unclutched.

>
> Compare focusing a camera with a VF screen to focusing a telescope with
> no screen, just an aerial image. There is no fixed point of focus, it
> varies with one's eyesight. In a camera we need a fixed screen to focus
> on, so the focus plane coincides with the film/sensor plane.

I have to use the diopter adjustment. My left eye has no astigmatism,
and is about -1.75 (and is my dominant in any case). I can get that
sharp with the diopter or with the angle finder (with the angle finder I
need to put the VF diopter to 0, of course).

Thanks for your feedback, Colin.

I'll be testing other lenses tonight or tomorrow and we'll see how it goes.

Cheers,
Alan.

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 15, 2005 2:46:10 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
>
> Owamanga wrote:
> >>100mm f/2.8:
> >>http://www.aliasimages.com/images/KM7D/BF/PICT0710a.JPG
> >
> >
> > Although the target is in focus, it's not centrally focused. There is
> > more behind it in focus than there is in front of it. At first glance,
> > this appears to be very slight back focus.
>
> eh? In this one, there is more in focus further away. This follows the
> usualt 2/3 in focus beyond the plane, 1/3 focus camera side of the plane.
>
I understand that the 1/3 - 2/3 rule doesn't apply at very close
distances.

Colin
Anonymous
March 15, 2005 2:46:11 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

> I understand that the 1/3 - 2/3 rule doesn't apply at very close
> distances.

Never heard that... but I hear new things all the time! :-)

Got any references?

Cheers,
Alan

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Anonymous
March 15, 2005 4:45:16 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

On Mon, 14 Mar 2005 17:54:09 -0500, Alan Browne
<alan.browne@freelunchVideotron.ca> wrote:

>4. Owamanga's comment above, is in fact wrong. The image is correctly
>focussed with the right about of focuse on each side of the focus plane
>(1/3 on near side, 2/3 on far side).

No it isn't, read Colin's math.

:-)

The words Owamanga and wrong just don't sit well together.

--
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http://www.pbase.com/owamanga
Anonymous
March 15, 2005 7:07:49 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
> Graphing it turns out to be impractical (eg: I'm lazy), but it is
clear
> that Owamanga / Scott (and anyone else who wants to crow) are right.

I will admit that I did some plots on MathCad just to be sure.

Scott
Anonymous
March 15, 2005 7:57:31 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
>
> Colin D wrote:
>
> > I understand that the 1/3 - 2/3 rule doesn't apply at very close
> > distances.
>
> Never heard that... but I hear new things all the time! :-)
>
> Got any references?
>
> Cheers,
> Alan

Yep. Some simple math should do it. All dimensions in millimetres.

First, we need the hyperfocal distance for a given lens. Let's say we
will use a 50mm lens set at f/8. The HD for such a lens at that stop is
given by F^2/(f:number x diameter of chosen circle of confusion) (coc).
Coc generally accepted for 35mm film is 0.03mm, so:

50^2/(8 x 0.03) = 10,416.6 mm - say 10,000mm

OK. The near distance for acceptable dof is given by (Hyperfocal dist. x
focus distance)/(hyperfocal dist. + focus distance),
and the far distance for acceptable dof is given by (Hyperfocal dist. x
focus distance)/(hyperfocal dist. - focus distance).

Let's set the lens focus distance at the hyperfocal distance. Then we
have:

Far distance: (10^4 x 10^4)/(10^4 - 10^4), which is 10^8/0 = infinity
Near distance: 10^8/(10^4 + 10^4), which is 10^8/(2 x 10^4), which is
5,000mm. So, our dof in this case is 5 metres to infinity. The near
depth is 5 metres, and the far depth is infinite, which is what we
expect when focused on the HD of 10 metres.

Now, let's do it again for a close distance, say about 10 inches, or
250mm, and we'll switch to metres to keep the zeroes under control.

Near distance is (10 x 0.25)/(10 + 0.25), = 2.5/10.25, = 0.244 metres.
Far distance is (10 x 0.25)/(10 - 0.25), = 2.5/9.75, = 0.256 metres.

Since we focused on 0.25 metres, the near dof is 0.25 - 0.244, =
0.06mm, and the far dof is 0.256 - 0.25, = 0.06mm.

The dof is equal either side of focus.

In fact, the near/far dof ratio varies with distance from infinite at
the HD to practically 1:1 at close distances, and is more or less 1:2
only at some intermediate distance.

Congrats if you managed to wade through that lot, Alan!

Colin
Anonymous
March 15, 2005 7:57:32 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

> Congrats if you managed to wade through that lot, Alan!

The math doesn't bother me ... but it's 10 past midnight so I'll do a
spreadsheet tomorrow and look at the front-DOF/BACK-DOF ratios as
function of distance.

I assumed that the 1/3 - 2/3 was one of those unchanging properties.

Thanks!

Cheers,
Alan.

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Anonymous
March 15, 2005 7:57:32 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

On Tue, 15 Mar 2005 16:57:31 +1300, Colin D
<ColinD@killspam.127.0.0.1> wrote:

>
>
>Alan Browne wrote:
>>
>> Colin D wrote:
>>
>> > I understand that the 1/3 - 2/3 rule doesn't apply at very close
>> > distances.
>>
>> Never heard that... but I hear new things all the time! :-)
>>
>> Got any references?

<massive math that proves the point snipped>

Colin, I came to the same conclusions in 20 seconds using a DOF
calculator, but couldn't get exact numbers for the close-ups, thanks
for that.

:-)

--
Owamanga!
http://www.pbase.com/owamanga
Anonymous
March 15, 2005 9:13:14 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

<snipped>
> varies with one's eyesight. In a camera we need a fixed screen to focus
> on, so the focus plane coincides with the film/sensor plane.

And on it goes. I've discovered that the ruler I have has a slight bow
in it. So I've printed a new target and I'll be positioning it on a
piece of glass, and clamped flat in place.

With the 28-70 @ 70mm, it appears ever so slightly to be focusing a
little close to the camera, but so little as to be crazy (a real world
object in focus at the same distance looks crisp).

The other thing is repeated focusing like this, critically, tires the
eye out quickly (holding breath Ox-dep).

Further, it's very hard/subjective to focus at 50mm and less on a target
this fine at a severe oblique (about 20 degrees elevation).

More to follow.

Cheers,
Alan.

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
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-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 16, 2005 1:46:12 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Owamanga wrote:
>
> On Tue, 15 Mar 2005 16:57:31 +1300, Colin D
> <ColinD@killspam.127.0.0.1> wrote:
>
> >
> >
> >Alan Browne wrote:
> >>
> >> Colin D wrote:
> >>
> >> > I understand that the 1/3 - 2/3 rule doesn't apply at very close
> >> > distances.
> >>
> >> Never heard that... but I hear new things all the time! :-)
> >>
> >> Got any references?
>
> <massive math that proves the point snipped>
>
> Colin, I came to the same conclusions in 20 seconds using a DOF
> calculator, but couldn't get exact numbers for the close-ups, thanks
> for that.
>
> :-)
>
> --
> Owamanga!
> http://www.pbase.com/owamanga

Aaahh, yes, sometimes math - like a dentist - is the only way to go.
{:-)
Alan is going to graph it as a function in Excel, I think. I did think
of that, but it was easier to just use two examples, certainly for the
NG.

Colin
Anonymous
March 16, 2005 1:46:13 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

>
> Owamanga wrote:
>
>>On Tue, 15 Mar 2005 16:57:31 +1300, Colin D
>><ColinD@killspam.127.0.0.1> wrote:
>>
>>
>>>
>>>Alan Browne wrote:
>>>
>>>>Colin D wrote:
>>>>
>>>>
>>>>>I understand that the 1/3 - 2/3 rule doesn't apply at very close
>>>>>distances.
>>>>
>>>>Never heard that... but I hear new things all the time! :-)
>>>>
>>>>Got any references?
>>
>><massive math that proves the point snipped>
>>
>>Colin, I came to the same conclusions in 20 seconds using a DOF
>>calculator, but couldn't get exact numbers for the close-ups, thanks
>>for that.
>>
>>:-)
>>
>>--
>>Owamanga!
>>http://www.pbase.com/owamanga
>
>
> Aaahh, yes, sometimes math - like a dentist - is the only way to go.
> {:-)
> Alan is going to graph it as a function in Excel, I think. I did think
> of that, but it was easier to just use two examples, certainly for the

Graphing it turns out to be impractical (eg: I'm lazy), but it is clear
that Owamanga / Scott (and anyone else who wants to crow) are right.

DOF ratios 0.025

mm
FL 100 100 100 100 100 100 100
DIST 200 400 800 1600 3200 6400 12800
A 11.0 11.0 11.0 11.0 11.0 11.0 11.0
s-f 100 300 700 1500 3100 6300 12700
h 36364 36364 36364 36364 36364 36364 36364
Dn 199 397 785 1537 2949 5455 9487
Df 201 403 816 1669 3498 7741 19670
Dof 1.1 6.6 30.8 132.2 549.6 2286.2 10182.9
Nearside -0.5 -3.3 -15.1 -63.4 -251.4 -945.1 -3313.2
Farside 0.6 3.3 15.7 68.8 298.2 1341.2 6869.6

Near % 50% 50% 49% 48% 46% 41% 33%
Far% 50% 50% 51% 52% 54% 59% 67%



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-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 16, 2005 3:40:21 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
>
> Colin D wrote:
>
> >
> > Owamanga wrote:
> >
> >>On Tue, 15 Mar 2005 16:57:31 +1300, Colin D
> >><ColinD@killspam.127.0.0.1> wrote:
> >>
> >>
> >>>
> >>>Alan Browne wrote:
> >>>
> >>>>Colin D wrote:
> >>>>
> >>>>
> >>>>>I understand that the 1/3 - 2/3 rule doesn't apply at very close
> >>>>>distances.
> >>>>
> >>>>Never heard that... but I hear new things all the time! :-)
> >>>>
> >>>>Got any references?
> >>
> >><massive math that proves the point snipped>
> >>
> >>Colin, I came to the same conclusions in 20 seconds using a DOF
> >>calculator, but couldn't get exact numbers for the close-ups, thanks
> >>for that.
> >>
> >>:-)
> >>
> >>--
> >>Owamanga!
> >>http://www.pbase.com/owamanga
> >
> >
> > Aaahh, yes, sometimes math - like a dentist - is the only way to go.
> > {:-)
> > Alan is going to graph it as a function in Excel, I think. I did think
> > of that, but it was easier to just use two examples, certainly for the
>
> Graphing it turns out to be impractical (eg: I'm lazy), but it is clear
> that Owamanga / Scott (and anyone else who wants to crow) are right.
>
> DOF ratios 0.025
>
> mm
> FL 100 100 100 100 100 100 100
> DIST 200 400 800 1600 3200 6400 12800
> A 11.0 11.0 11.0 11.0 11.0 11.0 11.0
> s-f 100 300 700 1500 3100 6300 12700
> h 36364 36364 36364 36364 36364 36364 36364
> Dn 199 397 785 1537 2949 5455 9487
> Df 201 403 816 1669 3498 7741 19670
> Dof 1.1 6.6 30.8 132.2 549.6 2286.2 10182.9
> Nearside -0.5 -3.3 -15.1 -63.4 -251.4 -945.1 -3313.2
> Farside 0.6 3.3 15.7 68.8 298.2 1341.2 6869.6
>
> Near % 50% 50% 49% 48% 46% 41% 33%
> Far% 50% 50% 51% 52% 54% 59% 67%
>
> --
> -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
> -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
> -- [SI] gallery & rulz: http://www.pbase.com/shootin
> -- e-meil: there's no such thing as a FreeLunch.

Yep, I did think of writing a quick proggie to tabulate a selection of
ratios, and that's what you did, so I won't do it now.

As for crowing, only idiots crow. I prefer to think you've added to
your store of knowledge.

Regards,

Colin
Anonymous
March 16, 2005 3:40:22 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

>
> Yep, I did think of writing a quick proggie to tabulate a selection of
> ratios, and that's what you did, so I won't do it now.
>
> As for crowing, only idiots crow. I prefer to think you've added to
> your store of knowledge.

I do enough crowing that payback is always a bitch in waiting!

But indeed, "accepted wisdom" has been shattered by the glaring truth in
this case.

Cheers,
Alan

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
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-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 16, 2005 10:41:50 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

On Mon, 14 Mar 2005 15:17:18 -0500, Alan Browne
<alan.browne@FreeLunchVideotron.ca> wrote:

>
>I tested the backfocus of the Maxxum 7D. Links below.
>
>For the 100mm lens, all seems well.
>
>For the 50mm lens, it seems to focus camera side of the target.
>etc...

Great thread. Particular thanks to Colin and Alan for the education.

ta

Ken
Anonymous
March 17, 2005 12:43:39 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
>
> Colin D wrote:
>
> <snipped>
> > varies with one's eyesight. In a camera we need a fixed screen to focus
> > on, so the focus plane coincides with the film/sensor plane.
>
> And on it goes. I've discovered that the ruler I have has a slight bow
> in it. So I've printed a new target and I'll be positioning it on a
> piece of glass, and clamped flat in place.
>
> With the 28-70 @ 70mm, it appears ever so slightly to be focusing a
> little close to the camera, but so little as to be crazy (a real world
> object in focus at the same distance looks crisp).
>
If the shot shows that the focus is closer to the camera, there are only
three explanations: 1. That you didn't have the VF focus right after
all, or 2. The camera is out of alignment, i.e. the lens - VF screen
distance is different than the lens - sensor distance, or 3. the lens
exhibits a focus shift when stopped down. Some lenses shift focus
slightly when stopped down, and you are then reliant on the dof
accommodating the error -which becomes noticeable when doing tests such
as you are doing.

If you are getting differing results from two lenses, then it leaves
either the VF focus, or a different focus shift with different lenses
when stopped down as the source of the error.

You may be able to eliminate focus shift by focusing at the stop you are
going to take the shot at, but not all cameras will allow you to do
this. You might have to hold the dof button down - could be a bit of a
fiddly job.

Sometimes it's better to not probe too deeply into these things - you
end up disappointed, when in actual fact the device is performing to
spec. Good luck.

Colin
Anonymous
March 17, 2005 12:43:40 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:

>>With the 28-70 @ 70mm, it appears ever so slightly to be focusing a
>>little close to the camera, but so little as to be crazy (a real world
>>object in focus at the same distance looks crisp).
>>
>
> If the shot shows that the focus is closer to the camera, there are only
> three explanations: 1. That you didn't have the VF focus right after
> all, or 2. The camera is out of alignment, i.e. the lens - VF screen
> distance is different than the lens - sensor distance, or 3. the lens
> exhibits a focus shift when stopped down. Some lenses shift focus
> slightly when stopped down, and you are then reliant on the dof
> accommodating the error -which becomes noticeable when doing tests such
> as you are doing.

First of all, 'ever so sligthly' is less than a mm from over a meter
away on a difficult to focus target (oblique at a thin printed line), so
the error is just as likely to be human as anything else.

All of my tests, to date are wide open. And breaking news is that if I
place a 3-D object right at the focus line, and then focus on the
junction 'tween the object and the paper, they're looking bang on. I
think a lot of the error is duw to just how hard it is to focus a wide
angle lens (28-80 at 28 and 50mm FL) on a thin line that is about a
meter away.

>
> If you are getting differing results from two lenses, then it leaves
> either the VF focus, or a different focus shift with different lenses
> when stopped down as the source of the error.

This is strange... if there is an error in the VF or sensor position,
then all lenses should show the bias. After all what is focused on the
VF is ... well , focused on the VF...


>
> You may be able to eliminate focus shift by focusing at the stop you are
> going to take the shot at, but not all cameras will allow you to do
> this. You might have to hold the dof button down - could be a bit of a
> fiddly job.

If there is an error 'tween VF and backplane, then DOF can take care of
it, but not in all sits. I have other cats to skin today, but testing
shall resume.

>
> Sometimes it's better to not probe too deeply into these things - you
> end up disappointed, when in actual fact the device is performing to
> spec.

I have been having my doubts about all this process just raising
non-concerns. IAC, the shots of the meter and the Columbian 500 note
were closeup and certainly in sharp focus. DOF on those was on the
order of 1 cm.

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-- e-meil: there's no such thing as a FreeLunch.
Anonymous
March 17, 2005 12:43:40 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

In rec.photo.equipment.35mm Colin D <ColinD@killspam.127.0.0.1> wrote:
> >
> If the shot shows that the focus is closer to the camera, there are only
> three explanations: 1. That you didn't have the VF focus right after
> all, or 2. The camera is out of alignment, i.e. the lens - VF screen
> distance is different than the lens - sensor distance, or 3. the lens
> exhibits a focus shift when stopped down. Some lenses shift focus
> slightly when stopped down, and you are then reliant on the dof
> accommodating the error -which becomes noticeable when doing tests such
> as you are doing.

There are also -
* lens have slightly different focal lengths to those marked on
the lens body (so 70mm might be 68.9 or similar).
* the actual focal distance at any lens focal length (whetever a
focal length setting with a zoom lens or a prime lens) will
have slightly different focal lengths depending on how far
you are focusing the lens. The one maarked really refers to
infinity focus.
* getting lens manufacuring dolerances down to where the difference
in focal lengths would be undetectable is not so easy, and its
worth more making sure differences in optical quality aren't
detectable.

>
> Colin

--
Sander

+++ Out of cheese error +++
Anonymous
March 17, 2005 12:43:41 AM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Sander Vesik wrote:
> In rec.photo.equipment.35mm Colin D <ColinD@killspam.127.0.0.1> wrote:
>
>>If the shot shows that the focus is closer to the camera, there are only
>>three explanations: 1. That you didn't have the VF focus right after
>>all, or 2. The camera is out of alignment, i.e. the lens - VF screen
>>distance is different than the lens - sensor distance, or 3. the lens
>>exhibits a focus shift when stopped down. Some lenses shift focus
>>slightly when stopped down, and you are then reliant on the dof
>>accommodating the error -which becomes noticeable when doing tests such
>>as you are doing.
>
>
> There are also -
> * lens have slightly different focal lengths to those marked on
> the lens body (so 70mm might be 68.9 or similar).

If the shot is in focus on the VF, it should be in focus on the film
(sensor) plane. So the true FL does not metter.

> * the actual focal distance at any lens focal length (whetever a
> focal length setting with a zoom lens or a prime lens) will
> have slightly different focal lengths depending on how far
> you are focusing the lens. The one maarked really refers to
> infinity focus.

See above.

> * getting lens manufacuring dolerances down to where the difference
> in focal lengths would be undetectable is not so easy, and its
> worth more making sure differences in optical quality aren't
> detectable.

See above. If it focuses on the VF, it should focus on the film plane.

IAC, as I've improved my setup and testing technique, the focus is
looking very close to dead on. It is just very hard to see the test
pattern with the wider angle lenses. At longer lengths, the error is zero.

Cheers,
Alan


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Anonymous
March 17, 2005 12:37:31 PM

Archived from groups: rec.photo.equipment.35mm,rec.photo.digital.slr-systems (More info?)

Colin D wrote:


> The program doesn't need to be installed, it will run by simply opening
> it wherever it is.

Thanks, but I'll stick to fcalc, free from www.tangentsoft.net

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!