Backfocus - different lenses give different focal planes?

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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.

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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

 

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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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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

 

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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

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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

 

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Ken Ellis wrote:

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

Confusion on my part...

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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

 

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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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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 +++

 

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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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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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