Backfocus - different lenses give different focal planes?

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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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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

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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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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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