Re: Auto-focusing and IR focus shift

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 14 Sep 96 at 22:11, Jeff Spirer wrote:

> >> Not only that, but the camera seemed to adjust the focus point for the lens
> >> with the filter in place.  I think that since the autofocus system is
> >> sensitive to IR light, then if I allow only IR light to pass through the
> >> lens, then the focus shift should be handled automatically.
> >
> >Tricky subject. I did notice the same thing about a year ago, with a
> >EOS 100/2.8 and EOS-1 and at that time I thought it didn't refocus
> >for IR, it seemed to hit the same focus point over and over again.
> >This was with a #87C btw. It has also been mentioned in FotoMagazin
> >8/95, a German mag....
> 
> 
> I fail to see how this could be considered anything except a really serious
> bug.  If the focus point changes as IR content changes, the focus point
> should change, regardless of film, with any filter that changes the IR to
> non-IR ratio.  How would the camera know the difference between Tri-X with
> a 25 filter and HIE with a 25 filter?  In other words, as filters get
> progressively darker, the focus point will shift regardless of the
> desirability of that shift.  Even a yellow filter could cause some focus
> shift.

You overlook the fact that the image taking optics are not the only 
optics involved....the AF-sensors have their own small optics as well.
So film focus is not neccessarely the same as sensor focus.
Suppose the refractive index of those differ from the image 
taking lens....any deviation, in any direction is possible IMO.
It would be rather unlikely that  film focus and sensor focus match 
for every film/lens combination.

And what about APO lenses? Those are supposed to be error free in IR
and UV (although the definition of APO is ment for the visible
spectrum, and Andy once pointed out that it would not be impossible
that the focus deviation sky rockets outside that range, more than
normal corrected lenses (I have some nice ASCII art about this from
him on my homepage btw). Yet the sensor optics could still add some 
IR focus shift, spoiling the perfect matching focus of the lens.

Like Andy and I agreed on in our last conversation....this subject 
raises more questions than it solves....8-))
 
--
Bye,

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      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 24