Re: Auto-focusing and IR focus shift

[jeffs@xxxxxxxxxxxxx (Jeff Spirer)]

At 11:23 AM 9/14/96, Willem-Jan Markerink wrote:
>On 13 Sep 96 at 21:17, Daniel Cardish 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.




Jeff
jeffs@xxxxxxxxxxxxx
http://www.hyperreal.com/axiom/



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Topic No. 22