Re: Focusing IR

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

On 13 Apr 98 at 8:47, Marc Turner wrote:

>      
>      >I must disagree here....:-))
>      >One can very well look through #89B, at least for composition with 
>      >wide angles. And since AF systems don't seem to have much trouble 
>      >with focusing through an IR-filter (I even get focus locks through a 
>      >very opaque #87C!), it might even work with teles (fast f2.8 lenses 
>      >preferred of course, to squeeze out every bit of light for 
>      >composition).
>      
>      This makes me wonder - if the autofocus system works through a 
>      visually opaque IR filter, then it seems like it would be trying to 
>      autofocus using only the IR end of the spectrum.  It seems like this 
>      would automatically compensate for the different focus point and 
>      handle the IR focus correction automatically!  Anyone have any insight 
>      on this?  Does it work, or if not then why?  I have a lot of pain (as 
>      I suppose others do) with these autofocus lenses that have no IR focus 
>      marks.  Mostly I end up shooting with smaller aperture for DOF.

Both Andy Davidhazy and I did some testing when this was discussed on
the PhotoForum list many moons ago. His results were confirming your
theory, mine were inconclusive/mixed....some of my lenses focused in
the right direction, others in the wrong direction. On one hand this
seems bizar, OTOH it is impossible to predict what the tiny
AF-sensor optics themselves do in terms of IR-focus. They could 
compensate, or aggrevate the problem....regardless how well-corrected 
the lens is....even a perfect APO would then still be subject to an 
error in either direction....8-))

I guess the only safe method is testing a specific camera with a 
specific lens....



- -- 
Bye,

Willem-Jan Markerink


      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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