Re: Nikon - Dot or no dot ?

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

On  8 Jun 99 at 3:37, danzig@xxxxxxxxxxxxxx wrote:

> >This topic, especially the macro-aspect, could keep optical 
> >engineers talking for hours, without any hard rules for your 
> >specific lens, so ... from WJ
> 
> 
> Actually WJ, I BEG to differ ;) 

Begging won't get you far with me....:-))
 
> Focus on your subject, the stripe represents visible light focus.  
> Move the focussing ring by the distance that lies between dot and 
> the stripe ( works better if you actually mark the area on the 
> focussing ring directly in front of the stripe, so you know exactly 
> how much to move the focussing ring  ) Move it that distance 
> towards the dot . That is the correct focus for IR. 

Mmm....not really, at least not 100% accuracy with 100% certainty.
The problem is that the IR dot is a compromise, affected/fouled by 
several factors, like: 
- film sensitivity (absolute limit in nm; note that Konica Hexar has 
two AF correction settings, one for 750nm, one for 850nm)
- filter used for final image (relative limit in nm)
- filter used for composing (correction applied by human eye)
- object distance (correction can differ with distance, the more complex 
the design, the harder to estimate)
- focal length (note that some APO zooms are only true APO at one end 
of the zoom range, confirming this theory)

And all this is what caused Leica to remove the IR-dot on their 
modern lenses....they felt that it gave a false sense of accuracy.





 

--                 
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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