RE:Relationship between IR focal length adjustment and focal length.

[cameron shaw <cameron@xxxxxxxxxxxxx>]

> >> Another question:  "Is there any relationship between the focal length of a
> >> lens and the need to perform an IR focusing adjustment?"  I ask this
> >because
> >> the brochure that came with my Sigma 70-210 f/2.8D AF APO zoom lens  says
> >> that IR focusing adjustment is only needed at the short (70mm) end of the
> >> lens.
> >
> George responded by writting:
> >The shorter the focal length. the greater the depth of field.  The above
> >sounds
> >backwards to me.
> >As I normally shoot with the lens stopped down to f11-f16, depth of field
> >takes care of everything and I do not refocus.
> 
> I agree with your comments George and I rechecked the instructions that came
> with my Sigma APO 70-210 f/2.8D Nikon Mount lens. However the instructions
> still state that no IR correction is needed at the longer focal length of
> 210mm, only at the 70mm end.  I find this very interesting.
> 
> Peace Rolland Elliott
>
	I have used a Nikon zoom lens that had infrared focus correction 
adjustments marked on the lens, one for 35mm the other fot 70 mm. The 35 mm 
adjustment was three times the angular displacement of the focusing ring when 
compared to 70mm.
Cameron Shaw


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End of Infrared-Digest V0 #288
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