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

[Rolland Rainbow Elliott <X93ELLIOTT1@xxxxxxxxx>]

>Rolland Rainbow Elliott wrote:
>>
>> 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
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End of Infrared-Digest V0 #276
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