[original from EOS-list, CC to IR-list)
On 17 May 02 at 10:09, [EMAIL PROTECTED] wrote:
> Hi all,
>
> Just got the 70-200 4.0 and found something missing from the manual:
> On the distance scale, there seem to be two IR correction marks: one marked
> "70", the other marked "100".
> I assume that this refers to the focal length. Is that true? I can't think
> of another reason, but if so, why is there no mark for 200mm?
>
> It's not of much importance to me since I don't shoot infrared, but I do
> like to understand the markings on the lens.
It's not uncommon for semi high-end *zoom* lenses to be partially
IR-corrected, for only part of the focal range....in this case 200mm.
Also note that even if they *could* have designed it the other way
around, it's far easier to make the focus correction at 70 or 100mm
than at 200mm.
Further note that I still hope that Canon will one day add one or
more CF's, SCF's or PCF's, to automatically adjust AF for
IR-correction....depending on film-type and filter-color....:))
Last but not least: while one can correct focus for black&white IR,
this is not possible for color-IR....b&w sees only part of the
spectrum, hence you correct focus for that particular part of the
spectrum....color-IR however sees nearly the entire spectrum
(minus-blue, as the recommended Wratten dark-yellow #12 is also
often called), so any correction for IR means an error on the
low-end visible end....only a true APO lens can tackle this problem,
or stopping down a lot (but that on itself can cause problems, since
IR (800nm infrared vs 400nm visible blue) is twice as susceptible for
diffraction effects (bending of light along the aperture blade
edges)....
--
Bye,
Willem-Jan Markerink
The desire to understand
is sometimes far less intelligent than
the inability to understand
<[EMAIL PROTECTED]>
[note: 'a-one' & 'en-el'!]
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