Archived posting to the Leica Users Group, 2006/01/18
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>Henning,
>
>Thank you for your explanation. So, the compensation needed would
>be for the wavelengths beyond 900nm. Interesting.
>
>Bob
>
Actually, Bob, the compensation needs to be for the peak sensitivity
that the filter/film combination provides. This tends to be somewhere
between 720 to 870nm or so. Older lenses that are not corrected as
well for longitudinal chromatic aberration in this region have
significantly different focal lengths for these wavelengths, and
therefore the focus has to be compensated (and the lens stopped down,
if you want things sharp)!
>> >The filter controls what wavelengths get through, of course.
>>The filter >therefore can be ground to compensate the focus point
>>for those >wavelengths. What the film records should not be a
>>factor, since it can >only record what gets through to it.
>>>
>>>Bob
>>>
>>
>>All filters let through a large range of wavelengths; most IR
>>filters pass wavelengths well beyond 900nm while few films are
>>sensitive in that region. Therefore compensation differences due to
>>the emulsion matter.
>>
>>Since the main issue for the IR filters is the low cutoff to match
>>it to subject matter, film sensitivity and thus exposure time, the
>>upper end is usually left free to take advantage of whatever
>>sensitivity the film may have.
>>
>>--
>> * Henning J. Wulff
>> /|\ Wulff Photography & Design
>> /###\ mailto:henningw@archiphoto.com
>> |[ ]| http://www.archiphoto.com
>>
>>_______________________________________________
>>Leica Users Group.
>>See http://leica-users.org/mailman/listinfo/lug for more information
>>
>
>
>
>_______________________________________________
>Leica Users Group.
>See http://leica-users.org/mailman/listinfo/lug for more information
--
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com