Infrared film is not all the same. Kodak used to make "real" IR film
with response to the far IR. This had a gap in the spectral response
around green. This film was used with far IR filters that are opaque to
visible light. These filters will not work on near IR film or films with
extended red response like the late, lamented, Technical Pan because
their IR response does not extend much below the visible. I think this
is true of most of the modern IR films, they are not true IR film. True
IR film is somewhat perishable, the deep IR sensitivity makes the film
vulnerable to fogging from heat. Kodak made it in small amounts. True IR
film was commonly used for aerial survey work and some scientific
photography. With a true IR filter it will discriminate between
deciduous and evergreen trees, the former appearing light and the later
dark. It was also used occasionally for "day for night" B&W photography
because skies appeared completely black. Some cameras leaked enough IR
to be useless with it. Some bellows, for instance, were semi-transparent
to IR.
Lenses are generally corrected for two colors, both in the visible
spectrum. Often they have a shift in focus between visible and IR. Some
lenses in focus mounts have a correction, usually for near IR. There
were special lenses corrected for far IR, mostly for scientific
photography and aerial survey work. Also, some types of optical glass do
not transmit well for far IR so lenses intended for this work must have
suitable glass types.
I am digging this stuff from memory but think its all correct.
The film Carlos used is probably sensitive to near but not far IR.
It will still show the difference in reflectivity of different types of
plants.
On 8/26/2017 12:23 PM, Don Williams wrote:
At 07:00 AM 8/26/2017, Carlos wrote, in part:
Images taken last week end in the farm with the Rolleiflex 2,8 FPlanar, Kodak Tmax 400, Romek PQ7 1+3 developer:
The Rollei red filter:
https://flic.kr/p/XPtV65
I think I used some "IR" film in the long-ago and the pictures were similar to the red filter picture.
Also, I have some vague recollection that cameras used to have a second focus index mark for IR. Do I remember correctly?
DAW~
