Infrared, TLR's and viewing lens correction diopters.

Here's one for the optician types. I am using a Mamiya TLR with 80mm and
135mm lenses for Infrared portraiture and figure work. Quite happy with
viewing and focusing without a filter and leaving the IR filter on the
taking lens, BUT wish I could get a correction diopter for the viewing lens
to make the focal length just enough longer that the planes of focus for the
unfiltered viewing lens and the filtered taking lens would match. I know it
would need to be a weak negative diopter but have no idea how to calculate
it. I am quite happy using MACO 820 IR so we could spec a frequency of 820
nm for the IR peak transmission and a cheap IR filter (perhaps a B+W deep
red?). ( assume the results would be pretty good then for Ilford SFX a 750
nm and Kodak at 900+ if I want to experiment further but your welcome to
spec out three correction diopters or just give me the math tools to do it.



--
darkroommike

----------

"Mike King" wrote in message
...
Here's one for the optician types. I am using a Mamiya TLR with 80mm and
135mm lenses for Infrared portraiture and figure work. Quite happy with
viewing and focusing without a filter and leaving the IR filter on the
taking lens, BUT wish I could get a correction diopter for the viewing
lens
to make the focal length just enough longer that the planes of focus for
the
unfiltered viewing lens and the filtered taking lens would match. [...]

The difference between the corrected lower and viewfinder is way to small to
effect framing, so you must be desiring a correction for the IR focus. Well,
don't obsess about it. Make the correction Maco suggests and be happy. It's
not that much with that film and the proper filter for that film.

jjs wrote:

"Mike King" wrote in message
...
Here's one for the optician types. I am using a Mamiya TLR with 80mm and
135mm lenses for Infrared portraiture and figure work. Quite happy with
viewing and focusing without a filter and leaving the IR filter on the
taking lens, BUT wish I could get a correction diopter for the viewing
lens
to make the focal length just enough longer that the planes of focus for
the
unfiltered viewing lens and the filtered taking lens would match. [...]

The difference between the corrected lower and viewfinder is way to small to
effect framing, so you must be desiring a correction for the IR focus. Well,
don't obsess about it. Make the correction Maco suggests and be happy. It's
not that much with that film and the proper filter for that film.

The correction depends on the lens, not the film. Many SLR lenses have a
red mark for IR focus, but for TLR lenses that isn't feasible. You could
experiment, e.g. by shooting a tape measure at full aperture, and
examining with a loupe which distance is sharp.

-- Lassi

"Lassi Hippeläinen" wrote in
message ...
jjs wrote:
[... the subject is B&W infrared ...]

The correction depends on the lens, not the film. Many SLR lenses have a
red mark for IR focus, but for TLR lenses that isn't feasible.

First, the film does make a difference. The deeper the IR, the more the
focal length is increased. B&W film is not strictly IR sensitive, so if the
author were to use a 25A rather than deep IR, he could make less of a
correction. Second, an IR mark is quite feasible on some TLRs - right on
the focusing rack. I cannot recall if the C220 or C330 had it, but it's
still possible to figure and add one.

My request was for a formula to calculate negative diopter correction lenses
to modify the focal length of the viewing lens so that the back focus would
match that of the taking lens when using IR (which has a longer back focus
than visible light).

--
darkroommike

----------
"Lassi Hippeläinen" wrote in
message ...
jjs wrote:

"Mike King" wrote in message
...
Here's one for the optician types. I am using a Mamiya TLR with 80mm
and
135mm lenses for Infrared portraiture and figure work. Quite happy
with
viewing and focusing without a filter and leaving the IR filter on the
taking lens, BUT wish I could get a correction diopter for the viewing
lens
to make the focal length just enough longer that the planes of focus
for
the
unfiltered viewing lens and the filtered taking lens would match.
[...]

The difference between the corrected lower and viewfinder is way to
small to
effect framing, so you must be desiring a correction for the IR focus.
Well,
don't obsess about it. Make the correction Maco suggests and be happy.
It's
not that much with that film and the proper filter for that film.

The correction depends on the lens, not the film. Many SLR lenses have a
red mark for IR focus, but for TLR lenses that isn't feasible. You could
experiment, e.g. by shooting a tape measure at full aperture, and
examining with a loupe which distance is sharp.

-- Lassi

try http://medfmt.8k.com/mf/diopter.html for formulas on lens focal length
changes, that said, you won't find any standard diopters that are close to
the tiny shifts needed. However, a filter typically causes a shift of
about a third its thickness, and a thin filter behind the lens might work?

quoting above URL:
new F = (F/1+DF)


whe
F is focal length in meters
D is power of diopter lens
note D can be + or - diopter value


For example, a 180mm (.18m) focal length normal lens becomes the
equivalent of a 220mm (.22m) lens when a -1 diopter lens is used.
Conversely, a +1 diopter lens generates a 150mm (.15m) equivalent lens
value.
end-quote:

hth bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

Mike King wrote:

My request was for a formula to calculate negative diopter correction lenses
to modify the focal length of the viewing lens so that the back focus would
match that of the taking lens when using IR (which has a longer back focus
than visible light).

--
darkroommike

You need to know the colour dispersion curve of the lens. Every lens
design has its own curve, and only the lens maker knows it. Therefore I
suggested testing yourself.

The correction is so small that you won't find a proper lens anyway, so
you just have to focus first and then defocus with the calibrated
amount.

-- Lassi

----------
"Lassi Hippeläinen" wrote in
message ...
jjs wrote:

"Mike King" wrote in message
...
Here's one for the optician types. I am using a Mamiya TLR with 80mm
and
135mm lenses for Infrared portraiture and figure work. Quite happy
with
viewing and focusing without a filter and leaving the IR filter on the
taking lens, BUT wish I could get a correction diopter for the viewing
lens
to make the focal length just enough longer that the planes of focus
for
the
unfiltered viewing lens and the filtered taking lens would match.
[...]

The difference between the corrected lower and viewfinder is way to
small to
effect framing, so you must be desiring a correction for the IR focus.
Well,
don't obsess about it. Make the correction Maco suggests and be happy.
It's
not that much with that film and the proper filter for that film.

The correction depends on the lens, not the film. Many SLR lenses have a
red mark for IR focus, but for TLR lenses that isn't feasible. You could
experiment, e.g. by shooting a tape measure at full aperture, and
examining with a loupe which distance is sharp.

-- Lassi