how to accurately focus view camera with IR film ?

[freecitizen]

[]

 

[Dr Croubie]

I asked this once, can't remember where.
The answer was along the lines that IR film today (SFX200, Rollei IR400S) is only 720-820nm or thereabouts, and it just isn't infrared enough to warrant needing different focussing.
It was only in the days of 900nm+ film that a separate focussing scale was needed.
Or something like that, someone else will chime in soon enough to confirm or deny.
If you're really worried, focus with a deep red 8x filter on, and stop-down a lot...

 

[freecitizen]

Sorry everyone ... my apologies. My original post did not appear as I intended.. Here it is in a more legible form ...........

My 4x5 cameras are a Speed Graphic with Optar 135mm lens and a Sinar Norma with lenses which I think came originally from a Linhof Technika ………. 65/8 Super Angulon, 135/5.6 Symmar and 210/5.6 Symmar.
I want to shoot with 4x5 Rollei IR400 sheet film using a R72 filter ( held in front of the lens ).

How do I focus correctly ? I know how to focus using a lens with distance scale and depth of field marked on the barrel such as on a rangefinder or SLR ….. but not with a view camera. After focusing normally and then placing the R72 filter in place, I expect I need to move the lens a little bit closer to the subject but do not know how much or how to measure it properly in the field.

The inherent narrower depth of field in large format makes focusing even more critical and I do not have APO lenses. I don’t want to just rely on depth of field by stopping down. I need to be able to nail the focus as I want it to be.

Any ideas are much appreciated ….

 

[Sirius Glass]

I asked this once, can't remember where.
The answer was along the lines that IR film today (SFX200, Rollei IR400S) is only 720-820nm or thereabouts, and it just isn't infrared enough to warrant needing different focussing.
It was only in the days of 900nm+ film that a separate focussing scale was needed.
Or something like that, someone else will chime in soon enough to confirm or deny.
If you're really worried, focus with a deep red 8x filter on, and stop-down a lot...

The focus for these films are close enough that one can just focus normally and let the depth of field will handle the rest. That was my experience with HIE at ISO 400 with a Red 25 filter metered through the lens and filter.

 

[DWThomas]

I've not attempted to precisely test, but my feeling is the current "IR" films extend such a small amount into the IR range an adjustment likely isn't necessary. Somewhere just recently I vaguely recall seeing the adjustment in the old days was 0.25% of the focal length which surely isn't very much -- 1mm on a 400mm lens -- and as Dr Croubie says that was film that went out to 900nm. If you're getting that critical, perhaps a test is in order with your specific lenses.

 

[freecitizen]

Thankyou very much for your replies.

I think I need to do some testing. I'm trying to photograph flowers and similar, so I will do a focus test with and without R72 filter to see what focus shift, if any, is present when doing closeups.

Many Thanks .............. freecitizen

 

[wildbill]

It completely depends on the lens used. All lenses are not the same.
After getting the info from schneider optics, I decided to focus normally with both kodak HSI, and efke infrared films. My negs are sharp.