I have seen a question in the "alternative" forum, about IR focusing shift. It made me think that I have a couple of Kodak IR 4x5 packs in my freezer and intend to use it someday. Does someone have a formula that will help me focus my camera? For example, when I am shooting with a 150mm lens and my bellows are 200 mm long when I am focused for the visible light rays, hoa will I find the correct bellows length for IR ? Must have something to do with the IR wavelength and the lense's diffraction factor, but how do I calculate it ? Don't tell me to use F45 and shoot it like I see it focused.

-- George Papantoniou (papanton@hol.gr), April 06, 2002

Answers

There might be data for *average* lens but the correction for individual lens design differs from that for others.Try to find the data related to the lens' longitudinal chromatic aberration.

-- Ryuji Suzuki (rsuzuki@rs.cncdsl.com), April 06, 2002.

I don't have a formula for you, but infrared focuses a slight bit shorter than does normal daylight. A good source of information is at http://www.cocam.co.uk/CoCamWS/Infrared/INFRARED.HTM

-- Howard Dvorin (HowardDvorin@cs.com), April 06, 2002.

The Ilford Maunal of Phorography (not sure who the current publisher is) contains lots of data on all aspects of film photography.

On a 35mm lens, there's usually a red dot indicating the infra red infinity. On zooms there's usually a couple of red lines.

Having said that, if you stop down a couple of stops, any focussing error will be negligible. IMO, unless you intend to use macro images then forget about the IR mark.

Try doing a test shot first and see whether I'm right. I'll beinterested to hear your experiences. BTW I use IR film in a 35mm camera occasionally.

-- Rhys (bd0574@msn.com), April 06, 2002.

Ryuji, you're right, different lenses will react differently, due to their different chromatic abberation correction, I guess. I've got ten lenses for my 4x5, could I ever be able to find my way with those calculations ?

Howard, do you mean that I should be using a shorter bellows length ? Since IR is diffracted at a smaller degree than visible light, it sounds strange to me. My first guess would be to use a longer bellows, if I am not mistaken.

Rhys, I know that closing down should take care of the problem, I just want to be more accurate with my focusing and not to worry so much about using a small aperture.

-- George Papantoniou (papanton@hol.gr), April 08, 2002.

George, you brought up a good point. I mean a slightly shorter distance. That would require a slightly longer bellows. Try looking at a lens for a 35mm camera. It might have the red infrared focusing mark. If it does you will get a good graphic on what to do.BTW, I work in infrared in 35mm. I use a 24mm lens at f8 and use the hyperfocal distance for my focus setting. That might give you a direction to go in for your large format work.

-- Howard Dvorin (HowardDvorin@cs.com), April 08, 2002.

Different lenses are corrected for different wavelengths. APO lenses are corrected for extra points along the wavelength to minimize aberration over wider ranges of wavelength. So it's hard to generalize here. If your lens came from a decent manufacturer, you might even be able to find technical data for your particular lens. Otherwise, I would make a test exposure with the lens focused at 5m target with visible light, and other targets sitting at various known distances. When I adjust rangefinder cameras or screen position of SLRs, I can use ground glass at the film plane but I usually make sure the focus is accurate using film with a procedure like this. Once you find where the lens was focused at with infrared light, you can formulate a correction factor to add to the bellows movement.

-- Ryuji Suzuki (rsuzuki@rs.cncdsl.com), April 08, 2002.

Ryuji, all my 4x5 lenses are either Schneider or Rodenstock. I don't know if you call them decent, but I haven't found any information on IR focusing in their data sheets. If you've got something, tell me about it.

Howard, yes, but the hyperfocal distance for IR will not be the same as the one for visible light. I've got the formulas to calculate the normal one, but how about the IR one ? Well, I am sure all this is too much fuss about focusing precision that might sound too perfectionistic for many people, but there are lads that worry whether their 4x5 sheets stay flat in their chassis when shooting, aren't there ?

-- George Papantoniou (papanton@hol.gr), April 09, 2002.