IR focusing formula

[george_papantoniou]

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.

[ryuji_suzuki]

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.

[howard_dvorin]

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

[rhys]

The Ilford Maunal of Phorography (not sure who the current publisher

is) contains lots of data on all aspects of film photography.

 

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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.

 

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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.

 

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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.

[george_papantoniou]

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 ?

 

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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.

 

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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.

[howard_dvorin]

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.

[ryuji_suzuki]

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.

[george_papantoniou]

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.

 

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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 ?