Focusing LF in the Infrared Sprectrum

[Bryan Lemasters]

[QUOTE=David A. Goldfarb;386812]I've got my 4x5" set up and pointed out the window, so I just tried this little experiment--

Focus a 150mm lens at infinity, but leave room to back it up (i.e., set the lens a bit back from the infinity stop, if you've got infinity stops). 1/70 the focal length is a little over 2mm. Now back up the lens 2mm and what does the horizon look like? Now stop down, and how far do you have to go to get acceptable sharpness. I thought it was okay again at f:32.

Here is a paragraph from Paduano's Art of Infrared Photography:

"On a 4x5 view camera where there is no focusing mark for infrared, extend the bellows by 1/4 of 1% of the focal length of the lens. Focus adjustments are unnecessary if the lens is stopped down to f/32 or higher."

Sounds consistent with David's experiment.

[C. D. Keth]

I'm a bit confused. One source cited says to extend the bellows 1/70th of the focal length and one says 1/400th of the focal length. Why such a huge discrepancy?

[gregstidham]

Thomas,
I use a couple lenses with the Efke IR820 film and a Hoya R72 filter.

My Rodenstock Sironar-N 150mm needs no correction for focus. I use it at f5.6 often and have never had a focus shift.

I also use an older 8 inch Cooke Series II Anastigmat. It needs a small amount of focus correction for accurate focus when I use it wide open at f4.5. The correction is a tiny amount however, maybe 1 or 2 mm. It hasn't been a problem.

I'd experiment with your own lenses to get a feel if any correction is needed. It is the best way since not all lens designs need correction for the IR spectrum.

[Kodachrome25]

Old Thread Alert....

The experimentation can be maddening at times which raises a question I guess..

I shoot Rollei IR400 in both 120 and 4x5 and I do have quite a nice supply of IR820 in those formats as well. I use a R72 filter 100% of the time. When I did tests with my 135 Apo Sironar and 180 Apo Symmar, I did not need to make corrections so I just went to town with the film with my 65, 90, 135, 150 and 180 lenses, no issues. I also did not need to correct my 240 Fujinon A but confirmed that with a test I did yesterday wide open and it was tack on the focus point.

But my new Schneider 350mm F/11 Apo-Tele-Xenar is off by a good bit. I did two ranges yesterday and between the two, I think I can figure out the correction factor based on what I saw on the film tonight.

The question I have is does the amount of measured bellows correction change as one focuses at subjects of closer distance or is it a fixed number? In other words if my correction for infinity is say, 3mm of additional extension, will it be the same for something that is 20 feet away?

Can't wait to get this dialed in, because frankly I can not stand endless testing and experimentation, it gets in the way of making great photos happen.

[vinny]

Several years ago I emailed schneider about this (for a particular lens, 150 apo symmar) and they gave me the correction factor. I don't remember what it was but not worth messing with on a field camera. They also said that there isn't a formula that works for all lenses. Sounds like a pain in the ass with that 350mm and I don't have an answer.

[Sevo]

The question I have is does the amount of measured bellows correction change as one focuses at subjects of closer distance or is it a fixed number? In other words if my correction for infinity is say, 3mm of additional extension, will it be the same for something that is 20 feet away?

No - it will be proportional to extension. That is, if that 350mm lens needs 3mm at infinity, it needs around 1% extra extension for infrared. Which would amount to 6-7mm (twice that 3mm extra) at 1:1. Given that barely any large format camera has a vernier scale to make sub-mm adjustments by number rather than by sight, it will not be worth the considerable bother of adding extra increments to that 3mm for magnifications smaller than 1:8 or even 1:4 (where you'd still deal with slightly less than a mm extra). But you will have to account for it once you get into the macro range.

[ataim]

I’ve shot with IR film (Efke IR820) with a 720nm filter. With the dark cloth completely covering my head and GG, and waiting about 30 seconds for my eyes to adjust I can barely see an image and will focus on that. Very few images are out of focus. I have a harder time with exposure.

[evan clarke]

I use APO lenses with the Efke Aura and it's always fine, no adjustment..

[Kodachrome25]

I’ve shot with IR film (Efke IR820) with a 720nm filter. With the dark cloth completely covering my head and GG, and waiting about 30 seconds for my eyes to adjust I can barely see an image and will focus on that. Very few images are out of focus. I have a harder time with exposure.

I thought I might give it a try as I wait for a reply from Schneider / New Jersey. So I just got done with a rather eye fatiguing session under the dark-cloth. What I found on the ground glass adds up to what I found on the test shots, the required additional bellows extension for a Schneider Apo-Tele-Xenar Compact 350mm F/11 with an R72 filter is exactly 1.25mm at infinity...and it really is required as even a .5mm nudge of the focus sends things into soft land really fast with this lens. This measurement was taken with digital calipers.

So I carefully placed a small piece of aluminum tape on the side of the rail of my 45N-2, it is exactly where it needs to be and will help me keep my sanity. This should work fine as the rear standard is always at max extension with this lens, the variable being focus.

Evan, I have 4 other APO lenses that are fine too, it's the 350 that is the pain and often it is a lens I go to first for a specific commission that I bought it for...

[uphereinmytree]

I use efke IR more than any film and I never bother with focus shift and haven't had any issues. I use Imagon lenses, a verito, a 6" petzval, and modern lenses. I use a B+W 092 filter which isn't as dark as a hoya r72 filter. It may be for me that the slightly lighter filter lets in enough of the visible spectrum so that the IR focus shift isn't an issue. Also, I think Efke being a weaker IR film, isn't as affected by IR focus shift. I do use the index marks for medium format lenses when shooting 120 IR film. The 092 B+W filter lets in a little more light for shadows where IR is weak and helps fend off ridiculous reciprocity failure calculations. I shoot urban landscapes with deep shadows and less foliage so the lighter filter works for me.

To stay on track here, Shooting wide open with a verito and efke IR, I didn't see any issue with focus shift and rarely consider it. Even an achromat imagon was well focused.