Focusing LF in the Infrared Sprectrum

[tgtaylor]

Hi all,

Two quick questions:

What's the best way to focus LF using an IR filter? The 35mm and MF lens have an IR focusing offset, but those are lacking on LF since you focus with the bellows.

I have a Cokin Z007 IR filter on the way and intend to experiment first with a 35mm camera with Ilford SFX. However MF ad LF are my main camera's and both the Rollie and Efke products look promising and the latter is available in 4x5, 5x7 and 8x10.

Finally, your input on IR filters suitable for current IR film would also be appreciated. I purchased the Cokin filter because it is an 89B 4x4 filter that would work with all my lens, was relatively inexpensive ($69.95 with free shipping), and was available. The Hoya R72 was recommended most often in the literature but it and the B&W 92 are round filters which, with suitable step-up rings, would work for most of my lens except for the WA MF and LF lens that I have.

Thanks in advance,

Thomas

[David A. Goldfarb]

According to Ansel Adams' _The Camera_, for IR you should extend the bellows by 1/70 the focal length. If you can focus on both standards on your camera, I suppose you can mark a zero point and then an IR focus correction for each lens on the bed or the rail of the camera for the standard you don't usually focus with.

[vinny]

It depends on the lens. If its a modern lens, contact the manufacturer for specs and they'll tell you how much to compensate. In other words DON'T WORRY ABOUT IT!
This has come up before, try a search too.

[Daniel_Buck]

extend the bellows by 1/70 the focal length.

for those of us not very mathematically inclined, I think that might roughly translate to: "nudge the front standard forward, and stop down" :D

[David A. Goldfarb]

You could also make a little ruler with 1/70 the focal length marked for each lens you carry.

[Bryan Lemasters]

Focus shift is not nearly as great on Ilford SFX as it would be on, say, Kodak HIE. With the dark red filter you are still exposing some visible light. Focus as you normally would and stop down to f16 or 22. You should be OK. My 2 cents.

Bryan

[Marko]

Focus shift is not nearly as great on Ilford SFX as it would be on, say, Kodak HIE. With the dark red filter you are still exposing some visible light. Focus as you normally would and stop down to f16 or 22. You should be OK. My 2 cents.

Bryan

The longer the wavelength, the greater the shift. Ilford SFX is mostly getting the deepest red and barely scratching the IR, and the HIE is good past 900 nm, depending on the filter.

Using an Apochromatic lens should also help a bit, especially when stopped down.

[Ash]

By the looks of things, unless you're going shallow focus, I wouldn't worry.

If you've stopped down the lens to have the majority of the scene in focus, the offset for IR will be compensated for anyway.

[Brian Ellis]

I used a lot of 35mm and 4x5 HIE 4x5 before it was discontinued. I haven't used any of the other infrared films. With Kodak's infrared I didn't find that the focus shift was worth worrying about so I stopped bothering with it even when using 35mm lenses that had the red adjustment mark on them. Adams' advice was useless to me, I'm incapable of making adjustments in increments of 1/70th of the focal length of the lens. But if you feel compelled to make some adjustment, do what Daniel suggests. I did the "nudging" routine for a while before concluding that I didn't need to bother with it.

[David A. Goldfarb]

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.

Maybe some of that "infrared glow" is just being out of focus. It wasn't very hard to measure 2mm of bellows travel using an ordinary ruler.

[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.

[Brian C. Miller]

From Kodak publication F13 (http://www.kodak.com/global/en/professional/support/techPubs/f13/f13.pdf):

Lenses do not focus infrared radiation in the same plane as visible radiation. Because infrared radiation is longer in wavelength than visible radiation, the focus point is further from the camera lens. Therefore, the lens must be moved slightly farther from the film to focus an infrared image. This focus difference is most critical when using filters to block all visible radiation from the film.
Most camera lenses have an auxiliary infrared focusing mark. Consult your camera manual for the appropriate use.
For best definition, make all exposures at the smallest lens opening that conditions permit. If you must use large apertures and the lens has no auxiliary infrared focusing mark, establish a focus setting by trial and error. Try extending the lens by 0.25 percent of its focal length beyond the correct focus for visible light.
For example, a 200 mm lens would require a 0.50 mm extension as 200 mm x 0.0025 = 0.50 mm.

As the instructions say, "trial and error." The Wikipedia article states that APO lenses are already corrected into the near IR, so they don't need to be refocused.

The last time I was out using Efke, I simply stopped down a bit, and the results were adequate. (Just remembered: I also shot my Nikkor 210 wide open, and adjusted the lens forward just a "tweak." No problems under the loupe.)

As for current sheet film IR emulsions, you have a choice of Rollei, Rollei, and Rollei. No more HIR, no more Efke. If there's no more Rollei, then there's Ilford SFX in a roll film holder.

(BTW, when someone says "Verito" or "Petzval," could you please not mock us for using older lenses? Thanks. After all, what's the point of arguing the merits of Nikkor 210 vs Caltar 210 vs Fujinon 210 in the infrared spectrum? As the Kodak instructions state, "trial and error.")

[Kodachrome25]

Trial and error is fine, but as these films dry up and become more expensive, less trial and error is desired. I have info for a specific lens, contributing to what should be a more open minded, not one size fits all mentality to problem solving. I have no idea what a Vertigo or a Pretzel or a Pizza lens is for that matter. I am not against learning what those are, but one of this site's biggest issues in becoming all it can be besides an aging population often displaying a country club attitude is assumptions based on the usual suspects who post here without regard to newcomers.

I did a solid DAY of google searches on this before I decided to re-open this topic with my specific issue. And what I see most of here are non-answers in that it is NOT desirable to crank the aperture down into the diffraction zone in every shot and leave where the focus falls to chance, especially with longer lenses. So I still think a better way to go about this would be to forgo the "It works for me, don't know what your problem is" kind of answers and instead, create a database of lens specific focus compensation with the darkest filter one might use with the films that are still available.

[Sevo]

So I still think a better way to go about this would be to forgo the "It works for me, don't know what your problem is" kind of answers and instead, create a database of lens specific focus compensation with the darkest filter one might use with the films that are still available.

Tables by filter never were needed - at any rate, not even Kodak ever had any, and some of their films had a much bigger IR range than we can get now.

As far as tables are concerned, it is pretty easy to build a table once you have found the IR correction for a single distance (by trial and error, spec sheet or whatever) - it is merely a wavelength dependent variation in focal length (that is, a 300mm lens needing 5mm IR correction at infinity is a 305mm IR lens), so that the usual formulas for focal length apply.

[Andrew O'Neill]

When I used Kodak IR film and the #89 (opaque) filter, I compensated for focus. With the #25 (which I used 99% of the time), I focused through the filter. With Efke IR, which just barely touches IR, I don't bother giving any focussing compensation. I usually shoot at f/11-f/16. Example provided is Efke IR 8x10, #89 filter, at f/16.