Large format IR shooters

[colrehogan]

I have found that stopping down with the Maco IR has traditionally been enough at least in the 120 size. I guess I never really looked hard at my few 4x5 shots and until I tried it in 8x10 and compared it to a Tri-X neg of the same scene (shot with a red Lee filter, I think) side by side (all shots were done with a 420 mm Repro-Claron and I developed them in Pyrocat-HD), I never really saw how 'fuzzy' (for lack of a better term) my IR negatives were.

Do you just stop down your lenses a lot or do you actually do a focus adjustment for the IR light with your LF camera? Hope that makes sense.

 

[dtomasula]

Diane -

Focus shift should not be a problem with LF IR film, particularly if you are using small aperatures in the f22-64 range. That should give you plenty of depth-of-field and take care of any focus shifting.

Is it possible your "fuzziness" is a photographer-related focus problem and not a focus-shift problem? Maybe camera shake?

 

[colrehogan]

I don't know. I will try again this weekend if I get some film loaded tonight. I just looked at my notes and my exposure was at f/16. I will try at a smaller aperture. As for camera shake, I doubt it, as the wind hadn't picked up and the Tri-X negs which I shot just before the IR stuff was sharp.

 

[jjstafford]

Focusing 'shift' for IR is important with LF's long lenses. I don't know how someone could find otherwise. Heck, using just a 25A on Pan film shifts focus. With your film, you are working in the low IR range - maybe around 750 or 800 - but still significant. I would have to find my old book, and will if you ask, but I seem to recall a figure of a 3% (to possibly 5%) extra bellows extension is needed.

 

[KenS]

I would have to find my old book, and will if you ask, but I seem to recall a figure of a 3% (to possibly 5%) extra bellows extension is needed.

A movement to the "rear" (ie increasing in bellows extension) of 0.25% of the distance from the nodal point of the lens to the film plane is all that is theoretically required.

Ken

 

[jjstafford]

A movement to the "rear" (ie increasing in bellows extension) of 0.25% of the distance from the nodal point of the lens to the film plane is all that is theoretically required.
Ken

Aw, that pesky decimal thing. So, if a person is using a 250mm lens, then it requires an extension of another 6mm (6.25mm) - that is 1/4" which is significant. If your groundglass or film holder were off that much, you would be deeply concerned.

 

[colrehogan]

A movement to the "rear" (ie increasing in bellows extension) of 0.25% of the distance from the nodal point of the lens to the film plane is all that is theoretically required.

Ken

Does that mean .25% times the physical distance from the lensboard to the ground glass?

 

[colrehogan]

jjstafford,
If you could find your book, it would be appreciated. Actually, the film is 820 nm.

 

[Ole]

Diane, the max is 820nm -- that is where the sensitivity drops below 50%. The minimum depends on the filter, with the one I use I get the range from 710nm to 820nm (between the 50% points for filter and film). Center of the sensitivity is then 765nm, so that is the wavelength should be corrected for.

In addition to the filter, the correction will depend on the lens used. I found that without correction the results were far sharper using an APO-Lanthar than a Symmar or even a Zeiss Planar (at that time I had a 210mm APO-Lantar, a 135 Planar, and a 150 Symmar). 0.25% of 210mm is only about 0.5mm (not 5!), which isn't really all that much and might be difficult to do precisely with most cameras. For 150mm the correction would be less than 0.4mm, which I didn't even think of attempting.

It turns out that it's impossible to make general statements about how much the correction should be. It depends on the (general type of) lens construction, the corrections made, the type of corrections made and so on.

 

[colrehogan]

Hi Ole,
I see. I suppose I wouldn't have even noticed if it wasn't a post that I felt should have been in focus in the first place (and was in the Tri-X negative). Oh well.

 

[Ole]

Stopping down is probably a good idea anyway. I noticed the difference between my various lenses when shooting within a stop or two of full aperture - so most of them were used at f:5.6!

There was hardly any difference to be seen at f:16, which is where I take most pictures. But it might be worth noting that most lenses are not particularly well corrected for IR anyway, so some extra softness must be expected.

One thing to look for in "soft" IR negatives is whether there is some other point which is sharper than where focus was intended. If there is, that's a strong indication that the focus should be shifted a little. If not, it won't help at all...

 

[JohnArs]

Hi Diane

First the IR films are not as sharp as the normal ones because they are tuned for IR not for sharpness but I always work for focusshift issues in the range of f 22-64 and my negs looking really sharp an the kodak HIE wich I still have some boxes in my fridge works quit well on XTOL!

 

[jjstafford]

> 0.25% of 210mm is only about 0.5mm (not 5!),

Oh crap. Now I know that I'm a total innumerate. So be it.

--
"The mind is always the second thing to go."

 

[colrehogan]

Thanks Ole. The out of focus things were in the foreground and the tree I was focusing on is in focus. I suppose I just wasn't expecting the stuff in the foreground to be that out of focus. Thanks to all who replied.

 

[KenS]

Does that mean .25% times the physical distance from the lensboard to the ground glass?

Yes.

so..... with your 250 mm lens (at infinity), you have to "increase" the bellows distance by 1/4 of 1%. So.... it is .25 multiplied by 2.5mm = .626 mm... not a Hell of a lot. It is probably within the depth of focus. If, however, you are focussing closer than infinity then it should be the 0.25% of the focal length of the lens plus the bellows extension. Don't forget that your lens is not always the focal length marked... When "working", the 210mm Rodenstock I used to use on the Sinar P2 was "actually" a 212.5 mm focal length. The only lens with which I do not have to "make IR allowances" is my quartz/fluorite 85mm lens on my Pentax Spotmatic