Infrared focus point for Hasselblads

[keithwms]

Newer APO/ULD/ED glass will likely not require correction with the current crop of near-IR films, especially if you are only filtering to the red or deep red. If you use a deeper cutoff filter (rm72 or deeper) then you may well need to correct a bit and/or stop down preventively.

With older lenses you may wish to focus a bit nearer than normal (to the near point of your composition) and then stop down as much as needed to bring your far point back into focus. In other words, enclose your near/far points by a safety margin. I explained why to focus nearer in some old post somewhere... it has to do with the refractive index as a function of wavelength for typical (non-apo/uld) glass.

If you want to use shallow DOF then IMHO there is no option but to shoot apo glass... even if you are using the near-IR films. (Mind you, with the IR I do, I am generally using a #87, which is quite a high cutoff and I gather that most people don't go that far with the current films)

With superpan and a deep red filter I can report that I saw no focus shift, shooting wide open at ~f/3.5 on a mamiya 6.

I can also report that I have needed no refocus with the RB 210 KL apo, with Rollei IR and superpan.

My main advice with IR film is, if you have doubts: shoot, bracket exposure, bracket aperture, refocus, and shoot some more, and keep doing that until you gain confidence.

[2F/2F]

I assume that opaque or near-opaque filters will be used, because otherwise, no IR film made now gives anything like the "classic" IR look. Using any current b/w IR film with a #25 or #29 filter won't look all that different than using the same filters on a regular panchromatic film, and certainly will not give the "classic" IR look.

Info on the necessity of focus shift using Rollei IR with an R72: http://www.digitaltruth.com/products...d_film_007.php

In short, if you want the sharpest image, you should shift focus.

The same should hold true Efke IR with an R72, as it has about the same max sensitivity of 820 nM.

Here is the index page for all the IR film info on this Website (and info on Rollei ATP): http://www.digitaltruth.com/articles.php.

IMO, these short reads on the current b/w IR films are the most useful to be found on the WWW. They are simple, straight forward, and practical. I would read them all to get some helpful starting information.

So, in summary, IMO:

1. There is little-to-no point in using any of the current IR films with anything but an opaque or near-opaque filter, if a "classic" IR look is desired.

2. Focus should be shifted when using IR films with these filters, if you want the plane of focus to be right where you focused in the viewfinder (though this should not be necessary if using them with shallower red filters such as #25 and #29).

P.S. Ilford SFX is not an IR film, so is not included in these comments. It will work with an R72, however, and this will give the most extreme effect with this film.

[scinysnaps]

Yes, I am planning to use an 87 IR filter with the Rollei IR film
Great stuff guys
Thanks a lot.. I will read those articles
Cheers

[Q.G.]

Howevertheless ... I think IR indices are no longer needed/ appropriate ... evidenced by their omission by Hasselblad/ Zeiss.

Well, no.

The earlier lenses do indeed not have the index mark, because such a thing depends so much on dominant wavelength.
They were, and are, still needed, but not appropriate to put just one single mark on the lens.

But, despite that all, later lenses do have such a mark.
Market pressure, people asking about it, perhaps?

Hard to explain adding an IR index mark as evidence - by omission - that such a thing is no longer needed, isn't it?

And 2F/2F is absolutely right: you must correct for the wavelength.

[Q.G.]

Would this be with the 25 red only or does it apply to the IR 87/87C as well?

That would be the correction needed to bring light having a wavelength of 800 nm to a focus in the plain were the film is.
No matter what filter.

As mentioned before, the spread in focal length for different parts of the (near) IR spectrum is quite large, and you have to decide (and control) which part of the spectrum to use, and which bit to bring to focus on the film.

I did mention that the way to do it was "the hard way", didn't i?


The easy way is to use a Sonnar-Superachromat. It brings all light to a common focus, from deep blue to 1000 nm IR.
Two drawbacks. A 250 mm focal length. And rather expensive.

[polyglot]

Stopping down might cure any focus errors but be aware that you will do a real disservice to your image sharpness - the long wavelengths of IR mean that you will get diffraction effects at larger apertures. Blue is about 480nm, the near-IR range these films are sensitive to is usually 720-820nm, which is about 50-70% longer. So you will get softening due to diffraction a whole 1-1.5 stops earlier.

Of course, some IR films can be pretty crunchy so the extra softness may well be hidden by the coarseness of the grain and you can go ahead and stop down.

And you people ragging on SFX for not being an IR film? Yes, you, 2F. BS. It records a distinct (if tiny) frequency band that the human eye is insensitive to, as evidenced by its showing the Woods Effect on green foliage and an R72 filter. Sure there are other films with better sensitivity and to longer wavelengths, but SFX definitely extends outside the human range of sensitivity.

[2F/2F]

SFX is a lovely film. I did not "rag" on it. I said that "Ilford SFX is not an IR film." Additionally, Ilford does not claim that it is, and would make the same statement I made if asked. It is a near-IR/extended-red-sensitivity film, through and through.

I also said: "It will work with an R72, however, and this will give the most extreme effect with this film."

Ilford SFX is sensitive to almost exactly the same frequency range as the human eye/brain. It doesn't see anything we can't see...it just sees it in a different ratio to the more easily visible wavelengths than we do. Whatever "Woods" (:rolleyes: - His name was Robert Wood, with no S) effect you see with SFX is visible to the eye...if you block all the other wavelengths from hitting your eyeballs in order to keep them from overwhelming the deep red ones. When I look through a camera with an R72 filter mounted on a bright and contrasty day with nice puffy clouds, I see a dark sky, white clouds, and light plants (with only an f/2.8 lens, no less)...and so does Ilford SFX.

What is it with the uppity Australian residents who can't seem to read lately? Is it coincidence that the last person who started BS with me over a very moderate statement, and quoted implications to me that I did not make is also located there?