Infrared Focusing On Hasselblad

[John Simmons]

Could anyone who uses a hasselblad explain how to achieve proper focus when using infrared film. Help would be much appreciated. Thanks.

 

[Robert Hall]

How close do you have to be?

The closer you are, the more there is a need to worry about such things.

If not too close, just set your aperture to F/11 and shoot away.

Otherwise the correct adjustment is .25% of the focal length. [closer, btw]

What are you shooting?

 

[Helen B]

***

Otherwise the correct adjustment is .25% of the focal length. [closer, btw]

***

Is there such a thing as a 'correct' value? It's just a rule of thumb, isn't it? The way in which focal length changes with wavelength varies from lens to lens - for example apochromats can be better or worse than achromats in the infrared, in terms of change in focal length. It will also depend on which film and which filter is being used (ie what the mean wavelength of the image-forming radiation is). Sidney Ray, in Applied Photographic Optics, suggests 0.4% of the focal length (f/250) as a rule of thumb.

What film and filter do you intend to use? If you are using a deep red filter that passes some visible light, and film that isn't particularly sensitive to infrared, then you may be able to focus visually. In general lenses are not as sharp in the deep red and infrared as they are in the rest of the visible spectrum, so focussing errors are less obvious. You could use three or four frames of your first roll of film as a test: use the depth of field markings as a guide, and move the lens forwards from the visual (or measured) focus point a little at a time.

Best,
Helen

 

[Robert Hall]

.25%

This is just the simple shift from visible light to Near Infrared. If I had my Wood book close by, I could get you a reference page.

As with all things physical, they can be measured. This being the case, a little math on the wave length and Bob's your uncle.



Bracketing is a good idea. Keep in mind that our light meters are not calibrated to near IR, only to visible light. I don't really want to go down that path again on the ins and outs of metering.

Remember that you want to expose for shadow. The deeper the red filter, the deeper the shadow.

The highlights always get enough exposure. The trick is not over developing them. One can't bring up in the shadows what was never there. Over developing to try and bring up shadow detail is folly.

 

[Helen B]

.25%

This is just the simple shift from visible light to Near Infrared. If I had my Wood book close by, I could get you a reference page.

As with all things physical, they can be measured. This being the case, a little math on the wave length and Bob's your uncle.

***

Hi Robert,

It isn't simple. There is no correct answer that can be applied to all lenses, there are only rules of thumb. There are far too many variables.

I'd be interested in the reference for 0.25% as a 'correct' value. Arthur Cox, in Photographic Optics, suggests 0.4% and Rudolf Kingslake, in Optics in Photography suggests 0.5%, but these are only rules of thumb - there's no reason for them to be any more useful than 0.25%.

I'll dig out some data for specific lenses, if you wish.

Best,
Helen

 

[PatTrent]

Robert:

There should be a little red line on the depth-of-field scale on your lenses. That little red line (only on the right-hand side as you view the lens from above), is the "Infrared Index" mark. My CF lenses have it, and yours should also. But it's pretty tiny, so you have to look hard for it.

After you focus normally, note where the distance reading is on the lens barrel. Then shift your focus ring so that that distance indicator (say, for example "20 feet") is no longer aligned with the center mark (on your d.o.f. scale) but is instead aligned with that little red Infrared marker.

In other words, this works the same way as for 35mm-system manually focused lenses.

Pat

 

[Helen B]

There should be a little red line on the depth-of-field scale on your lenses.
***

Do all Hasselblad lenses have the IR focussing mark? I didn't notice any marks on the ones that I looked at today, mostly the CFE and CFi lenses, I think. I assumed that John asked the question because there were no marks on his lenses.

Later: I just had a closer look - now I see the broken lines. I must get new glasses.

Thanks,
Helen

 

[PatTrent]

Helen:

I have only CF lenses, and they have the marks. Your point, though, since your own Hassy lenses don't have the marks. Interesting. Hmmmmm.

Pat

 

[Helen B]

Pat,

See my edit to my previous post. I didn't notice the broken lines at first glance. They are my neighbour's lenses, I use an SL 66.

I looked at the 120 mm Makro-Planar as a numerical example of the offset. The broken line is at f/8. That corresponds to shift of about 0.64 mm, or 0.5%, if my maths is correct.

Best,
Helen

 

[Ed Sukach]

I've been mulling the "IR Line" on the Hasselblad lenses for some time, now. If you have been the victim ... uh ... had the assignment ... of "ray tracing" (divulging age here) it is obvious that the wavelength of light does indeed affect where the focal plane is located (see: Snell's Law).
However ... If a filter is applied to remove those wavelengths shorter than those desired for IR photography, only those left will reach the groundglass, and be ued in focusing in SLR cameras. To focus with the filter on, and then re-focus to the IR line - will cause an OUT focus condition ... something I've already noticed in practice. It is my guess the Hasselbled has already noticed the *most* IR photographers using Hasselblads focus with the "milder filters" ... and the IR Line was causing more grief than it was worth.
It can be demonstrated: Focus without the filter. Add the filter and check the focus again. Of course this is only applicable if there IS a visible image getting through the filter, say with a #25. The more "opaque" the filter, the greater the "shift" would be, so it may well be an important factor with these filters ... especially in LF photography.

BTW -- It really does NOT have anything to do with lens design. Snell's Law and the effects of wavelength apply to any and all configurations.

 

[Helen B]

***

BTW -- It really does NOT have anything to do with lens design. Snell's Law and the effects of wavelength apply to any and all configurations.

Ed,

The relative amount of focus shift is dependent upon lens design.

Best,
Helen

 

[Ed Sukach]

Ed,
The relative amount of focus shift is dependent upon lens design.
Best,
Helen

Why?

This gets complicated, but the relative amount of refraction due to differing wavelengths remains constant in the only characteristic that would affect the ray trace - the refraction index of the glass itself.
Hopefully, this makes sense. Reminds me of trying to describe "pinhole diffraction" in three words or less.

Uh ... quietly, with hat in hand ... been there - done that.

 

[gr82bart]

Umm .... I just 'nudge' the focus ring over a little bit. Seems to work fine for every image I've taken in IR so far.

Regards, Art.

 

[Helen B]

Why?

This gets complicated, but the relative amount of refraction due to differing wavelengths remains constant in the only characteristic that would affect the ray trace - the refraction index of the glass itself.
Hopefully, this makes sense. Reminds me of trying to describe "pinhole diffraction" in three words or less.

Uh ... quietly, with hat in hand ... been there - done that.

Ed,

Why? Because of chromatic correction. That messes up the simple relationship between wavelength and focal length that exists for an uncorrected lens. How much detail would you like me to go into?

Best,
Helen

 

[Ed Sukach]

Ed,
Why? Because of chromatic correction. That messes up the simple relationship between wavelength and focal length that exists for an uncorrected lens. How much detail would you like me to go into?
Best,
Helen

When you say "chromatic correction", are you talking about correction for "Chromatic Abberation"?

 

[Helen B]

When you say "chromatic correction", are you talking about correction for "Chromatic Abberation"?

Yes. Chromatic correction is a general term that does not define the degree of correction. Terms such as achromatism can be taken to infer a certain degree of correction of chromatic aberration. At least that is how Alexander Conrady used the terms in Applied Optics and Optical Design.

Best,
Helen

 

[Ed Sukach]

Why - oh WHY do these involved discussions always appear only when I am flooded with darkroom work?

If you understand the nature and design techniques for Chromatic Abberation correction - briefly put: incorporating glasses of differing indices of refraction and various lens contours and configurations, they are done with one object in mind - to cause a broad spectrum of light to converge on the same plane; i.e. ALL be in focus at the same place. If we were to remove all light of one particular wavelegnth (I know, I know ... it will not be "perfect"), the focal plane location will NOT change - at least not as long as that spectrum is within that specifed in the design criteria. Not much, anyway... and that "not much" comes from a former Metrology Lab Technician ... "not much" = "damn little".

Most of the IR film we have today is really "visible red to near infrared" and it is doubtful that there will be anything like a great deal of focus change. Possibly with the "old" Kodak HIE and fillters that attenuated all - or most - light below 900 nm or so there would be a noticeable difference - IF original focus was determined without the filter ... but, even then a concrete "all inclusive correction value" would be nearly impossible to determine - the lens would be used outside of its design criteria, and to me that translates to "all bets are off". It would be possible that the theoretical IR correction could go either way ... closer than indicated, or further away.

Again, as I've said - focus the Hasselblad through the filter, if possible - and given that it is possible, leave the focusing where it is, ignoring the IR Index mark. If the filter is opaque to ALL visible light, the only thing left would be trail and error.

 

[DrPablo]

I hate to make a trite suggestion, but what's wrong with just stopping down? I shoot some IR film on 4x5, and even with near subjects at f/22 I have front to back sharpness. I'd think that the increase in depth of field will atone for a subtle focus error.

 

[Dan Fromm]

Why - oh WHY do these involved discussions always appear only when I am flooded with darkroom work?

If you understand the nature and design techniques for Chromatic Abberation correction - briefly put: incorporating glasses of differing indices of refraction and various lens contours and configurations, they are done with one object in mind - to cause a broad spectrum of light to converge on the same plane; i.e. ALL be in focus at the same place. If we were to remove all light of one particular wavelegnth (I know, I know ... it will not be "perfect"), the focal plane location will NOT change - at least not as long as that spectrum is within that specifed in the design criteria. Not much, anyway... and that "not much" comes from a former Metrology Lab Technician ... "not much" = "damn little".

Most of the IR film we have today is really "visible red to near infrared" and it is doubtful that there will be anything like a great deal of focus change. Possibly with the "old" Kodak HIE and fillters that attenuated all - or most - light below 900 nm or so there would be a noticeable difference - IF original focus was determined without the filter ... but, even then a concrete "all inclusive correction value" would be nearly impossible to determine - the lens would be used outside of its design criteria, and to me that translates to "all bets are off". It would be possible that the theoretical IR correction could go either way ... closer than indicated, or further away.

Again, as I've said - focus the Hasselblad through the filter, if possible - and given that it is possible, leave the focusing where it is, ignoring the IR Index mark. If the filter is opaque to ALL visible light, the only thing left would be trail and error.

Ed, first off very few lenses have absolutely no chromatic aberration in the visible part of the spectrum. Secondly, very few lenses are corrected for the invisible non-photographically useful parts of the spectrum. Some enlarging lenses are, according to their makers' propaganda, well corrected for 350 nm - 700 nm, few taking lenses are well-corrected for as wide a range.

There are old taking lenses which worked well enough on orthochromatic emulsions (not very sensitive at the short end of the visible spectrum) but that worked poorly on panchromatic emulsions (better sensitivity at the short end) because of poor corrections for blue. That's why so many semi-nice old folders have built-in yellow filters and that's why the take sharper b/w pictures with the filter in place.

Why don't you admit that you've overstated your position or made exagerated claims for how wonderful modern lenses are?

 

[Helen B]

Why - oh WHY do these involved discussions always appear only when I am flooded with darkroom work?

If you understand the nature and design techniques for Chromatic Abberation correction - briefly put: incorporating glasses of differing indices of refraction and various lens contours and configurations, they are done with one object in mind - to cause a broad spectrum of light to converge on the same plane; i.e. ALL be in focus at the same place. If we were to remove all light of one particular wavelegnth (I know, I know ... it will not be "perfect"), the focal plane location will NOT change - at least not as long as that spectrum is within that specifed in the design criteria. Not much, anyway... and that "not much" comes from a former Metrology Lab Technician ... "not much" = "damn little".

Most of the IR film we have today is really "visible red to near infrared" and it is doubtful that there will be anything like a great deal of focus change. Possibly with the "old" Kodak HIE and fillters that attenuated all - or most - light below 900 nm or so there would be a noticeable difference - IF original focus was determined without the filter ... but, even then a concrete "all inclusive correction value" would be nearly impossible to determine - the lens would be used outside of its design criteria, and to me that translates to "all bets are off". It would be possible that the theoretical IR correction could go either way ... closer than indicated, or further away.

Again, as I've said - focus the Hasselblad through the filter, if possible - and given that it is possible, leave the focusing where it is, ignoring the IR Index mark. If the filter is opaque to ALL visible light, the only thing left would be trail and error.

Ed,

You now appear to be agreeing that the amount of IR focus shift is dependent upon lens design, and that there is no single relative focus shift value that applies to all lenses, even if one IR wavelength is chosen. Is my interpretation of your current opinion correct?


Thanks,
Helen

 

[PatTrent]

Pat,

See my edit to my previous post. I didn't notice the broken lines at first glance. They are my neighbour's lenses, I use an SL 66.

I looked at the 120 mm Makro-Planar as a numerical example of the offset. The broken line is at f/8. That corresponds to shift of about 0.64 mm, or 0.5%, if my maths is correct.

Best,
Helen

They're tiny alright. When I first started using my Hassy I didn't even notice the mark, even though I was used to seeing them on my Canon and Nikon lenses. :rolleyes: I think though that at f/8 (or f/11 on MF) it would be a very minor focusing adjustment for a scenic.
Pat

 

[Ed Sukach]

Ed, first off very few lenses have absolutely no chromatic aberration in the visible part of the spectrum. Secondly, very few lenses are corrected for the invisible non-photographically useful parts of the spectrum. Some enlarging lenses are, according to their makers' propaganda, well corrected for 350 nm - 700 nm, few taking lenses are well-corrected for as wide a range.

I disagree. *NO* lenses have "absolutely no chromatic abberation" in the visible (or anywhere else) part of the spectrum. Every design involves a compromise ... cost and sheer possibility are always determining factors. The only question is "How much deviation from pefect correction of chromatic abberation ... or spherical, or coma ... is acceptable."

Why don't you admit that you've overstated your position or made exagerated claims for how wonderful modern lenses are?

??? I did? My point was that the IR focusing line on the Hasselblad lenses was probably omitted fpor a reason ... and that working without it, focusing from the filtered image on the groundglass ... was probably the way to go with modern IR emulsions.

I oversated how "good" something was??? I haven't done that for some time ... well before I started working in a Metrology* Lab.

* No, not "Meteorology" (weather). Metrology is working with eye-watering "fine" measurements.

 

[Helen B]

They're tiny alright.
***
Pat

The current lenses appear to use a broken line instead of a red line - one of the DoF lines is dashed instead of solid, and it isn't red. At first glance, I thought that it was just a little dirt or chipped paint!

Ed,

As you'll notice from my original post in this thread, I agree with you about focussing with a filter that passes some visible light when combined with the type of 'IR' film currently available in 120. I got the clear impression, perhaps erroneously, that you were disagreeing with my suggestion that focus shift depends on lens design.

Best,
Helen

 

[Ed Sukach]

Helen...

To tell the truth, I don't know if I was "agreeing" or not. I only tried to add what little I know about lenses and optical pheonomena. Chromatic abberation - "color correction" - happens to be one of those characteristics widely misunderstood ... usualy one hears something like, "I don't need a Color Corrected lens ... / Chromatic abberation doesn't affect me ... etc.,... because I don't use color film - I only work in black and white." That particular belief is so ingrained among some that it is nearly impossible to convince them that ALL imaging is degraded by the lack of correction in that area. Another widely held - and erroneous belief - is that chromatic abberation has something to do with color fidelity - that overall color balance will be affected ... which is only remotely "true" ... I was going to write what would be a pun - the image would be only affected around the "fringes"...

One "Idea" I'm working on now ... removing the lens from a Hasselblad 503Cx body ... lens mounting flange to film plane will be something like ~ 80mm; taping an approriate pinhole over the flange surface; adding a #25R (or ...?) filter - and blasting away with Dynalite 1000mx strobes - full power onto Konica or MACO IR film.

Something tells me the Polaroid back is going to be used - extensively - here, to BEGIN to determine exposure. I wonder how much IR the Dynalites kick out ... ?