Infrared Focusing On Hasselblad
Could anyone who uses a hasselblad explain how to achieve proper focus when using infrared film. Help would be much appreciated. Thanks.
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?
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.
Originally Posted by Robert Hall
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.
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.
Originally Posted by Robert Hall
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.
Last edited by Helen B; 01-03-2007 at 10:25 PM. Click to view previous post history.
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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.
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.
Originally Posted by PatTrent
Later: I just had a closer look - now I see the broken lines. I must get new glasses.
Last edited by Helen B; 01-03-2007 at 11:53 PM. Click to view previous post history.
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.
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.
Last edited by Helen B; 01-04-2007 at 12:49 AM. Click to view previous post history.
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.
Ed Sukach, FFP.