Discussion in 'Large Format' started by troyammons, Feb 26, 2006.
Does anybody have a link to a good method.
Most older lenses have a published correction factor (not only in the original literature, but sumamrized in books like Morgan and Morgan's Graphic Graflex Photography).
Troy: There's some good info here: http://www.cocam.co.uk/CoCamWS/Infrared/INFRARED.HTM
When this question is asked, mostly people suggest extending the focus by a small fraction, repeating advice from old books. That is the suggestion of the link that Lee gave for most LF lenses. I don't think that any simple rule will work. There are many lenses designs out there today, using many different types of glasses. Many modern lenses have reduced chromatic aberration compared to the old designs, on which the rule from the old books was based. But the manufacturers aren't telling us what to do. And I don't think the differences are as simple as achromat versus apochromat, as suggested by the article that Lee gave a link to. Nor is the name of the lens a good indication of whether the lens is truly an apochromat -- sometimes the word "apo" in a lens name is just a marketing ploy. But there are LF lenses using ED glasses which reduce the focus difference between colors. (To be technical, glasses with anomalous partial dispersion that reduce the secondary spectrum.) Rodenstock publishes graphs of longitudinal chromatic aberration for their LF lenses (some at http://www.linos.de/en/prod/obj_analoge_fotogr.html, more in the paper datasheets). These graphs show that the Apo-Sironar-S series, which use ED glass, have reduced longitudinal chromatic aberration compared to the Apo-Sironar-N series. But it's not simple, the various series of Rodenstock lenses have different shape curves of longitudinal chromatic aberration, e.g., Apo-Sironar-S versus Apo-Grandagon. And unfortunately for this question, the curves end at 678 nm, so we have to guess at an extrapolation into the IR. My experience is that no correction is necessary. I think that the depth of focus at typical taking apertures (f22 or smaller) covers up the focusing shift. The only currently available LF IR film that I know of is from Maco -- its spectral sensitivity doesn't go as far into the IR as the discontinued (in sheets) Kodak HIE, so less of a correction is needed. The sensitivity of the Maco film is only to 820 nm. This doesn't require much of an extrapolation of the Rodenstock curves. My guess for the focus shift is only a couple tenths of a mm. In the usual situation, this is lost in the uncertainty in where to place the focus. The depth of focus at f16, using a circle of confusion of 0.1 mm (a typical value for 4x5, but it depends), is +/- 1.6 mm, so an error of a few tenths of a mm doesn't matter.
At the small apertures typically used for LF work you don't need to make any focusing adjustment for infrared film. Depth of field will take care of any minor focusing errors that otherwise might show up, at least that was my experience with Kodak infrared back in the days when they made it.
The shift from visible to IR focus depends on the color correction of the lens on use; as many above have mentioned. With aerial lenses the focal distance from mounting flange to film is might be engraved on the lens; for the mean of "visible"; and IR focus. With my Kodak 153mm F2.5 Aero Ektar Type I (6 inch); the lens RR1699 is engraved F.D. 5.512 IN. ; I.R.F.D. 5.521 IN. The IR focus is 9 mils longer. With my 4x5 Phase One scan backs; when one leaves the IR filter off; one mostly picks up IR. I have found the IR offset rules often not needed; or not enough. Thus I often do a prescan and check the focus point on the monitor of the computer.
The question arises are you just shooting IR; and not visible; or visible PLUS IR? One might have to stop down for both.
Keep track of the lenses you use for IR stuff; and make your own tests if you are using your lenses not stopped down alot. With the scan back; I just do a scan with and without the IR filter over the lens; and see where the best focus point shifts to.
The context of past work with IR focusing is sometimes lost with time. The offsets may apply to their situation; and not yours. The IR response varies with the emulsion too; which muddies up the equation.
Excellent answers. I agree that there is no need for focus corrections for IR. Any focus shift is within the focus errors of LF work. When small apertures are used, there is no need for any corrections.
In astrophotography; folks use to use IR and visible films with the 178mm F2.5 Aero Ektar wide open; with the rig on a clock drive. The rig I used had my Edmund reflector telescope removed; plate/board for mounting a cluster of cameras. A small say 2" long refractor was used as the guide scope; closeing the tracking loop on a guide star. With a wide open fast LF lens; there is little error for focus.
Here is the 178mm F2.5 Aero Ektar @F2.5 after sunset; with IR and visible, this is an exposure at night that took about 1/2 hour:
Here is the same lens at F11 during the day with the Phase One 4x5 scan back and no IR filter. The buildings accross the lake are about 4km away; the cell tower about 6km.
When I used the mid 1930's 12cm F6.8 Angulon wide open for some IR shots; the focus shift from ground glass focus to IR focus is alot more than when the better corrected Aero Ektar is used.
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