How do you adjust focus for infrared film? Or do you just stop down and hope for the best? Thanks!
How do you adjust focus for infrared film? Or do you just stop down and hope for the best? Thanks!
In 35 mm world, many lenses have marks for IR. I've been told that these marks allow you to shorten your focal lenght by about 1/400th, which is the difference required for IR light. So, my suggestion would be to shorten your focal lenght just a wee, wee bit after having focused, to shoot relatively distant objects at small apertures and to hope for the best. You should get satisfactory results very quickly, but obviously, some experience will help. I never shot LF IR myself, but in 35 mm the real issue is not focus, it is light metering.
I don’t make any focus adjustment when shooting LF infrared. This is especially true if you are shooting through a dark red filter, like a number 29, because much of the light that is getting through this filter is still in the visible range and if you adjust your focus for infrared light you will end up with pictures that are out of focus. Try using 35 mm infrared film, a dark red filter, and adjusting the focus using the infrared marks that come on some lenses. You’ll get fuzzy pictures.
Even using a filter that is totally opaque to visible light I still don‘t adjust focus because I‘m usually shooting at f22 or f32, which seems to work just fine for the types of pictures I take (mostly landscapes). However, I suppose if you are shooting portraits or pictures with a very shallow depth of field you should experiment with adjusting the focus in order to hit the intended focus point as accurately as possible. I would try placing a ruler or scale at a 45 degree angle to the camera, focus on the mid point, develop the film, and check the negative to see how much depth of field you have and which way the focus is shifting.
Good luck with the IR film. It can be frustrating, but I really like the results when it works. Like Phillip says though, the exposure is usually more of a problem than the focus.
At typical LF taking apertures (roughly f16 to f32), IR focus shift doesn't seem to be a problem. Naturally, it will depend on the lens.
Rodenstock publishes in their paper brochures for their LF lenses curves of Longitudinal Color Aberration, more frequently known as Longitudinal Chromatic Aberration. These curves show the focus shift versus wavelength. The ones from Rodenstock don't directly answer your question since they cover the only visual wavelengths, from 438 to 678 nm. With the Macro 820c IR film and a deep red filter, the pertinant wavelength range is approx. 650 to 820 nm.
Extrapolating from typical Rodenstock curves (extrapolation is always a bit of a risk), the guessed change in focus from visual to the IR range of Macro 820c is circa a several tenths of a mm. This would explain why the effect tends not to be a problem -- the typical landscape photo with a 150 mm lens will have subject-distance variations causing best-focus position variations of several mm, i.e., about a factor of 10 larger.
If I interpret the sign of the y-axis of the graphs correctly, the correction for IR should be to move the lens away from the film. This fits with the behavior of a simple lens, which focuses red light farther from the lens than blue light.
As Tom says, you could try some test photos.
manyt thanks to all!
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