focus for i/r when lens does not have marks
My F/50 standard 35-80 zoom lens does not have any i/r marks on it. How do I focus when I use i/r film in this camera?
John A. Lind
This is a _tough_ question! Modern AF camera bodies, especially with zoom lenses, are not user friendly with IR film. The focus compensation often shifts with the focal length, and can be different depending on which IR film you are using.
However, I believe there are some work-arounds that may work for you. I don't have any AF bodies, but here is what I've read in the past about IR films, AF bodies and zoom lenses. I don't know how familiar you are with the various IR films, so there's information included about each of the IR films I know about.
1. To reduce the effect of any focus error, use f/8 or f/11 apertures. This maximizes your depth of field. I recommend _not_ using f/16 or narrower apertures! The wavelength of IR is longer than visible light. Thus, IR will encounter diffraction limiting, which degrades resolution, sooner than visible light as you stop the lens down. The limit for IR with most 35mm lenses is about f/11 before you start to encounter diffraction.
2. Kodak's Ektachrome EIR:
3. Kodak B/W HIE (a.k.a. 4143)
4. Konica Infrared 750:
5. You are going to have to experiment some with whatever film you decide to use. Keep in mind the only IR film I'm aware of with an anti-halation layer is Konica's. This extra layer on the film (which is removed during processing) keeps light . . . or in this case IR also . . . from reflecting back into the film emlusion from the pressure plate that holds the film flat against the rails. Some pressure plates are more reflective of IR than others and I don't know how reflective the F50 pressure plate is. Your first roll of anything other than the Konica should tell you the answer. Also, many modern light meter sensors have IR filters on them as they are a photo-diode type. Your F50 metering may or may not work very well for a proper exposure and you will have to experiment with this also.
Thank you for your comprehensive response to my Q about i/r film, much appreciated. Will begin experminting with your suggestions on the weekend.
If you check B&H's web site. They have an infrared B&W film called Macophot 820c that goes for about 10 bucks.
It has an anti-halation backing.
I havent tried it yet myself, but intend to soon as I found out about the pressure plate issue while using my Maxxum 9000.
I got the 9000 specificaly for IR photos thinking it would be OK since it had a manual film advance. I found out about the pressure plate issue afterwards.
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