Infrared Photography, Part 3: Focusing with Infrared Film

Special consideration must be given when focusing the camera using infrared film. Some lenses have an infrared guide to help. Other lenses may require focusing tests.

A reader’s question prompted me to think that I had treated focusing with infrared film a little too lightly, in the last article on Infrared. With that in mind I would like to discuss this subject a little more in depth.

50mm lens showing range scales, depth of field scale and red "R" mark.
50mm lens showing range scales, depth of field scale and red “R” mark.

 

While it may seem counter intuitive, light waves of different wavelengths do not focus on the same plane when passing through a simple lens. If you could put a blue filter over an older camera and focus and then switched to a red filter, you would find that the image through the red filter was slightly out of focus.

Modern lens makers have tried to compensate for this phenomenon with the introduction of apochromatic lenses. Through design, these lenses force light waves of different wavelengths to focus on the same plane. As you can imagine this results in a far sharper image.

Most lenses, however, can’t tackle infrared wavelengths and make them focus on the same plane as visible light. This makes it necessary to shift the focus of our lenses when we attempt to shoot infrared film. To make this easier, lens makers have put an infrared mark on most of their lenses to aid in this shift. Exceptions are most auto focus lenses and many zoom lenses.

If you are new to infrared you may not have seen this focusing aid so let’s look at what and where it is. As you can see in Fig. 1, there is a red “R” located on the depth of field scale on the lens. Check your lens. This may be a red dot or an “R.” As many newer lenses don’t have depth of field scales, they probably won’t have this feature either.

To focus using this, focus as you usually do, and then look down at your lens. In the middle of your depth of field scale you will see a centering mark. (See Fig. 1 again) This centering mark will be pointing at a distance on your range or distance scale opposite of the depth of field scale.

After noting the spot where this mark is pointing, simply rotate your lens so that this spot is now lined up with the red “R” instead of the centering mark. For Kodak IR film this will probably be in focus. I say probably because there is room for error when you rotate the lens. You may want to test this by shooting a roll with a wide open aperture, to verify that it really is in focus.

Konica is a near red film. That is, it is sensitive to infrared very close to visible red light. Visible red light ends at about 700nm. Konica IR peaks at about 750nm. Kodak on the other hand is sensitive to light up to around 950nm.

Because Konica is sensitive to a much shorter wavelength, the amount of focus compensation is not as great. In my experience I have found that if I move the spot on my distance scale to half way between the red mark and the centering mark, my pictures are most often in focus. This may take some validation on your part until you feel comfortable with where exactly that mark is on your lenses.

Fig. 2: A possible “shooting range”
for focus calibration.

One way I have found to help find this, is to a series of tests with a target. In Fig. 2, I have shown you a possible “shooting range” to test focus. The idea is to pick a card in the middle of the range, and by bracketing your focus, find out where it comes into focus.

This requires careful note taking on your part. You should also use a tripod so that you eliminate camera movement, both to and fro, as well as shake when you actually shoot, as a variable in your test. I would also suggest shooting at a fairly wide aperture so that depth of field is kept at a minimum.

It might also help to carefully mark the distance between your centering mark and the red mark into fourths (if there’s room!) to help quantify the distance between the two marks. This can be done with a very fine tip pen with an ink that can be seen against the color of your lens. Keep this pen handy when you do your test. Use it to mark the spot on the distance scale where your eye tells you your target card is in focus.

That way you will have a spot to move against the four you put on the depth of field scale so you don’t forget what you’re doing.

After you develop your film you should be able to find in which frame the target card is in focus and by checking your notes be able to tell where on the depth of field scale the best spot for the focus shift occurs. If it is the red spot great! If not, especially if you are using Konica, you will have a new spot to use when you shoot the film in the future.

What if you have an older or newer lens without a red mark? What do you do then? Take a look at figure 1 again.

Do you see how the red mark replaces one of the f-stop marks on the depth of field scale? Also notice that it is on the side of scale that when you shift your focus to that mark you are extending the lens or focusing closer.

Let’s say you are using an old twin lens camera with a 75mm lens. Find a lens with that same focal length that has the red mark on it. What f-stop on the depth of field scale has been replaced by the red mark? Using your lens’s depth of field scale what is the f- stop mark to which you will extend your lens to be in focus for Kodak IR film? This mark will also provide a starting point to do the tests for Konica as I have outlined above.

In any case, I would suggest using the smallest aperture opening, and greatest depth of field possible when shooting either of these films. If depth of field is a consideration in your composition, (and it should be) and you need a shallower depth of field, consider bracketing your focus to make sure you get one that is right on.

This is particularly important for subjects that are relatively close to the camera such as with a portrait.

Go to Part 1…Infrared Photography, Part 1: Debunking Myths
Go to Part 2…Infrared Photography, Part 2: How to Correctly Use the Film

by Michael Fulks
All text and photos: © Michael Fulks. All Rights Reserved.

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