How do you focus accurately when shooting with an infrared or UV pass filter?
Asked 10/7/2014
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2 answers
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I’m looking into infrared and ultraviolet photography with filters such as the Baader-U. Since these filters block visible light, the optical viewfinder and normal live view can become very dark or unusable. I also understand that IR and UV light can focus at a different point than visible light, so focusing first and then attaching the filter may shift focus.
How do photographers get sharp focus for IR or UV images when they can’t easily see through the filter? Are there reliable methods for focusing before or after the filter is attached?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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There are basically two ways to do this:
Autofocus works properly with an IR-pass filter in place, as does auto-assisted manual focus (you turn the focus ring, the camera tells you if things are in focus). You are effectively focusing in a low-light situation, however, as the autofocus sensor has an IR-blocking filter just like the image sensor does.
Many lenses, especially older ones, have an "IR focus" mark: a red line slightly offset from the visible-light focus mark. You can either focus using visible light, attach the IR-pass filter, and then turn the focus ring so the point that was lined up with the visible-light mark is now lined up with the IR-focus mark, or you can estimate the distance to the subject and set the focus ring directly.
If your camera has a live preview drawing from the image sensor, you can try focusing using that, but as noted above, it's a low-light situation and the display may not be clear enough to use.
Originally by user26921. Source · Licensed CC BY-SA 4.0
user26921
11y ago
0
Generated from our catalog & community — verify before relying on it.
There are two common ways.
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Focus through the filter: Some cameras can still autofocus, or at least provide focus confirmation, with an IR-pass filter attached. This works like focusing in very low light, so performance may be slower or less reliable.
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Use the lens’s IR focus mark: Many older lenses have a small red infrared index mark. Focus normally in visible light, then attach the filter and shift the focus ring so the original focus point lines up with the IR mark. You can also estimate or measure subject distance and set focus manually.
For UV/IR work, live view may still help if the sensor can see enough of that spectrum, but it depends on the camera, filter, and exposure conditions.
In short: either focus with the filter on if your camera can manage it, or focus in visible light and compensate using the lens’s IR index mark. The exact amount of focus shift varies by lens, so testing your setup is important.
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