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I'm researching the world of photography outside of the visible spectrum and I'm really curious to get my hands on some filters, the Baader-U for example, to try shooting in these spectra. However, I noticed there's a problem -- these filters completely obscure visible light, making the viewfinder and liveview useless. Moreover, it's not simply possible to focus the image then apply the filter, since IR/UV converges at a different distance than visible light. The image will go out of focus when the filter is applied.

How then, do people get tack sharp images if they can't see before they shoot? For example, this or even this video

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  • For IR at least, many lenses contain additional focus marks (typically in red) that you could use to manually focus. Also see this answer. – mivilar Oct 7 '14 at 18:51
  • Concerning that video I'm pretty sure they manually focussed on a fixed point and had people walk into the focal plane (see e.g. 2:25). – mivilar Oct 7 '14 at 19:00
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    Actually, liveview with exposure simulation/magnification works pretty darn well for me for focusing with a Hoya R72 filter on my Panasonic micro four-thirds cameras. :) – inkista Oct 7 '14 at 21:59
  • If your camera supports it, focus bracketing can be a useful tool too. – PlasmaHH Oct 16 '14 at 13:58
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There are basically two ways to do this:

  1. 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.

  2. 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.

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