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07-27-2012, 02:43 PM   #1
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How do you focus for IR?

The modern lenses have no IR compensation setting on the focus scale. How should I figure IR compensation?

If I focus at something that is 3 feet away, should I set the focus at 4 feet or 2 feet or what?


07-27-2012, 03:35 PM   #2
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If your camera has lv use that. Some cameras will af ir if the proper filter is used to get rid of daylight. If manual focusing close lens down for more dof and test focus with multiple shots.
07-27-2012, 03:47 PM   #3
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I don't have any live view. Hard to tell until I get it up on the screen. Wondered if there is some sort of forumla for IR correction?
07-27-2012, 04:02 PM   #4
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Lenses used to have focus marks for IR. New ones may have no markings at all...
You may be able to work out your own formula though, by trial and error, which is quick and free with digital
I expect it depends what IR wavelength you are working with too.

07-27-2012, 04:23 PM   #5
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The IR focusing shift will be different for each lens/focal length. I would tend to use an old MF Pentax lens.

07-27-2012, 05:09 PM   #6
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Your IR photos (that I've seen) have been in focus.

If your system autofocuses ok, then manual focus should be ok too if you rely on the CIF beep & green hexagon to detect focus because that's the way autofocus does it..

Otherwise, how did you get such good focus so far?

You could also change the shims on the viewfinder screen so that viewfinder focus agrees with actual focus. Mount an old lens with the red IR focus mark, then focus on infinity (set to the IR mark), then adjust the focus screen shims until the scene is in focus through the viewfinder.

The best way would be to get & convert a live view camera - then there'd be no problems with setting white balance, getting precise focus, etc. I don't know which Pentax models were the first with live-view.

Dave in Iowa

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