Napioa - Wind Origins

Napioa - Wind Origins
by octopus                

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Below are images I've shot recently while experiminting with IR photography. The first is an IR image with +2 stops of exposure and a custom white balance based on the bright part of the tag on the fence. The second image is a shot taken 1 minute later without the IR filter, also with +2 stops of exposure and the white balance calculated by the camera.

As you can see, in the IR shot there are lots of spots. At first I thought these were dust spots but they aren't apparent in the visible light shot. What are they, and how can I avoid them?

Adjustments: Lightroom 4 (Exposure and white balance)
Camera:      Canon 60D
Lens:        Canon EF 24-105mm f/4L IS USM
IR Filter:   Lee Polyester 87 (730nm)

IR shot of a fence

Visible light shot of a fence

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It looks like your filter is dirty but maybe someone will offer a better explanation. – Itai Jun 10 '12 at 19:03
Looks like a bit of bokeh. Might be some dust on the filter, possibly even the lens. The IR shot is much darker relative to the full color shot, which might be helping OOF spots produced by dust particles on the filter to show up better. – jrista Jun 10 '12 at 19:05
@Itai Ahhh, I did check the lens and the filter, but I only checked the front of the filter! I'll take another look. – Damian Powell Jun 10 '12 at 19:23
up vote 7 down vote accepted

These spots are clearly some point-like IR light sources out of focus (When I said sources it may also be some reflecting stuff). You may check that changing the f-number will change the size of the spots.

You can see from the left part of the image that some of the spots are in front of the wood wall so they do not originate from the sky.

One can conclude that you have some dust on your optics / filter.

Actually most of dust reflects IR much more than visible light. (This was used to discriminate dust back in the time we where scanning B&W film.)

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Sherlock Holmes would be proud of you :) – MattJ Jun 13 '12 at 9:42

From my experience multi-coatings on lenses, when the IR filter over the sensor is removed, reflects the aperture in the form of white spot(s). On some lenses (Nikkor 35/2.0 which have very pale coatings) I don't have the white spots, and the picture is generally much sharper. On macro and lenses with intensive multi-coatings, I notice that the effect is much stronger, and I don't use a larger aperture (5.6 or wider).

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