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IR leakage?

Infrared Lens
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#1 montanawildlives

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Posted 06 June 2021 - 16:02

I took this pic a few weeks ago (Fujifilm xt3, full spectrum, Fujifilm 60mm f2.4, Kolarivision UV pass filter). I set white balance in post off the sky (which was blue in real life). I know this is far from optimal and have since purchased a piece of virgin PTFE. (I also found that asphalt worked as one member suggested).

I just wanted to get a little feedback on a couple of things. I had kind of decided that the Kolarivision filter had serious IR leak, but when I look at this pic the foliage and grass is quite dark, not bright as I would expect with IR. On the other hand, you can see the lack of contrast in the center of the image, which in fact IS what I would expect with IR leak because this lens is known to have a diffuse hot spot in IR, which I have been told does not really materialize in UV photography (hot spots are not a common or typical problem in UV, so I hear). The hot spot gets worse and more defined as I stopped down the aperture as would happen with IR (leak). So, I guess my question is...if this has IR leak, why are the foliage and grass dark? This was a full sun day and shot with the sun at my back.

Thanks!

Attached Image: DSCF2094.jpg

#2 Stefano

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Posted 06 June 2021 - 16:17

If the filter leaks some IR, the foliage is still dark, but non as dark as it should be. Also, seeing green in a white balanced UV image (unless you are below like 350 nm) is an alarm bell. I think you have a leak.

#3 Andy Perrin

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Posted 06 June 2021 - 16:42

If this is an image from FILM, film will not behave like a digital Bayer image so I don't know that we can judge by the colors at all here. It's possible there is an IR leak or visible leak, especially given that the Kolari filters seem to have issues in recent times. If you have a leak, the image could still be mostly UV (it looks mostly UV) but with lighter grass than a pure UV image would give you. I would get some S8612 2mm and put it on there if you want to be certain there is no leak.

Edited by Andy Perrin, 06 June 2021 - 16:42.


#4 Stefano

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Posted 06 June 2021 - 17:27

Which program did you use for the white balance? IrfanView gave me green instead of black with UV LEDs.

#5 Bill De Jager

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Posted 06 June 2021 - 17:50

View PostAndy Perrin, on 06 June 2021 - 16:42, said:

If this is an image from FILM, film will not behave like a digital Bayer image so I don't know that we can judge by the colors at all here.

The Fujifilm XT-3 is a digital camera.
Studying the botany and plant geography of California and western North America for almost 50 years.

#6 nfoto

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Posted 06 June 2021 - 18:57

I think this boils down to the combination of an unsuited w/b, a lens not passing UV well, and substantial IR leakage. In some way this reminds of the very first version of the Baader U (1/2" size) on an non-modified camera. The overall outcome was nothing like UV at all, yet not IR either.

#7 Stefano

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Posted 06 June 2021 - 18:59

Half inch size?

#8 nfoto

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Posted 06 June 2021 - 20:23

Sorry should be 1.25" -- blame intermittent memory failure.

#9 Andy Perrin

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Posted 06 June 2021 - 22:13

View PostBill De Jager, on 06 June 2021 - 17:50, said:

The Fujifilm XT-3 is a digital camera.
Unkind of them to confuse me! :grin:

#10 Reed F. Curry

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Posted 06 June 2021 - 23:16

If you already have IR capacity, why not just put a longpass 720nm filter over the Kolari. If anything comes through, you have an IR leak.
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#11 OlDoinyo

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Posted 06 June 2021 - 23:48

Uv hot spots are not so incredibly rare. The Minolta A 50/1.4 has a particularly egregious one (not that I use that lens for UV much anyway.) The Contax/Zeiss 24 shows an annular hot spot when stopped down, due to back-reflection off the iris!

#12 Reed F. Curry

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Posted 07 June 2021 - 00:04

It could be a green leak in the 524nm range.
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#13 OlDoinyo

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Posted 07 June 2021 - 01:03

Green leaks would be very unusual for a Wood's-glass based filter--that is one of the ranges of greatest opacity for such a filter.

#14 Cadmium

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Posted 07 June 2021 - 02:03

I have seen green foliage before in tests with various UV-Pass filters, even the Baader U.
I have not see this for years, but below is an example with a UG11 stack (left) and a Baader U (right). It may have something to do with white balance. And no, this was not using the older Baader U that Birna is talking about above. I can't say what causes this, so I can't blame it on the the filter, or the lens. The lens used here was a Kuribayashi 35mm.
One could try to isolate the green, but it may have something to do with something entirely different.

Attached Image: UG11_BG40_VS_BAADER_2.jpg

#15 OlDoinyo

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Posted 07 June 2021 - 16:59

I have seen a faint green tint on foliage in some of my photos as well. But I assume actual green light probably has little to do with it. The absorbance of chlorophyll is not perfectly uniform across the UVA spectrum; it declines to a minimum somewhere near 325 nm. Perhaps this explains it.

#16 Stefano

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Posted 07 June 2021 - 18:12

Hard to say. UV-green is a very hard color to see, unless you have a "UV-green" bandpass filter or a "UV-green" LED. The sensitivity of our cameras at 325 nm is much less than at 360-370 nm, so it's unlikely that a weak signal there is visible in UV images.

Here (especially in the printed boxes) you can see how things appearing UV-yellow in TriColour UV (absorbing at around 323 nm, peak of Bernard's bandpass filter) are just slightly lavender if not white in the typical UV images. So I doubt any UV-green around 325 nm would be that visible.

#17 Reed F. Curry

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Posted 07 June 2021 - 22:39

In 2017, Andrea did a comprehensive test for the visible light leak in a 1.5mm thick ZWB1 https://www.ultravio...e-leak-ir-leak/
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#18 Cadmium

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Posted 08 June 2021 - 03:02

The only stacks I have detected any visible light leaks with are those using thin U-340, like 1mm U-340, which does leak more green than the UG11 at the same thin thickness, but the U-340 green leak is still fairly hard to see,
and doesn't look green as much as it just slightly fogs up the blacks. It is hard to demonstrate, but I would not recommend U-340 1mm. Use 1.5mm or 2mm instead and the leak will be gone (assuming stacked with S8612...).
The green shown in photos above 'I don't think' is that kind of green leak, unless you can isolate it and prove that it is.

Edited by Cadmium, 08 June 2021 - 03:03.


#19 Stefano

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Posted 08 June 2021 - 09:27

Based on Jonathan's graphs here (won't share the raw data as he didn't want to), at 0.5 mm U-340 has a 0.2% peak in green light, and U-360 is below 0.1%. This means that at 1 mm thickness they have OD 5+ blocking (U-340) and OD 6+ blocking (U-360) in visible light. Perhaps OD 5 is not enough?

Anyway, it is either a leak or a white balance issue, in my opinion. So trying the usual longpass test is the best thing.

#20 nfoto

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Posted 08 June 2021 - 10:23

It's all about poor lens for UV, filter leakage in IR, and unsuitable w/b. There is no magic in getting "green grass" for assumed UV captures with these limitations in mind. I managed that twenty years ago :smile: thus not surprised my experience can be duplicated.