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Subjective comparison of three common IR filters

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Alex H View Drop Down
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Post Options Post Options   Quote Alex H Quote  Post ReplyReply Direct Link To This Post Topic: Subjective comparison of three common IR filters
    Posted: 24 July 2011 at 11:45
Recent discussion with some of my friends at DYXUM stimulated me to do this simple and not very scientific comparison between three most common filters used for infrared photography. This test will also be posted on my blog.

The idea was to show how three different filters will render the vegetation and the sky in a very simple picture. The filters tested are "Hoya R-72", "B+W 092" and "B+W 093". The setup for the test was very simple - some sky and some vegetation on the sunny day in southern Sweden. So please do not critique the composition, as there is none. The camera used was a full-spectrum modified Sony A100 with Sony SAM 35mm F/1.8 lens, mounted on the tripod. All pictures were taken at F/8 and the exposure was set manually. For every filter, a series of exposures were made and the "properly" exposed picture (with no clipped highlights) was used for further examination. Also, in camera white balance was pre-set for the grass, and the IR-white standard (polytetrafluoroethylene = PTFE = Teflon) was used to further adjust the WB in the post-processing according to the recommendations and tests of Dr. K. Schmitt.

The need for an IR-white standard is caused by the fact that not all RAW-processing software (that I could try) has an option for an "in-camera" white balance. For example, Capture One Express 6, which I use for all RAW processing, applies "custom white balance" to the pictures, which is far off from the "in-camera" settings, thus requiring additional corrections. On the other hand, the other two RAW-converters used in this comparison, the Sony IDC ver. 3 and the Adobe Camera RAW ver. 6 (as implemented in Adobe Photoshop Elements) render the "in-camera" white balance properly.

But there is one advantage in using Capture One Express for RAW conversion - it allows to "turn off" the specific ICC camera profile. Why is that important? The ICC profile is partly based on the transmission properties of the internal cut filter in the camera, and applies some color correction to the image to make it look more "realistic". However, when the internal cut filter is removed from the camera, applying the ICC profile does not make much sense from the technical point of view. In certain cases additional color correction may be beneficial, like in the IR shots shown below. But in other cases, especially in technical UV-photography it has a negative effect.

Hoya R-72 filter is probably the most commonly used and often rather cheap comparing to other infrared and near-infrared filters. The
official data says that the filter transmits only from 720 nm but in fact it leaks some deep red from below 700 nm and allows for some false IR-colors. The chart below shows the same picture taken through the Hoya R-72 filter processed with three different RAW-converters, and in case of Capture One Express - with and without the default Sony A100 ICC profile. Similar chart will be shown for other two filters below. It should be mentioned also that the pictures can be "clicked" and opened in larger size.

Hoya R-72 filter



B+W 092 is classified as "dark red" by the manufacturer. It blocks the visible light up to 650 nm. Its transmission is around 50% for under 700 nm, and over 90% in the range from 730 nm to 2000 nm. Which means, it leaks some red, and provides a better palette for false-IR colors.

B+W 092 filter



B+W 093 is classified as "IR black" by the manufacturer. Its transmission starts only at 800 nm and increases to 50% only at 900 nm. It basically produces monochrome images with custom white balance, but as You can see from the pictures below, ICC-profiled shots do have some color.

B+W 093 filter



The next series of charts show three pictures of the same scene taken with three filters and processed in identical way, compared side-by-side. The first chart shows pictures, which were opened in Capture One Express, calibrated with PTFE IR-white standard and without ICC profile. It clearly shows the difference in initial color rendition between three different filters.

Converted to TIFF in Capture One Express (no profile)



The second chart shows pictures adjusted in Photoshop Elements. The white and black points were defined in "levels".

Levels adjusted in Photoshop Elements



The last chart shows pictures after the "channel swap" in GIMP, in order to achieve a blue color for the sky. Here You can see that the B+W 092 filter gives much more natural colors without excessive color correction. Similar effect can be achieved also for the pictures from Hoya R-072 filter, but, in my limited experience, it requires much more processing, including "color variation" and selective adjustment of "hue" and "saturation", among others.

Channel swap in GIMP



Concluding this post I would say, that I personally prefer the B+W 092 for false-color infrared and the B+W 093 for the grey-scale (black-and-white) photography. Hoya R-072 seem to be a "Jack of Both Trades". Is it good enough for You? Only You can decide.

Please remember that this comparison show only the most basic editing of the infrared or near-infrared images. Definitely, there are many more ways to alter the false-colors in post processing, and achieve breath-taking results, even for pictures taken with "deeper" filters. And there are a lot more filters out there, with different spectral properties.

I only hope that this post will help newcomers to IR-photography to choose their tools, depending on what their vision of IR photography is, and their personal preferences.

Thank You, Alex


Edited by Alex H - 24 July 2011 at 12:29
 



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Micholand View Drop Down
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Post Options Post Options   Quote Micholand Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2011 at 17:29
Alex, thanks for posting this informative comparison

And also for updating the IR+UV collective thread in the KB
/Michael

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photoman View Drop Down
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Post Options Post Options   Quote photoman Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2011 at 17:41
Thanks for the information Alex, I've been interested in getting into IR photography and have often wondered about the differences between filters. Is there any chance you could also repeat this test using a camera that has not been modified for taking IR photos? I've also wondered what kind of exposure settings and results one can expect with an unmodified a700 and any of these IR filters.

Edited by photoman - 24 July 2011 at 17:48
A700, Sig 10-20, 17-70, 24 f2.8 400 f5.6 telemacro, Min 85 f1.4, 50 f1.7, 70-210, Cosina 100 macro,
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Alex H View Drop Down
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Post Options Post Options   Quote Alex H Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2011 at 17:55
Thanks Michel and Photoman!

Yes, I can repeat the same test with unmodified A700 a little later. May be with a more interesting subject. I should also update the post exposure data. Will do that in few days.

(I am currently moving from one apartment to another and have most of my camera gear packed, so please excuse me if it takes long for me to fulfill Your requests).

Alex
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