Adjustable IR filters

[Craig]

I was looking for an IR cutoff filter in the 720nm range and I came across an adjustable one that is supposed to be able to adjust the cutoff between 530 and 750nm. It looks similar to a polarizer, in terms of turning the filter changes the effect.

Has anyone used this for B&W IR film and is it effective? Or would I be better off buying either a #29 or IR720 filter?

 

[Andrew O'Neill]

I've seen them as well, but never tried one. If you want strong IR effect like Wood, buy it, and pass on the #29.

 

[Sirius Glass]

I was looking for an IR cutoff filter in the 720nm range and I came across an adjustable one that is supposed to be able to adjust the cutoff between 530 and 750nm. It looks similar to a polarizer, in terms of turning the filter changes the effect.

Has anyone used this for B&W IR film and is it effective? Or would I be better off buying either a #29 or IR720 filter?

Depending on the Wood effect that you want you can use the Red23, Red25, Red29 or 720.
Read the following 4 urls to see photographs with comparison with several films:

• https://www.digitaltruth.com/products/product_tests/infrared_film_004.php
• https://www.digitaltruth.com/products/product_tests/infrared_film_004-2.php
• https://www.digitaltruth.com/products/product_tests/infrared_film_005.php
• https://www.digitaltruth.com/products/product_tests/infrared_film_010.php

 

[Andrew O'Neill]

I ordered one. I'll play around with it when I get home from Japan in August.

 

[DWThomas]

(Perhaps it's my aging brain, but) At a quick look, these apparently sandwich a red filter and two polarizers and I can't quite see how the summation of a variable neutral density filter plus a low pass filter is going to move the cut-off frequency. Dunno, maybe the actual polarizers are less effective at longer wavelengths?

At any rate, it appears this topic came up in 2016, and there is an outfit that does IR conversions of digi-cams that didn't sound very enthusiastic about these filters. Of course, it's pointed out that compared with top of the line filters, they are cheap enough to experiment with.

 

[Craig]

Thanks for the link, I had missed that. Looks like a true red filter will be better for my needs.

 

[reddesert]

I think these are a variant on a concept (or gimmick) that Spiratone used long ago in filters called "Colorflow." (Took a while to dig around and find that name. Other companies also had similar things.) The two-color Colorflow filters had a pair like yellow/red and rotating the adjustment would change from full yellow, to intermediate, to full red. I believe the way these work is that the adjustment uses a pair of polarizers to blend between the two filters, although I can't at the moment come up with a fully coherent explanation of how that works. The Lifepixel link doesn't really explain it correctly. And of course they have a business interest in selling their own IR filters.

But I think Lifepixel's criticism is legitimate. You can get weird effects from the crossed polarizers, and the other issue that would concern me is transmission leaks. If the adjustable filter isn't perfect, you may get some light blueward of 720nm leaking through. Normally a small out-of-band leak doesn't matter for pictorial use, but when you are trying to use only the very red end of the film's sensitivity, even a few percent of blue leak may dilute the effect.

I would trust a cheap fixed-wavelength IR filter more than these things. Consider just buying two filters like a 720nm and either a Red 25 or 23. You will probably find that two filters covers most uses and you don't need continuous adjustment.

 

[Helge]

I think these are a variant on a concept (or gimmick) that Spiratone used long ago in filters called "Colorflow." (Took a while to dig around and find that name. Other companies also had similar things.) The two-color Colorflow filters had a pair like yellow/red and rotating the adjustment would change from full yellow, to intermediate, to full red. I believe the way these work is that the adjustment uses a pair of polarizers to blend between the two filters, although I can't at the moment come up with a fully coherent explanation of how that works. The Lifepixel link doesn't really explain it correctly. And of course they have a business interest in selling their own IR filters.

But I think Lifepixel's criticism is legitimate. You can get weird effects from the crossed polarizers, and the other issue that would concern me is transmission leaks. If the adjustable filter isn't perfect, you may get some light blueward of 720nm leaking through. Normally a small out-of-band leak doesn't matter for pictorial use, but when you are trying to use only the very red end of the film's sensitivity, even a few percent of blue leak may dilute the effect.

I would trust a cheap fixed-wavelength IR filter more than these things. Consider just buying two filters like a 720nm and either a Red 25 or 23. You will probably find that two filters covers most uses and you don't need continuous adjustment.

There is such a thing as color specific polarizers.
I was very confused when I heard about a polarizer LC filter that could turn a black and white CRT into a high quality color screen.
The key is exactly those colour specific polarizers.

 

[MCB18]

Honestly, I would just buy a cheap 720mm filter. My Zomei filter has worked extremely well. Would recommend.

 

[Ivo Stunga]

Please do report how that filter works, because my gut feeling tells me it's fake Chinese dreams.

 

[reddesert]

I don't think it's necessary to disparage a country or people over a filter. Chinese companies are perfectly capable of building color filters and polarizing screens. That's how most laptop displays and cell phone screens work. The question is whether this particular filter is suitable for pictorial IR use.

I dug up that the Vivitar version of polarizable two-color filters was called "Cromo Blend," and that led to this post where someone (embarrassingly, it was me) found the explanation of how they work: https://www.photrio.com/forum/threads/vivitar-chromo-blend-filters.184489/post-2427786
see explanation at: http://www.the-ultraphot-shop.org.uk/bicolor.htm

Essentially, they use a dichroic filter that causes a varying polarization angle as function of wavelength (similar to what Helge mentioned), a blocking filter to exclude the wavelengths that are not transmitted at all, and a linear polarizer whose angle selects among the remaining wavelengths. So the Lifepixel link is not quite right; they aren't using two crossed polarizers like a variable neutral density filter. However, if there is already linear polarization in the scene, the effects are going to be somewhat unpredictable. Also the strength of the selective filtration depends on how strongly the dichroic polarizer polarizes (that is, if it transmits 95% polarized light, you might still get 5% of 600nm light leaking through at the 720nm setting). Anyway, since Andrew ordered one I look forward to seeing how his works.

 

[Maris]

It's years since I used a Spiratone Colorflow filter so my only recent experience with polarizer adjusted filters has been with the Neutral Density type. And that experience has been frustrating through lack of reproducible results. When polarizers are nearly crossed a small turn makes a big difference in transmission and it's hard to get the same exact value twice in a row. I've taped up my adjustable filters so they don't turn. Should have bought fixed value ones first up.

 

[Sirius Glass]

It's years since I used a Spiratone Colorflow filter so my only recent experience with polarizer adjusted filters has been with the Neutral Density type. And that experience has been frustrating through lack of reproducible results. When polarizers are nearly crossed a small turn makes a big difference in transmission and it's hard to get the same exact value twice in a row. I've taped up my adjustable filters so they don't turn. Should have bought fixed value ones first up.

I agree. Wise words.

 

[Ivo Stunga]

China is known to sell non-viable, fake products - they can stop doing so if the opinion of the world matters to them.

And what that poor man's variable ND filter, made of 2 interlocked polarization filters, has to do with blocking a specific part of EM spectrum, letting only the slice of interest through to expose film? Selecting light polarization via filter isn't EM filtering - am I misunderstanding things?

What NIR has to do with LCD monitors?


And when I chain up CPL with Hoya RG 72, the results are kind of unpredictable, exposure "floats" in the range of 2 stops or something, usually erring on underexposure.

 

[Andrew O'Neill]

Checked out a variable IR filter.. the FOTGA.

 

[Helge]

Checked out a variable IR filter.. the FOTGA.

Always worth my time.
Thanks.

 

[Andrew O'Neill]

You're very welcome, Helge. I'm glad I did this.

 

[DWThomas]

Thanks! I think you have confirmed my suspicions.

I find there seems to be enough variables in IR photography that one or two discrete steps in filter cutoff might should oughta be sufficient. That's especially so since the current films barely make it to the IR region anyway.

 

[Helge]

Thanks! I think you have confirmed my suspicions.

I find there seems to be enough variables in IR photography that one or two discrete steps in filter cutoff might should oughta be sufficient. That's especially so since the current films barely make it to the IR region anyway.

There seems to be some confusion about what really constitutes IR/where it begins. Some say 700nm, others 750nm.

Fact of the matter is 680nm is already barely visible. 700 requires 30 seconds or so for adaptation, and results in clear Woods effect. 720 is invisible for all practical purposes (although up to 1000nm is visible under certain special circumstances).
Most sources agree that IR starts at 700. Therefore current IR film is well into IR.

Because no filter is perfect it still makes a lot of sense to use a pol filter with a R72 filter. You take out the last pollution to get real dark skies and get the leaves extra white (the Woods effect is mostly diffuse light).
In my experience short infrared is also still affected somewhat by polarizers. So that is something to be mindful of and use consciously.

Trouble with these types of of variable filters is that you are not using the effect consciously. It’s purely dependent on happenstance

 

[Andrew O'Neill]

Funny... in all my years using IR films, I never used a polariser in conjunction with an IR filter... In fact, I can't even remember the last time I used a polariser. I know I have one, somwhere. Maybe I need to take a closer look at it...

 

[MCB18]

I’m interested to see how you guys calculate the exposures you need. With my film, I just meter at 6 ISO on a clear sunny day and the results are phenomenal! I can usually get sub-1 second exposure times to boot. Hearing some of you saying stuff like 30 seconds exposure makes me think I might be doing something wrong!

 

[Helge]

I’m interested to see how you guys calculate the exposures you need. With my film, I just meter at 6 ISO on a clear sunny day and the results are phenomenal! I can usually get sub-1 second exposure times to boot. Hearing some of you saying stuff like 30 seconds exposure makes me think I might be doing something wrong!

You should be able to do better than merely sub second. Handheld should be easily attained in good light.
ISO 6 for slow Aviphot and 10 - 12 for higher speed Aviphot derivatives.

 

[DWThomas]

Most sources agree that IR starts at 700. Therefore current IR film is well into IR.

Hmmm, OK. But I'm pretty certain IR films of yore were responsive out to 800 or even further, the late lamented Efke IR820 for example; Kodak HIE went to nearly 900. Most of the stuff today appears to be already running out of sensitivity at 720. I found about six additional stops exposure needed to go from a 720 to a 760 filter with Rollei Infrared 400 as an example.

We can get some Wood effect with Ilford's SFX200 and an 89B which is about a 695 cutoff, but that film is sometimes referred to as an "extended red" film. So I guess we have a different perspective on what constitutes "well into IR." That said, it doesn't matter much, as we have to work with what we can get.

 

[Andrew O'Neill]

My exposures can be anywhere from 1 second to 24 and even longer, depending on the light, and especially, what aperture I decide to use... Usually smallish, especially with 4x5. EI's are usually 3 to 6.

 

[Helge]

Hmmm, OK. But I'm pretty certain IR films of yore were responsive out to 800 or even further, the late lamented Efke IR820 for example; Kodak HIE went to nearly 900. Most of the stuff today appears to be already running out of sensitivity at 720. I found about six additional stops exposure needed to go from a 720 to a 760 filter with Rollei Infrared 400 as an example.

We can get some Wood effect with Ilford's SFX200 and an 89B which is about a 695 cutoff, but that film is sometimes referred to as an "extended red" film. So I guess we have a different perspective on what constitutes "well into IR." That said, it doesn't matter much, as we have to work with what we can get.

The suns spectrum filtered through the atmosphere drops by almost half energy from 720 to 1000nm plus there is a narrow but deep valley around 720.
So although it’s very nice with extended spectrum it also comes with a whole host of not very well advertised problems, like keepability, need to load in the dark, problems with even slight light leaks etc.
Including the fact that you don’t get more than an about a stop more of speed.

There is a slight difference in the strength of the different IR effects with film with sensitivity over 750, but it’s surprisingly little.
And of course you can use an orange see-through filter instead of a R72 filter with film with extended sensitivity, but again it matters to a surprisingly small degree, once you become aware of the possibilities. Like use a RF or a TLR, tape IR gel inside the film gate etc.