Color profile with 3.0 ND filter

[iklo]

I am not sure about the correct way to fix the color tint from 3.0 ND fiter (Heliopan). The red tint is more pronounced when the subject is bare rocks in hot afternoon pointing to a strong IR component. The tint also goes up with exposure time.
I tried the following:

I. Take a picture without the filter with Colorchecker target. Create custom profile in Phocus. Comparing factory profile WB to custom made there is almost no diferenee.
2. With ND filter on and exposare adjusted by 10 steps take another picture with grey card and the Colorchecker. Create and apply new custom profile. The results do not match those in step one.
3. Try applying profile from step 1 to the image taken in step 2. Stil no match.

Even ignoring color profile diferences, it seems that WB values also do not match. If I force them to match, the results are strange. The tint I am getting is no longer red but more like yellowish.

Ary suggestions ave greatly appreciated.

[Vieri]

I am not sure about the correct way to fix the color tint from 3.0 ND fiter (Heliopan). The red tint is more pronounced when the subject is bare rocks in hot afternoon pointing to a strong IR component. The tint also goes up with exposure time.
I tried the following:

I. Take a picture without the filter with Colorchecker target. Create custom profile in Phocus. Comparing factory profile WB to custom made there is almost no diferenee.
2. With ND filter on and exposare adjusted by 10 steps take another picture with grey card and the Colorchecker. Create and apply new custom profile. The results do not match those in step one.
3. Try applying profile from step 1 to the image taken in step 2. Stil no match.

Even ignoring color profile diferences, it seems that WB values also do not match. If I force them to match, the results are strange. The tint I am getting is no longer red but more like yellowish.

Ary suggestions ave greatly appreciated.

Hello iklo,

I might recommend two different solutions:

1. Carry a colorchecker target in the field, and take a reference shot with & without filter with the checker for each situation. I.e., if you shoot 2 minute exposures one after the other for 30 minutes in the same spot with the same light, you'll need only one filterless colorchecker shot and only one 2-minute colorchecker shot, you don't need to take one before each subsequent shots. Then, when post-processing your files, first take the filterless shot with colorchecker in it. Set the WB you'd like to use with your shots (i.e., cloudy, or your own, and so on). Sample the RGB values on the colorchecker 18% target and take a note of them. Then, take the 2 minute, filtered shot with the colorchecker in it. Adjust white balance until the RGB values in the 18% grey target match the ones from your filterless shot, the ones corresponding to your desired white balance. Take a note of the Kelvin / Tint values for your obtained WB. Finally, open your "normal" long exposure shots, those without the colourchecker in them, and apply your obtained Kelvin / Tint values to them. They should now all look as you like them;
2. Get a better (more neutral) filter and save yourself a lot of time and hassle;

Hope this helps! Best regards,

Vieri

[Vieri]

Thank you, Vieri.

If lighting conditions remain the same, shouldn't RGB readings from 18% card match in two shots- with and without ND filter? They don't and that is the puzzling thing. I'll give it a try dialing in tint and temperature following your recommendations.

Do you give any thought to the color profiling off the ColorChecker, or only use it for the WB?

Finally, the scenario I described - with strongly IR emitting subjects is the one where most of the tint is noticeable. Most of the time the effect is small and often even pleasing, but to avoid it completely an ND filter would require a very strong IR blocking layer. With that the WB will be accurate but image will blur. My filter is not the worst out there but rather top of the line multi-coated Heliopan. I like Heliopans, both ND and CP, because they do not degrade image sharpness.

You are very welcome. About your questions:

1. shouldn't RGB readings from 18% card match in two shots- with and without ND filter?

Yes they should - if the ND filter is neutral. The fact that they don't means that the filter is not neutral;

2. Do you give any thought to the color profiling off the ColorChecker, or only use it for the WB?

I never use the color checker in the field, neither for colour profiling nor for WB;

3. Finally, the scenario I described - with strongly IR emitting subjects is the one where most of the tint is noticeable. Most of the time the effect is small and often even pleasing, but to avoid it completely an ND filter would require a very strong IR blocking layer. With that the WB will be accurate but image will blur. My filter is not the worst out there but rather top of the line multi-coated Heliopan. I like Heliopans, both ND and CP, because they do not degrade image sharpness.

I don't think what you see is infrared. I think your filter have a magenta colour cast instead, but without seeing the filters myself I cannot be sure.

Hope this helps, best regards

Vieri

[Charles2]

IR distortion of colors occurs mostly in the shadows, if experience with a Leica M8 is any guide.

You could put an IR cut filter on top of the ND filter.

[JCM-Photos]

I too have an older circular Heliopan ND filter and it is really bad in terms of colors consistency, I could never really use it because of its lack of anti-IR filtering.
The problem is that  filter strength is too week in near IR against strong visible light density, that is what creates variable shifts in other visible colors.
It was also a problem with the very first Leica digital cameras, they were retrofitted with an anti-IR sensor filter to solve the problem.

Today you have some much better filter brands with an integrated anti-IR layer with a constant and soft color cast that is easily removable.

[JCM-Photos]

ND filters like mine that are IR coated have a slight magenta light reflexion when I show at them from the side.

My Heliopan that has no IR filtration shows a neutral light reflexion like any normal glass surface.

[Vieri]

By the way, starting today Formatt-Hitech has a special sale offering 20% off your purchases: just use code VIERIB20 at checkout to enjoy the discount. Hope this helps, best regards

Vieri