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sgreszcz

Workflow for testing ND filter color fidelity?

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I have a question, not strictly about color grading, but I'm trying to put together a workflow for testing my ND filters for color fidelity before starting a long-term documentary project. I'm going to compare two different ND fader types (Heliopan, Aurora) and my fixed ND filters (Breakthrough 3/6/10 stop).

Here is my plan, bur please tell me if I'm missing something or completely wrong:

1) Stick with a single lens, single camera with a set color profile (GH5, natural)

2) I'll be using my son as a model (for skin tone checking) and an X-rite color checker video passport.

3) Custom balance with an expodisk (or maybe better to just set the camera to a fixed sunny/5600k for consistency?

4) Expose the shots for the 28% grey at 50% IRE???

5) Shoot some video at a few apertures (vary the shutter speed) without any ND filters as a baseline.

6) Take some video at various apertures with the ND filters - start at widest aperture (darkest ND) and stop down by 1 each time (varying the fader to get the right exposure)

Here is where I especially need suggestions to see how "accurate" the images are:

7) Use vectorscope to see how well the colour patches "fit" as well as the skintones. How can I tell how much "shift" there is from the baseline?

Thank you so much for any advice!

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8 hours ago, sgreszcz said:

I have a question, not strictly about color grading, but I'm trying to put together a workflow for testing my ND filters for color fidelity before starting a long-term documentary project. I'm going to compare two different ND fader types (Heliopan, Aurora) and my fixed ND filters (Breakthrough 3/6/10 stop).

Here is my plan, bur please tell me if I'm missing something or completely wrong:

1) Stick with a single lens, single camera with a set color profile (GH5, natural)

2) I'll be using my son as a model (for skin tone checking) and an X-rite color checker video passport.

3) Custom balance with an expodisk (or maybe better to just set the camera to a fixed sunny/5600k for consistency?

4) Expose the shots for the 28% grey at 50% IRE???

5) Shoot some video at a few apertures (vary the shutter speed) without any ND filters as a baseline.

6) Take some video at various apertures with the ND filters - start at widest aperture (darkest ND) and stop down by 1 each time (varying the fader to get the right exposure)

Here is where I especially need suggestions to see how "accurate" the images are:

7) Use vectorscope to see how well the colour patches "fit" as well as the skintones. How can I tell how much "shift" there is from the baseline?

Thank you so much for any advice!

I'm no expert, but assuming that you custom-WB and set exposure every time you change anything then that should be a good approach.

Changing the aperture is more likely to only test the lens and not the ND I would have thought?  Still, it's useful for testing before using that lens for your film.  I know lenses that change colour at different apertures, so this is a thing.  If you're going to go to the trouble of testing one lens then test each of your lenses, preferably at wide open and at something neutral like 5.6 of 8.0.  Considering this is a test you should also ensure you've gotten a steady and high CRI light source, even if you use natural light it can be subject to subtle variation between shots, or even between doing the WB and hitting record.

I think if you go back and forward between non-ND and ND and look at Vectroscope and Waveforms then that should tell you everything you need to know.

In a sense, as long as the ND isn't too far off neutral then having colour accuracy kind of doesn't matter as long as you're consistent throughout the film (by using the same ND).  Colour grading often skews colours, for example cooling the shadows and warming the highlights, and it's more about how the final look matches the emotional tone of your film.  You can go pretty far from neutral and still be fine, think about The Matrix or any Soderbergh film, or music videos where coloured lights heavily skew skin tones.

Good luck! :)

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I guess the important part is how easy it is to match the clips afterwards and not necessarily how much of a colour cast each filter has (even though the two obviously correlates).

Lack of sharpness, contrast and uneven exposure (X-pattern) would be harder to fix than a -200 magenta adjustment. Assuming those factors are taken care of, does anybody have any tips on how to match clips with different filters?

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Sounds unnecessarily complicated. Just custom WB before each shot at your shooting exposure using a grey card or expo disk.  ND, vario nd and lenses all can have different colour casts so this will level the playing field but Vario ND filters because they are polarising will have a much greater influence on the look of the image esp skin texture than any colour cast. 

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@UncleBobsPhotography @Shirozina 

I think it depends on what level of lighting variety there will be between shots, the tolerance the OP is willing to take in the final edit, and the level of time, skill, and software capability available for colour matching in post.

I've spent a lot of time trying to match different cameras in post (made more difficult than this situation because my cameras also had different colour science and were pretty bad quality cameras) and I've found that it can be really difficult to get acceptable matching, even if you half know what you're doing in grading.  Obviously ND filters vary far less than entirely different cameras, but the differences I couldn't overcome were so large that they'd be completely unusable in most commercial settings, so for the OP the difference of NDs might be practically relevant.  The fact they're asking about how to do a test, and that their approach is detailed and seems sound would also indicate they might be creating a higher quality product where there is less room for variations.

I could be wrong of course - we all obsess over tiny little details that don't much matter in the grand scheme of things (like cameras, colour science, etc!) 😂😂😂

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3 minutes ago, kye said:

@UncleBobsPhotography @Shirozina 

 The fact they're asking about how to do a test, and that their approach is detailed and seems sound would also indicate they might be creating a higher quality product where there is less room for variations.

 

If the final output requires high quality color fidelity then the first place to start is to use a better camera. Nothing wrong with the GH5 but it's not the best in terms of accurate colour or even 'high fidelity' colour. For a start I'd use HLG and not 'Natural' as a profile as this is a recognised gamut and gamma that can be interpreted more easily by a good NLE. If you are going to stick with the GH5 then again it just doesn't need to be that complicated - custom WB before each take.

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On 4/22/2019 at 8:24 AM, Shirozina said:

Sounds unnecessarily complicated. Just custom WB before each shot at your shooting exposure using a grey card or expo disk.  ND, vario nd and lenses all can have different colour casts so this will level the playing field but Vario ND filters because they are polarising will have a much greater influence on the look of the image esp skin texture than any colour cast. 

Yeah, I should probably use the expodisk and manually WB more often.  I do do that for static/interview type shots, but for run-and gun and changing light (sunny/cloudy or sun/shade) I rely on AWB.  The reason I'm getting a bit paranoid about that is that I was doing some shooting with one of my faders and got some nasty colour shifts that I didn't really notice when shooting.  I'd like to stay consistent as possible.  As convenient as the faders are for getting good exposure, I think the colours are better with fixed NDs.

Also, I've been playing around with no ND at all and just cranking the shutter speed.  Seems to work OK unless there is fast motion or water/splashing, etc.

I just wish there were more internal NDs or lower ISOs.  I can get away with a couple stops by shooting my GH5 in 4k/60 and 100ISO too.

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3 hours ago, sgreszcz said:

Also, I've been playing around with no ND at all and just cranking the shutter speed.  Seems to work OK unless there is fast motion or water/splashing, etc.

This is more popular than you'd imagine, although different people see this to varying degrees too.  I recently noticed a shot with a low shutter angle in Peaky Blinders which is a very high production-value show, so although I have no idea what caused them to do that, it was even present there.

I'm not sure how much this will help you though, as changing light conditions will still necessitate either AWB, or worse, WB corrections in post.

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