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Archive 2017 · How to compare ND filters color cast

  
 
ATX353
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p.1 #1 · p.1 #1 · How to compare ND filters color cast


Hello all,

Not sure if this is the right sub-forum to ask this question in. I have two brands of ND filters I would like to compare to each other to see which is more neutral. Could someone explain to me what the best way to do this is? I saw a post on DPreview, where someone had basically taken: 1. picture of a color checker at a set and constant WB with no filter, 2. a picture with the same settings but filter brand #1, and 2. same as the former with the second brand of ND filter.

Is this the best way to compare two different ND filters?

Thank you in advance!



Nov 22, 2017 at 01:03 AM
jharter
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p.1 #2 · p.1 #2 · How to compare ND filters color cast


Are they square or round? in either case, take a shot of the same scene with one filter and then the other. Compare RAW images in LR to see which is warmer or different. I am sure there are more complicated, precise methods, but that is a simple way to start.


Nov 22, 2017 at 01:18 AM
ATX353
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p.1 #3 · p.1 #3 · How to compare ND filters color cast


Awesome, that's what I figured, just didn't know if there was a certain WB or something that mattered. They're both square.


Nov 22, 2017 at 01:27 AM
JBPhotog
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p.1 #4 · p.1 #4 · How to compare ND filters color cast


Best to use a known target that way you can evaluate the deviations in post.

I would use a grey card, set a custom WB in camera off the grey card and shoot a frame, this is your control exposure. Of course you want it in full sun, no shadows on the target and no need to focus the lens.

Then slip in your first ND, shoot a frame, then you second ND shoot a frame.

In post, evaluate your control shot with the eyedropper, check RGB values, if all went well with your custom WB they should be R:128, G:128, G:128 or at least equal across all RGB values making allowances for +/- exposure.

Then evaluate your first and second ND with the eyedropper, any deviation in RGB values will show you that filters bias.



Nov 22, 2017 at 01:21 PM
ATX353
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p.1 #5 · p.1 #5 · How to compare ND filters color cast


Got it, thank you @JBPhotog! Could you expound a little for a novice like myself... Take a shot of the grey card, and how would I use that to set a custom WB?

After I get the custom WB, shoot the frames as you said without any shadows etc. Where exactly would I use the eyedropper to check the RGB values? Across the frame or a specific spot?



Nov 22, 2017 at 01:42 PM
JBPhotog
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p.1 #6 · p.1 #6 · How to compare ND filters color cast


ATX353 wrote:
Got it, thank you @JBPhotog@! Could you expound a little for a novice like myself... Take a shot of the grey card, and how would I use that to set a custom WB?

After I get the custom WB, shoot the frames as you said without any shadows etc. Where exactly would I use the eyedropper to check the RGB values? Across the frame or a specific spot?


You don't say which brand of camera you are using but most high end and pro bodies allow one to use a target as a WB value. My Nikon's allow me to press the WB button and select PRE then I shoot a grey card, the camera then uses this value for a custom WB. Check out your camera manual or tell us here and that could help sort it out.

You will want to fill the frame with the grey card so no other colour references can influence the custom WB. Take an exposure and then in post I would sample in the centre of the frame, if your software lets you drag guide lines then place a horizontal and vertical guide so they intersect in the centre of the frame, that's your sample point for the eyedropper.



Nov 22, 2017 at 02:40 PM
ATX353
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p.1 #7 · p.1 #7 · How to compare ND filters color cast


JBPhotog wrote:
You don't say which brand of camera you are using but most high end and pro bodies allow one to use a target as a WB value. My Nikon's allow me to press the WB button and select PRE then I shoot a grey card, the camera then uses this value for a custom WB. Check out your camera manual or tell us here and that could help sort it out.

You will want to fill the frame with the grey card so no other colour references can influence the custom WB. Take an exposure and then in post I would
...Show more


Cool, thanks! I'm shooting on a D850, so the manual basically states the same thing. Looks like I will have to shoot it in the same lighting I plan on doing the test. Does that mean anytime you shoot a gray card to set the WB its only useful for the shoot you're doing?




Nov 22, 2017 at 04:01 PM
Roland W
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p.1 #8 · p.1 #8 · How to compare ND filters color cast


You can also skip the custom white balance, and just compare images in a post processing environment. I would suggest you have a gray card and a white card in the image, and a color checker if you want. I would equalize the exposures using the gray card, and avoiding any over exposure of the white card. Obviously shoot with similar image framing and exactly the same lighting for the comparison of no filter and each of the ND filters. Then, in post processing, be sure that you are set to view the RAW with no color correction or other adjustments. Then, using a measuring tool in the software, or just by eye, compare the images to look for color cast issues.

One thing to note is that some graduated ND filters give noticeable color cast if the image is being lit with light that is high in UV content. This lighting is common in landscapes that are shot at higher altitudes, like 5,000 feet and above. Images with blue sky and clouds as subjects are especially sensitive to this effect. It is a lot more work to set that test up by going to those kind of locations, but you might try some tests with light on your white card from a UV flashlight to look for UV related differences.



Nov 22, 2017 at 04:26 PM
JBPhotog
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p.1 #9 · p.1 #9 · How to compare ND filters color cast


ATX353 wrote:
Cool, thanks! I'm shooting on a D850, so the manual basically states the same thing. Looks like I will have to shoot it in the same lighting I plan on doing the test. Does that mean anytime you shoot a gray card to set the WB its only useful for the shoot you're doing?



Yes, always set a custom WB in the same lighting as your subject, if the lighting changes so does your WB. A simple way to handle changing light in post is to have the subject hold or place a WB target in the shot, then eyedropper that target and click the WB adjustment in post software, done. Now it isn't always perfect but it will get you as close as you can, proper direction of the target card is important.



Nov 22, 2017 at 05:03 PM
ATX353
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p.1 #10 · p.1 #10 · How to compare ND filters color cast


@JBPhotog, okay, so I tried it out in my house at the following settings:

f5.6
ISO 1000 @ 16mm

Set the WB, via grey card, then shot the first picture, 2nd with a Blackstone 10 Stop, and 3rd with a Breakthrough 10 Stop.

I sampled the colors from identical spots on the photos, basically selecting all the photos and then a spot.

Control:
R: 165
G: 162
B: 153

Blackstone ND 10 Stop
R: 170
G: 161
B: 144

Breakthrough ND 10 Stop
R:153
G:148
B:140

So, if I am understanding correctly, the Breakthrough is much more neutral than the Blackstone, is that correct or perhaps did I screw up the method of testing?

Thank you.



© ATX353 2017


Control





© ATX353 2017


Blackstone ND 10





© ATX353 2017


Breakthrough ND 10




Nov 24, 2017 at 09:22 PM
JBPhotog
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p.1 #11 · p.1 #11 · How to compare ND filters color cast


There are too many variables not controlled in this method, as in minor position errors in the sampling area.

Please reread my suggestions, “only shoot a grey card” for your eye dropper sampling with the two different ND filters. Any RGB bias will give you a more accurate reading as to the shift. Your sample images induce unknown RGB values from the coloured wall.

To reiterate:
Set a custom WB in camera, only controlled lighting source as in Full Sun, no clouds, or a constant light source like a tungsten filament, not a fluorescent or LED flickering light source. Make sure the grey card is being fully illuminated.

Using that custom WB shoot your grey card, exposure #1, this is your Control. If you did everything right in the above steps, your eye dropper sampling should be R:128, G:128, B:128 or close to that based on exposure bias. The “critical” aspect is, all RGB values should be the same number.

Exposure #2: shoot grey card, insert or attach ND filter adjust shutter speed only for proper exposure as aperture value changes can influence vignetteing.

Exposure #3 shoot grey card, insert second ND filter and repeat, adjusting shutter for proper exposure.

Evaluate all three exposures in post noting your control RGB values, ND1 and ND2 RGB values will reveal any colour shift bias. Example, if your;
Control is R:128, G:128, B:128
ND1 is R:128, G:131, B:128, slight green bias
ND2 is R:128, G:128, B:135, slight blue bias
Or
ND1 is R:137, G:128, B:135, is a magenta bias
Etc etc.



Nov 25, 2017 at 01:19 AM
ATX353
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p.1 #12 · p.1 #12 · How to compare ND filters color cast


Ah, understood. I misunderstood thinking that I only needed to shoot the greycard for the WB. With that being said, if test on an ultra wide angle, does the gray card need to be in focus? Just thinking that at 14mm or 16mm I would need a huge grey card.

And forgive my ignorance, but just for clarification when you say “shoot grey card, insert or attach nd filter...”. You mean shoot the grey card with the filter attached correct?


Thank you!

JBPhotog wrote:
There are too many variables not controlled in this method, as in minor position errors in the sampling area.

Please reread my suggestions, “only shoot a grey card” for your eye dropper sampling with the two different ND filters. Any RGB bias will give you a more accurate reading as to the shift. Your sample images induce unknown RGB values from the coloured wall.

To reiterate:
Set a custom WB in camera, only controlled lighting source as in Full Sun, no clouds, or a constant light source like a tungsten filament, not a fluorescent or LED flickering light source. Make sure the grey
...Show more



Nov 25, 2017 at 01:29 AM
JBPhotog
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p.1 #13 · p.1 #13 · How to compare ND filters color cast


ATX353 wrote:
Ah, understood. I misunderstood thinking that I only needed to shoot the greycard for the WB. With that being said, if test on an ultra wide angle, does the gray card need to be in focus? Just thinking that at 14mm or 16mm I would need a huge grey card.

And forgive my ignorance, but just for clarification when you say “shoot grey card, insert or attach nd filter...”. You mean shoot the grey card with the filter attached correct?

Thank you!



Use a longer lens, say 50mm to 200mm, that way the camera and lens will not cast a shadow or shade the grey card. Remember this is just a quick test, you don’t even need to mount the filter, just make sure it covers the front lens element.

Yes, shoot the grey card with the desired filter.

The best set up is put the camera on a tripod, grey card mounted on a stand or unmovable device.



Nov 25, 2017 at 11:33 AM
ATX353
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p.1 #14 · p.1 #14 · How to compare ND filters color cast


JBPhotog wrote:
Use a longer lens, say 50mm to 200mm, that way the camera and lens will not cast a shadow or shade the grey card. Remember this is just a quick test, you don’t even need to mount the filter, just make sure it covers the front lens element.

Yes, shoot the grey card with the desired filter.

The best set up is put the camera on a tripod, grey card mounted on a stand or unmovable device.


Understood! Thank you! I will give it a shot.

If one kept the WB the same and shot a scene, would that also be able to show the end user if there is any significant color cast? I understand now, that with all the different colors in a scene it's impossible to get an accurate judge of color cast, but if a filter had bad enough cast, wouldn't it show in a scene given the WB stays the same?

Thank you so much @JBPhotog!



Nov 25, 2017 at 11:38 AM
JBPhotog
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p.1 #15 · p.1 #15 · How to compare ND filters color cast


ATX353 wrote:
Understood! Thank you! I will give it a shot.

If one kept the WB the same and shot a scene, would that also be able to show the end user if there is any significant color cast? I understand now, that with all the different colors in a scene it's impossible to get an accurate judge of color cast, but if a filter had bad enough cast, wouldn't it show in a scene given the WB stays the same?

Thank you so much @JBPhotog@!


Yes and no. Here's the problem with shot of a scenic of full colour subject and trying to determine if the filter is neutral. There are way too many variables such as subject matter, what 'you' perceive as a colour cast and what other influences may be affecting the subject. I don't wish to sound pedantic but, you are essentially doing a science experiment and without a control you have no idea what the results are telling you. The benefit of shooting a known target such as a grey card is, you have a starting point at which you can influence colour bias in post.

Say your ND1 has a magenta bias, R:140, G:128, B:140 because you determined that through your shots of a grey card. Then in post you can eliminate that bias with R:012 and B:012 in a Hue and Saturation layer, essentially achieving neutral.

Okay, its all your now.



Nov 25, 2017 at 07:10 PM
GroovyGeek
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p.1 #16 · p.1 #16 · How to compare ND filters color cast


ATX353 wrote:
Hello all,

Not sure if this is the right sub-forum to ask this question in. I have two brands of ND filters I would like to compare to each other to see which is more neutral. Could someone explain to me what the best way to do this is? I saw a post on DPreview, where someone had basically taken: 1. picture of a color checker at a set and constant WB with no filter, 2. a picture with the same settings but filter brand #1, and 2. same as the former with the second brand of ND filter.

Is this
...Show more

Others have probably already pointed this out, but

a) I am yet to see ANY strong ND filter that is truly neutral. All that I have tried (B+W, Hoya, Haida, SR, Lee, Fotodiox), without exception, require custom white balance to get the color right, and all induce a color cast to some extent. This is particularly true of the strong filters, e.g. 10-stop

b) What matters far more, IMO, is not that the filter is neutral (in the sense that it does not change the image) but that it does not impart a color cast which can be near impossible to correct in PP. For example, B+W 10-stop filters tend to produce a purple cast. Using a grey card and then measuring the deviation from 128.128,128 will not help you in this respect, since you are only looking at one color.

c) If you really want to get into measurebating you should shoot a colorchecker card with and without the filter, correct WB on the middle-grey color, then measure the colors of all other sectors and plot the deviations from known values along each of the R, G, and B axes. The most neutral one will be the one that minimizes theses. You will have to come up with a metric of some sort to help you make a numeric decision or figure out how to plot in 4D :-)

d) Pigment-based filters and filters on resin supports tend to do the worst. The cheapish Cokin P filters tend to be particularly problematic. See my comment above about the older vintage B+W (FPRO series). Completely inorganic filters, for example sputtered Ti on high quality glass, tend to do better. Neither one is perfect.

e) There are plenty of reviews out there, people have done the measurebating for you, for example
https://fstoppers.com/bestfiltertest


My advice is to forget the measurebating. Take and image with the filter and then see what happens as you lower the temperature in PS. Is it possible to approximately recover the tonality of the image taken without the ND filter? If yes just go with it and don't sweat it too much. I find that the very reasonably priced Haida filters are perfectly adequate for landscape work, where you are likely to be contorting the tonality to a significant degree anyway. If you are doing product photography and need true colors then... ditch the NDs.



Nov 26, 2017 at 04:03 AM
ATX353
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p.1 #17 · p.1 #17 · How to compare ND filters color cast


GroovyGeek wrote:
Others have probably already pointed this out, but

a) I am yet to see ANY strong ND filter that is truly neutral. All that I have tried (B+W, Hoya, Haida, SR, Lee, Fotodiox), without exception, require custom white balance to get the color right, and all induce a color cast to some extent. This is particularly true of the strong filters, e.g. 10-stop

b) What matters far more, IMO, is not that the filter is neutral (in the sense that it does not change the image) but that it does not impart a color cast which can be near impossible to
...Show more

Awesome, thank you for your input. I agree, now that I've done a miniature test, I'm not going to sweat it.



Nov 26, 2017 at 09:48 AM
sjms
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p.1 #18 · p.1 #18 · How to compare ND filters color cast


i think, though, you are going to find out what fstoppers did. the BT is currently the "cleanest" of the bunch.

Edited on Nov 26, 2017 at 10:06 AM · View previous versions



Nov 26, 2017 at 09:54 AM
ATX353
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p.1 #19 · p.1 #19 · How to compare ND filters color cast


@sjms, yes that is what my test showed me in my other thread, although the Pro-Grey wasn't far behind.


Nov 26, 2017 at 09:56 AM
sjms
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p.1 #20 · p.1 #20 · How to compare ND filters color cast


they also took into account vignette and such too. the overall product


Nov 26, 2017 at 10:07 AM
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