Are Neutral Density Filters Really Neutral?

Peter Moretti · July 17, 2009

Besides the deep focus caused by stopping down a lot, very small apertures cause excessive diffraction that degrades the image.

The answer to these problems is to use neutral density filters. But are ND filters truly netural? Do they really not degrade the image or change the color blanace in any way, no matter how strong the filter is?

Thanks much.

Brian Drysdale · July 17, 2009

I once checked my Tiffen NDs with a colour temperature meter and there was a touch of green in there. I've never noticed it in practise, but perhaps it could be an issue if you stacked them up.

Schneider claim their filters don't have this effect or you could use Panchro filters to avoid it.

Tom Jensen · July 17, 2009

I've never noticed it. If it does, it's negligible. Color can easily be corrected.

Paul Bruening · July 17, 2009

I can see the color difference between my Tiffens and Hoyas.

Phil Rhodes · July 17, 2009

My Formatt polariser came with a printout of the filter's optical density as measured on some high-precision laboratory instrument.

P

Chris Keth · July 18, 2009

Schneiders look neutral to me while tiffen NDs look a little brown or warm. I haven't done any real testing of that, though.

Dominic Case · July 19, 2009

Consensus here seems to be that Tiffens are a little brown. Or green. Or that you can't spot the difference. Or that you can, (but not enough to specify what the difference is?).

Even if you don't see a difference by eye, a filter may be off-balance at the UV or IR end of the spectrum. Carbon filters are much more uniform across the entire spectrum than dye filters.

It is also possible that some dye filters will fade over time, so while a new one could be neutral, older ones could pick up a colour bias.

In practice, a slight colour shift can usually be corrected. Some lenses have a slight colour shift too.

K Borowski · July 20, 2009

In practice, a slight colour shift can usually be corrected. Some lenses have a slight colour shift too.

Well, if you want to be really picky about it, *nothing* is really neutral. This is the same impossible feat in nature as drawing a straight line, or a perfectly right angle.

But, in practice, with a nice, new filter that hasn't been subjected to excessive light exposure, the results are neutral-enough that the human eye usually can't discern any color cast.

For best judgement though, one should really consult with a woman that has 4-color vision. Ironically-enough, women who have this ability to discern 4 colors (two different types of green, IIRC) are the same ones that carry the gene for color-blindness.

John Sprung · July 21, 2009

For best judgement though, one should really consult with a woman that has 4-color vision. Ironically-enough, women who have this ability to discern 4 colors (two different types of green, IIRC) are the same ones that carry the gene for color-blindness.

These women are extremely rare. There's a Mrs. M in England who has been determined to be a true tetrachromat, and there may be a few others worldwide. The reason it's restricted to women is that the red and green color genes are on the X chromosome. (Blue is on chromosome #7).

For red and green there are three possibilities: normal, shifted color, or no sensitivity at all. In searching for tetrachromats, what the scientitsts do is look for males who have the shifted green or shifted red. Their moms and daughters have the potential to have one X chromosome with the shifted color and another with the normal color. If dominance happens cell by cell in their retinas, they can have a mix of normal and shifted green cones, or normal and shifted red cones. If their brains also develop the ability to process the difference, they see the world in four rather than three primary colors. But it's only very rarely that all the pieces fall into place, and a woman gets true tetrachromatic vision.

Some of the colorblindnesses are dichromatic, the person sees only two rather than three primaries -- sort of like early two color film. To the tetrachromats, all the rest of us are a little bit color blind, seeing only three rather than four primaries. Do the tetrachromats get some wonderful new experience of the world? Not really. The only practical effect seems to be that they can see mis-matches in re-touch paint work that seems to the rest of us to match.

And to top it all, consider that pigeons have pentachromatic vision -- they see in five primary colors.

-- J.S.

Leo Anthony Vale · July 21, 2009

Besides the deep focus caused by stopping down a lot...

You make it sound like deep focus is an abberation, instead of an artistic, stylistic choice.

Peter Moretti · July 21, 2009

Well I certainly didn't mean that.

Hal Smith · July 22, 2009

These women are extremely rare............Do the tetrachromats get some wonderful new experience of the world? Not really. The only practical effect seems to be that they can see mis-matches in re-touch paint work that seems to the rest of us to match.

I wonder if Natalie Kalmus was a tetrachromat? Apparently she used to drive Directors, Cinematographers, and Art Director nuts with her observations about what colors would and wouldn't work for three strip Technicolor photography.

John Sprung · July 22, 2009

Very unlikely that she was. In the two color days, they really needed someone who could tell them by experience what the colors would come out like -- for instance, that they were in danger of having the rocks in the BG look exactly the same as the cowboys' faces. Three strip had sufficient gamut for pretty much all practical colors, but they kept the consultant job mainly to protect Technicolor's "look" and reputation.

Later on they loosened up on that and let the DP's use it more creatively. John Alton did some amazing dance numbers for "An American in Paris" in 1951, no artificial restrictions on saturation or contrast.

-- J.S.

Tom Jensen · July 22, 2009

You make it sound like deep focus is an abberation, instead of an artistic, stylistic choice.

It's pretty obvious what he means since the topic is Neutral Density Filtration and the only reason you use them is to open the aperture. In this case, it is an abberration but the use of these filters is an artistic choice. It's all relative.

Edited July 22, 2009 by Tom Jensen