Correcting Neutral Density Filter "X" Factor

[Chrisd999]

I recently acquired a variable ND filter for my 5D3, which I am planning to use mostly for landscape still pictures. The version I have is the Rodenstock Digital Vario ND MC. Below are a set of pictures I took with varying shutter speeds to test the filter out. The first shot is without the filter, and the rest are with the filter on at increasing levels of darkness. After pixel peeping at 200%, I was amazed at how this filter maintained sharpness throughout the range, although there was a small loss of contrast noticeable after about 5 stops.

However, to my dismay, there appears to be some severe "X" shaped vignetting over the entire image which becomes noticeable at 4-6 stops. Beyond that it is horrible. It is more of an "hourglass" shape in the samples below, and I noticed about a 2 stop different in exposure between the center and sides of the image in the 6-7 ND stop range. I had read this was an issue with all variable ND filters, particularly at wide focal lengths, but I did not realize the effect would be this bad.

I know this can be manually corrected in Photoshop, but that seems like tedious work, and I was wondering if there are any easy solutions to correct this in PP?


Canon 17-40 at 17mm f/11 1/6s - No filter


Canon 17-40 at 17mm f/11 0.4 s - with filter - about 1.5 stops


Canon 17-40 at 17mm f/11 1s - with filter - about 3 stops


Canon 17-40 at 17mm f/11 4s - with filter - about 4.5 stops


Canon 17-40 at 17mm f/11 15s - with filter - about 6.5 stops


Canon 17-40 at 17mm f/11 30s - with filter - about 7.5 stops

 

[mikes]

I barely see the vignetting versus the flash exposure. To be fair, re-do the test with no flash on an evenly toned background, or wall inside the house. Vignetting will then be apparent.

 

[Chrisd999]

mikes wrote:

I barely see the vignetting versus the flash exposure. To be fair, re-do the test with no flash on an evenly toned background, or wall inside the house. Vignetting will then be apparent.

 

[mikes]

It is still best to shoot a big wall for these tests. If no flash was used then yes, I would be concerned as well. I have never seen vignetting like that before, and I have many polarizers and neutral density filters including a 10x. But none that exhibit that!

 

[tony brown]

Hundminen wrote:

I recently acquired a variable ND filter for my 5D3, which I am planning to use mostly for landscape still pictures. The version I have is the Rodenstock Digital Vario ND MC. Below are a set of pictures I took with varying shutter speeds to test the filter out. The first shot is without the filter, and the rest are with the filter on at increasing levels of darkness. After pixel peeping at 200%, I was amazed at how this filter maintained sharpness throughout the range, although there was a small loss of contrast noticeable after about 5 stops.

However, to my dismay, there appears to be some severe "X" shaped vignetting over the entire image which becomes noticeable at 4-6 stops. Beyond that it is horrible. It is more of an "hourglass" shape in the samples below, and I noticed about a 2 stop different in exposure between the center and sides of the image in the 6-7 ND stop range. I had read this was an issue with all variable ND filters, particularly at wide focal lengths, but I did not realize the effect would be this bad.

I know this can be manually corrected in Photoshop, but that seems like tedious work, and I was wondering if there are any easy solutions to correct this in PP?

There is an inherent problem with a double polarising filter due to selective colour alteration, internal reflection between the two filters and the fact that polarisers have directional characteristics. A circular polariser no less so. A circular polariser consists of a conventional linear layer at the front followed by a bi-refringent layer nearest the camera which outputs the light in two preferential waves at right angles to each other. (Rough description only - plenty on-line.)

In Rodenstock's catalogue, you can read the inevitability of a cruciform output exposure distortion and colour shift as follows:-



I use a homemade version using two polarisers - a linear followed by a circular (nearest the camera) but only for motion blurr effects as in waterfalls, etc. and only weak enough to get the effect and only VERY occasionally. I don't even know what stops reduction I get, as I just rotate to taste. Certainly, the colour bias does shift towards blue if I turn it up too much.

If the outputs are crucial to you, you would be better taking a small set of plain (non-polarising) Neutral Density filters and just adjusting the apertures to get any required intermediate values.

Cheers, Tony.

 

[christiangrunercom]

You can correct this issue with a LCC shot through a white plate.




This can be fed into Capture One, and it will remove the x-shaped vignette, colorcasts, and dust specs.




More info here: http://www.luminous-landscape.com/techniques/iq180_lcc_correction.shtml and here: http://help.phaseone.com/en/CO7/Editing-photos/Lens-Correction.aspx#item6

 

[Chrisd999]

tony brown wrote:

Hundminen wrote:

I recently acquired a variable ND filter for my 5D3, which I am planning to use mostly for landscape still pictures. The version I have is the Rodenstock Digital Vario ND MC. Below are a set of pictures I took with varying shutter speeds to test the filter out. The first shot is without the filter, and the rest are with the filter on at increasing levels of darkness. After pixel peeping at 200%, I was amazed at how this filter maintained sharpness throughout the range, although there was a small loss of contrast noticeable after about 5 stops.

However, to my dismay, there appears to be some severe "X" shaped vignetting over the entire image which becomes noticeable at 4-6 stops. Beyond that it is horrible. It is more of an "hourglass" shape in the samples below, and I noticed about a 2 stop different in exposure between the center and sides of the image in the 6-7 ND stop range. I had read this was an issue with all variable ND filters, particularly at wide focal lengths, but I did not realize the effect would be this bad.

I know this can be manually corrected in Photoshop, but that seems like tedious work, and I was wondering if there are any easy solutions to correct this in PP?

There is an inherent problem with a double polarising filter due to selective colour alteration, internal reflection between the two filters and the fact that polarisers have directional characteristics. A circular polariser no less so. A circular polariser consists of a conventional linear layer at the front followed by a bi-refringent layer nearest the camera which outputs the light in two preferential waves at right angles to each other. (Rough description only - plenty on-line.)

In Rodenstock's catalogue, you can read the inevitability of a cruciform output exposure distortion and colour shift as follows:-



I use a homemade version using two polarisers - a linear followed by a circular (nearest the camera) but only for motion blurr effects as in waterfalls, etc. and only weak enough to get the effect and only VERY occasionally. I don't even know what stops reduction I get, as I just rotate to taste. Certainly, the colour bias does shift towards blue if I turn it up too much.

If the outputs are crucial to you, you would be better taking a small set of plain (non-polarising) Neutral Density filters and just adjusting the apertures to get any required intermediate values.

Cheers, Tony.

Thanks for the info Tony. Nice to see the manufacturer forthcoming with the issue, but I wish the camera shop salesman shared that with me when he advised me to buy the variable ND. In hindsight, I think a fixed stop ND would have been a better choice.

 

[Chrisd999]

christiangrunercom wrote:

You can correct this issue with a LCC shot through a white plate.

This can be fed into Capture One, and it will remove the x-shaped vignette, colorcasts, and dust specs.

More info here: http://www.luminous-landscape.com/techniques/iq180_lcc_correction.shtml and here: http://help.phaseone.com/en/CO7/Editing-photos/Lens-Correction.aspx#item6

 

[MagicAngel]

Not only that, but I think you'll be really disappointed in variable ND overall. ND is very useful, when it is needed, but one or two ND filters is really all you will ever need, IMO. I think you will find that you either don't need it, or you need it on its full setting more often than not.

Personally, I always find myself thinking I need a bit more ND than I actually have. I don't think I've ever said, "Wow, if I only had a little less ND...".

As for the X, I'm thinking that it looks far worse on that scene than it will in general use. Just really bad luck that the X feel on that tree the way it did.

 

[MagicAngel]

Correcting it in LR4 is fine, but the big danger in that is that you are losing dynamic range in those areas. Some scenes are going to fall outside of your ability to fully correct them as you will lose the ends of your curve.

 

[The Rising Tide]

I have a Promaster Variable ND filter for my Canon Rebel T5 and once I get over 5 stops the X starts to appear. I've scoured the net for advice and I think the best thing to do is just stick with a ND filter that is not variable. I would just buy a 8, 9, or 10 stop ND filter and only use that, just like the person above me said, you are always wishing for more stops, so just get the highest and stick with that.