Operation of Variable Neutral Density Filters

[John Vickers]

Hi Folks,

Variable Neutral Density filters do - somewhat, see below - what they say.

They are primarily used for shooting video, particularly video that is somewhat "run-and-gun", where the scene cannot be blocked, and rapid changes in attenuation are required. An example might be wildlife photography, where the camera operator is following the subject through broken woodland. Some minor losses in sharpness and colour accuracy relative to good-quality fixed ND filters are accepted in exchange for convenience in the field.

The basic principle of operation is crossed polarisers: If the polarisers are parallel, they let light through; if they are crossed, they block light. [Some of you may see a problem already... ;-) ]

My understanding of how to use some kinds of variable ND ("VND") filters differs from some of the manufacturers recomendations, and from descriptions on the interwibble.

I'd be grateful if folk could tell be what I'm missing, or if I have the right idea. I'm a bit rusty on the maths and physics of polarisation.

These are VND filters screwed to the front of the lens.

To make life simpler, to avoid getting sidetracked in second order details, to start with at least, can we assume:

• Polarising filters work perfectly across the optical frequency band, understood to include all frequencies to which the sensor might be sensitive.
• Any Quarter-Wave-Plates ("QWP") provide a precise lambda/4 aka pi/2 phase difference between slow and fast axes across the optical frequency band.
• Where QWPs and polarisers are physically bonded to each other, the axes of the polariser perfectly bisect the fast and slow axes of the QWP.
• The two polarising elements can rotate only between parallel polarisation, and a few degrees short of cross polarisation. [Often there are no stops limiting rotation between the polarisers, but this assumption covers practical cases, and perhaps makes some arguments simpler]

Constructions for VND filters:

There seem to be a few common kinds:

1. Scene ->QWP->polariser->mechanical rotation->polariser->QWP->lens
2. Scene ->polariser->mechanical rotation->polariser->QWP->lens
3. Scene ->polariser->mechanical rotation->polariser->QWP->mechanical rotation->lens

The final QWP seems to be a concession to SLR autofocus and metering, though AFAICT VND filters are mainly used for video.

Marks on the holders of the first and second elements almost invariably advertise an attenuation scale: a "Minimum" mark indicates that the polarisers are parallel; a "Maximum" mark indicates that the polarisers are nearly crossed. Sometimes a numerical scale between "Min" and "Max" is present, usually marked in arbitrary units with constant angular spacing. The front element may rotate freely, or there may be stops at the "Min" and "Max" positions.

Examples:

1: Front and rear QWPs:

• Tiffen VND: https://www.ephotozine.com/article/tiffen-variable-nd-filter-review-20475
• Hoya NDX

2: Rear QWP only; orientation of rear polariser fixed if filter is screwed in:

• SRB VND: https://www.ephotozine.com/article/srb-variable-nd-filter-review-20451
• Recent Hoya (non-NDX) VND

3: Rear QWP only; polarisers can be rotated independently:

• Genustech POLARIZER ND Variable Filter: https://www.genustech.tv/Polarizer-ND-Variable-Filter-s/177.htm
• SLR Magic VND II. The SLR magic does not have rotating mount as such: the second rotation is on the filter threads: a locking ring is provided to allow locking the rotation of the second polariser.

Filters of type (3) are advertised as providing control of both polarisation and attenuation.

The polarising effect of filters of type (2) is not advertised. These are usually the cheapest VND filters.

Behaviour:

[Propositions I understand as true]

1. Given a scene reflecting unpolarised light towards the camera, all three constructions behave similarly: attenuation is adjusted by changing the angle between the two polarisers. The orientation of the polarisers relative to the scene does not matter.
2. Only filters of type (1) (front QWP) behave as polarisation-neutral VNDs. Any scene polarisation is removed (replaced with circular polarisation).
3. In any VND, selection of polarisation is only controlled if the first (front) element is a polariser, not a QWP.
4. If polarisation control is available (no front QWP), it is only available via rotation of the front element. [This may seem stunningly obvious, but is contradicted by both reviews, and some manufacturers' instructions].
5. If the orientation of the front polariser (no front QWP) is fixed, rotating the second polariser has a purely ND effect: there are no relative changes in intensity of different parts of the scene.
6. (No front QWP) If the orientation of the second polariser is fixed, rotating the front polariser in the marked direction of increasing attenuation may increase or decrease the intensity of scene elements, depending on the polarision of light reflected towards the camera by these elements. Similarly, rotating the front polariser in the marked direction of decreasing attenuation may decrease or increase the intensity of scene elements. Maximum transmission might occur half-way between the "Min" and "Max" attenuation markings. This is a manifestation of the "Three polariser Paradox": the user has a polarisation rotator, not a variable ND filter.

Have I got those right?

Implications for use:

1. In filters with no front QWP, rotating the front element while leaving the rear element fixed with the intent of changing overall scene attenuation will cause both hue shifts and relative intensity changes within a scene which reflects polarised light (i.e. much or most of the time).
2. In filters with no front QWP, and two rotating stages, control of both polarisation and attenuation can be acheived by (a) rotating the front element to achieve the desired polarisation effect, and then (b) rotating the rear element - while keeping the orientation of the front element fixed - to achieve the desired attenuation.
3. Filters with no front QWP, and only one rotator can be fairly easily converted to filters with two rotators by screwing the mount for a cheap circular polarising filter or ND grad onto the back (after removing the retaining ring and filter glass from the cheap filter). Amazon Basics Circular Polarizer works for me. [Oddly, the Amazon CIR-PL is really not bad. Not the very best, but better than many far more expensive filters]. The downside of this is increased hard vignetting with wide angle (never mind UWA) lenses.

Other comments:

1. AFAICT, what the user wants from a two-rotatator, no front QWP, VND is to set and lock the polarisation using the rear rotatable control, and then adjust the attenuation using the front control. Unfortunately the front element controls polarisation, and the rear attenuation, so while this may be physically possible, it requires a Rube Goldberg device [British English: "Heath Robinson Contraption"; French: «usine à gaz»] to make it work.
2. I've seen a few videos that review or discuss these things, but none of them demonstrate an understanding of what's happening. Here's a Genustech about the Genustech filter (no front QWP; 2 rotators):
   . If you recover from the facepalm at the start when they explain that their filter is not called "Solar Eclipse" any more, there's another facepalm when their "tech" guy explains how to use the filter.

 

[Bernard Delley]

thanks for making me aware of the issue with no front QWP !

I have not used my Hoya variable ND very frequently, since a ND usually requires also using a tripod. I hate to carry a tripod, easily 1kg or more, if I do not have specific ND plans on a hike.

The Hoya variable ND has no front QWP, so it acts as a polarizing filter in addition to its variable ND capacity. Knowing about the issue now, when polarization matters in an image, I can put it to use. I can set the desired attenuation first. Then, I can grab both rings and slightly unscrew the filter, by less than 180 degrees, to set the desired polarization.

 

[John Vickers]

The Hoya variable ND has no front QWP, so it acts as a polarizing filter in addition to its variable ND capacity.

It depends how old your Hoya VND is - it seems that older (non-NDX) Hoya VNDs have a QWP at the front, but current ones do not.

You can check by setting it to minimum attenuation, and looking through it from the back at an LCD monitor. If you can make it go black by rotating the whole assembly, you don't have a QWP.

More discussion of The two models, and how to quickly tell the difference (they have different markings) here (and in related posts): Re: VND filter recomendations

Knowing about the issue now, when polarization matters in an image, I can put it to use. I can set the desired attenuation first. Then, I can grab both rings and slightly unscrew the filter, by less than 180 degrees, to set the desired polarization.

That'll work. Personally, I find it easier to set the polarisation first (at minimum attenuation). Then I don't like AF-ON.

I'm a bit paranoid about the thing falling off if I leave the threads loose.

Hence my comments about dismantling an Amazon Basics CIR-PL, and using the rotating mount from that.

Unscrewing the filter retaining ring is a bit like undoing a stuck filter:

1. Find a top-quality flat-bladed screwdriver that fits well in the slots. Of course if you happen to have the right special tool for assembling filters, that's great.
2. Give one of the slots a push with the screwdriver. Not too hard, we don't want to slip and break something.
3. Tap the edge of the filter gently on a hard surface all way round the filter.
4. Give the other slot a push with the screwdriver.
5. Tap the edge of the filter gently on a hard surface all way round the filter.
6. Repeat 2-5, while gradually increasing the force of the screwdriver on the retaining ring. There's no threadlock.

Odd thing is, I did some tests on the Amazon Basics CIR-PL, and it's not bad at all. It has quite low attenuation, and the (white balanced) colour accuracy is good. I won't be binning my Hoya Fusion CIR-PL filters just yet, though.

 

[Bernard Delley]

The Hoya variable ND has no front QWP, so it acts as a polarizing filter in addition to its variable ND capacity.

It depends how old your Hoya VND is - it seems that older (non-NDX) Hoya VNDs have a QWP at the front, but current ones do not.

You can check by setting it to minimum attenuation, and looking through it from the back at an LCD monitor. If you can make it go black by rotating the whole assembly, you don't have a QWP.

Sorry, I got fooled by using hazy sky and and a non-metallic surface as light source. Using light through a PL or my MacBook Retina screen, its now clear that my Hoya 77mm is QWP_PL Pl_QWP.

More discussion of The two models, and how to quickly tell the difference (they have different markings) here (and in related posts): Re: VND filter recomendations

I got the "old" MINI MAX version of the filter. Depending on incoming pol orientation the white balance goes from yellowish to bluish.

Knowing about the issue now, when polarization matters in an image, I can put it to use. I can set the desired attenuation first. Then, I can grab both rings and slightly unscrew the filter, by less than 180 degrees, to set the desired polarization.

That'll work. Personally, I find it easier to set the polarisation first (at minimum attenuation). Then I don't like AF-ON.

I'm a bit paranoid about the thing falling off if I leave the threads loose.

Since incoming polarization is mostly a WB issue, it can be cleaned up in post processing. So no need for loose threads then.