PolarPro’s Peter McKinnon Edition Variable ND filters eliminate the dreaded X issue

April 12, 2019 by John Aldred 4 Comments

Last week, PolarPro announced their new Peter McKinnon Edition Variable ND filter. PolarPro has been at NAB 2019 this week and so has DIYP. So, we went over to talk to them and find out a little more about them.

The filter comes with both hard and soft cases, as well as one of PolarPro’s Defender caps designed specifically for the variable ND filters. They’re “slim” filters and as such, they have no front thread. This means you can’t just use your regular lens cap to protect the filter in your bag. The Defender goes on and comes off easily, though, and holds very strong when it’s attached.

I’ve been using the 2-5 stop version of the filter myself for a couple of weeks now and so far I’m very impressed. Each of the filters features hard stops at the 5 and 9 stop limits to prevent the dreaded X issues we often see with stronger or less expensive variable ND filters. They also claim the lowest refractive index of any variable ND filter on the market to further minimise this issue. Personally, I’ve not experienced it myself yet, even with the filter all the way down to 5 stops and relatively wide angle lenses.

The prices have changed slightly since the initial announcement. The 2-5 stop version is still $249.99, and the 6-9 stop version is still $299.99, but the package price for the pair is $449.99, and not $499.99 as originally reported.

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About John Aldred

John Aldred is based in Scotland and photographs people in the wild and animals in the studio.

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Steven Naranjo

I’d like to see it on the sigma 18-35 1.8.
Timothy Rucinski

Works with a lens hood?
Hmmm

Any reason that you decided not to describe to your readers of what the dreaded X problem is? Today’s journalists suck.
- Volker Bartheld
  
  Just in case you still bother to know the reason:
  
  The “dreaded x issue” relates to a behaviour that is inherent to variable ND filters very close to maximum extinction (=attenuation). There will be uneven absorbtion of light – often resulting in an x-shaped area, hence the name. There will be quite prominent color tints, typically purple, pink or blue, there will be a slight softness of the image, there will be a high sensitivity towards even the slightest touch of the attenuation ring and other evil things.
  
  This inevitable because of the way a variable ND filter works, the device essentially being two linear polarizers stacked together plus a so-called Lambda-1/4-plate (aka. wave plate or retarder), which slows down wavefronts parallel to its optical axis differently to those orthogonal to the optical axis, essentially creating circular polarized light from linar polarized light. This is necessary in order not to confuse the autofocus mechanism, which reacts sensitive to linar polarized light because of the reflective surface of the mirror being in the optical path. Mirrorless cameras might be immune to that, I don’t know.
  
  If you now rotate the one of the two polarizing filter out of the parallel position, light will be attenuated proportionally to the cosine of the two axes. So at 90° (=polarizers crossed), there should be theoretically 100% absorption, the image completely dark. In reality, the polarizers (made from a stretched plastic foil) can not polarize the light entirely, there’s always some leakage. Also, the molecules in the foil are not arranged perfectly parallel and in an equidistant manner. This results in an angle error. Furthermore, there’s typically a tiny warpage of the plate(s) in an either convex or concave way, because of the fixture and other reasons. And, last not least, polarization efficiency/behaviour also depends on wavelength, polarization being much less efficient towards the blue end of the spectrum.
  
  You typically can’t see the impact of all those effects, until being very close to maximum attenuation, hence the color tint and uneven illumination.
  
  Polarpro claims to have “solved” that problem, but in fact, they haven’t. In reality they just take better quality glass to guide the foils (yes, there still needs to be a foil – so far, I did not hear of any photographic polarizer that uses Herapathit cristals or a Nicol Prism nowadays because of handling difficulties and price) and then add a hard stop the limit the rotation/attenuation to a range where the problem isn’t that pronounced. Since the description says “2-5 stop” or “6-9 stop” and a polarizer foil – by design – attenuates the light by about 2 stops (it essentially filters out at least 50% all the time) – I would guess that the first variant is just a hard stop variable polarizer and the second one a hard stop variable polarizer where one of the glass plates has 4 stops of constant attenuation (ND4).
  
  That said: A variable ND filter (at least one based on two polarizer foils) is never a replacement for the dicrete version. As said before, there will always be polarization which changes the original image, espacially if blue sky or reflections from nonmetallic surfaces are involved. There is a reason why high quality fixed ND filters are quite costly, especially those with higher values. It is hard to produce dark optical glass that has an even attenuation across the entire surface in the entire visible spectrum without any parasitic reflections and light leakage.
  
  HTH.