ND filters

Posted on 30 May 2012

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There are a few choices to make here: fixed ND or vari ND, round or square, big or small, mattebox or screw-on? I’ll start with the first:

  • A vari ND is made of two polarizers that work together. This could have negative side-effects, especially with 3D recording. But it also works very fast to adjust lightning. Best choice for ENG-style shooting.
  • Fixed ND filters do not have the negative side-effects, but you will need different ones and might need to stack them, which  will slow you down and could lead to muddy images or internal reflections. Best choice for film-style shooting.

Fixed ND

To prevent muddy images you have to be aware that traditional ND filters have IR blocking included in each filter and the best of these cut 99% of the IR, 25% of the near-IR, and 12% of the red spectrum. Stacking 3 traditional ND filters cuts 99.999% of IR, 57.8125% of near-IR, and 31.8528% of the red spectrum (100%-[(100%-25%)^3], or 1 – .75^3). In this example, losing 31% of your red spectrum is what results in muddy images. That’s why there are IR and plain (no-IR) filters. You use one IR blocking filter and two plain (Tiffen indie) filters behind it if you need to stack your filters.

To prevent internal reflections between the filters, you’ll want to buy good filters: Schneider, Tiffen or another expensive brand. Or just buy a load of different cheap ND filters and avoid stacking them.

Vari-ND

I’m not sure if there are many negative side-effects of the more expensive vari-ND systems. Vari-ND’s will give the same uneven polarization on ultra-wide lenses as any circular or linear polarizer will do, like this wide-angle problem with skies. This might not even be solved with using a reversed circular polarizer as the front filter, but I will have to test that.

Also, the cheaper vari-ND will have a lot of color shifting or vignetting when turning it. There is a lot of useful information about vari-ND on 2filter.com. In general, they always take away 1 or 2 stops of your light (even the Schneider Optics is costing you at least one 1 stop), and you can then turn them to take out even more light. Prices for 82mm circular filters vary from $ 229 (Genus) to $ 501 (Heliopan).

Circular v.s. Linear polarizers

But I need  filters with at least 114 mm diameter. As far as I understand I should buy two circular polarizer filters, facing each other with their fronts so there will be no polarization effect. Circular is not to be confused with round, you can have a square circular filter. In polarization or vari-ND setups, circular means circular direction of the light waves.

This is because then first the quarter-wave retarder layer will depolarize all light, then the two polarizer layers will do their vari-ND thing, and then another quarter-wave retarder layer will make sure that there will be no problems with the light or autofocus system. This is what  _gl wrote about this: You can make a fader using two circular polarizers, facing each other. A circular is made of two layers, a linear polarizer, and a quarter-wave retarder. The retarder effectively unpolarizes light – so if you have the two circulars facing each other so that the retarders are on the outside, the incoming light is first unpolarized, then faded by the linears, then unpolarized again – so it should not filter incoming linear polarized light (like that from an LCD screen). Lineair polarizers are cheaper, but circular are needed for TTL and autofocus metering on DSLR camera’s. I’ve checked my idea with the technical department of Formatt and they confirmed that theoretically using the two circular polarisers as stated should work as a vari ND, and that you will not need the use of a Hot Mirror filter.

If I need polarization effect, I should get a third rotating polarizer in front of the vari-ND set. Or, maybe, turn around the first filter (not sure if it will still work as a vari-ND because of the quarter-wave retarder layer in between the two polarizer layers). Or, replace it with a linear instead of a circular filter. This I will have to test.

Sizes?

On the Schneider and Formatt websites I found that there are many different sizes of filters. Basically, you will have the screw-on filters up to 105 mm in size. Then you have the round filters (without screw-in ring) in different sizes like series 9, 4 1/2″, 105, 138 and 152 mm (6″) filters. These are designed to drop into a mattebox. And there are the square filters in 4×4, 5×5, 5,65×5,65, 6×6 and 6,6×6,6 inch, or the rectangular filters in 6×4 or 5,65×4 (panavision size) inch.

The last option would be to fit very small filters between the lens and the sensor. This is done in large format photography as well as with the ND filters in ENG camera’s. It would be nice to have this built into MFT adapters, but until then…

Screw-on, round or square?

The screw-on filters can be screwed on the front of the lens. This is not very useful with non-vari ND filters as you might want to change those a lot, and with the vari-ND it will only allow you to turn the front polarizer, since the back polarizer is fixed. If you are getting screw-on filters like these, you might want to consider buying the 82mm (or the 105mm Formatt Hitech Multistop screw-on filter) and put metal step-up adapter rings on all of your lenses. In that way, you can use your one filter on all of your lenses. Remember, when using filters or step-up on extra wide lenses, the ring width will possibly add to a vignetting issue. There are special step up rings for wide-angle lenses, that do add very little width to your lens. Also, make sure you buy screw-on filters that are able to take other screw-on filters on top, so you can stack them.

I highly recommend buying at least a clean glass screw-on filter for every lens you’ve got, as a lens-protection filters. You’ll also want a good IR and UV filter as well, to make sure your sensor is never reproducing strange colors from unseeable light. As you do not want to stack too many filters, you might want to look into combining these three functions into one screw-on filter for each lens, depending on prices.

Square filters can be fit in screw on filterholders like the Formatt, Cokin or Lee, as well as the rectangular gradual filters (you can slide them up and down inside the filterholder) and the 105mm round LEE filters (with the 105mm accessory ring that goes on top of the holder). Square and rectangular filters also fit in video/film matteboxes, I don’t know about round filters. But be warned: square filters are available in 1,5 mm (Hi-Tech), 2mm (standard for Cokin and Lee holders) or 4mm thick (for mattebox).

Materials

There are four types of material:

  • Gels. Cheap, optically good (Kodak Wratten), but not durable
  • Glass. If you buy good ones, these will be optically best, but easy to break.
  • Polyester (PET)
  • Optical Resin (CR-39).

Make sure all your filters have coating on both sides (multi-coated). This is mainly to prevent reflections between your filters and/or your lens. There are also special watercoatings you might be interested in.

Options

  • Cokin. Do not buy Cokin, is what I’ve read at several places.
  • Lightcraft. Do not buy (cross polarization after 6 stops and IQ drop off after focal length past 100mm). They say those problems are fixed in their mark 2 filters, but those have more internal reflection (not good).
  • Marumi (japan). Only has screw on filters in sizes up to 82mm. Not for me.
  • Tiffen Indie (take the expensive coated ones)
  • Heliopan (has hard stops, is said to be one of the best).
  • Hoya
  • $ 23.- Slim 72mm variND.
  • Schneider Optics or Century. 6″ Pola $ 549. This is a round filter, with or without (round) mount. It reduces light with 1-2/5 stops. They also have big sized square filters.
  • Formatt (UK). The HiTech filters are made of CR39 and only 100mm or 85mm. But Formatt also have glass filters at sizes up to 6,6×6,6″, at only £ 75 higher prices then the 4×4″.
  • Lee (UK).
  • Genus

I have tested the 5×5″ filters and they do fit with my lens. But as they were the same price I ordered the 5,65×5,65 inch Formatt glass filters.

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Posted in: Gear