The RotaPola

Posted by Dwight Lindsey on Nov 18th 2018

The RotaPola

Polarizers need to rotate, that’s how they do their magic. 

High quality rotating polarizers can effectively control reflections and increase color saturation. To do that, they need to rotate.

Cine cameras historically use cine matte boxes, which made rotating a polarizer slower and more complicated.

A Rotapola, also known as a RotaPol, makes it easy.

A Rota-Pol as a round polarizer mounted in a geared rotating matte box tray.

This makes rotating the polarizer smooth, fast, easy and repeatable

Before the RotaPola

Traditionally in cinematography, square, rectangular or round polarizers were used with matte boxes that had rotating stages.

The polarizer would be removed from its pouch, inserted in a filter tray and the filter tray would be inserted in a rotating stage. The rotating stage would then be unlocked and rotated to reduce reflections or increase color saturation and then re-locked.

This is a relatively slow procedure and there’s little chance of using the polarizer as a story-telling-device (more on this later). Pan the camera and the polarization might well not be optimum for the new view because changing polarization during the pan isn’t really possible with a rotating matte box stage.

The increased speed and flexibility offered by a Rota-Pol,
when motorized, allows it to be used as a story telling device
Action outside a window could be seen in the image reflected
on the window, then the polarization effect is pulled smoothly to
reduce reflections and reveal story elements behind the glass.

— Dwight Lindsey

A Closer Look at 4x5.65 RotaPols

Obviously a 4x5.65 Rota-Pol must fit in a 4x5.65 matte box, however each manufacturer makes different design choices which effect the way the Rota-Pola can be used.

The first choice is filter size and the first 4x5.65 RotaPols were made using the industry standard 4.5 inch round polarizing filters. We would call this a 4x5.65/4.5 RotaPol. This gives a round clear aperture of 4.26 inches because we lose about 3mm all around the filter for mounting. If we scribe a 16x9 rectangle in that circle, we get a useable aperture of about 94.41mm x 53.06mm. That’s a 3.71 inch horizontal aperture.

More recently, some manufacturers have been mounting 138mm polarizers in Rota-Pol frames. We call this a 4x5.65/138 RotaPol. This larger filter allows a 5.1 round inch clear aperture in the RotaPola. If we scribe a 16x9 rectangle in that 5.1 inch circle, we get a useable aperture of about 112.8mm x 63.69mm. That’s a 4.44 inch wide useable horizontal aperture.

Note that if you work in the old way with a rectangular 4x5.65 inch polarizer in a matte box with a rotating stage, your useable aperture may well be a bit less than 4 inches. The tray needs to rotate and when rotated to 90 degrees the horizontal aperture will be about 3.75 inches

So clearly a 4x5.65/138mm Rota-Pol has a larger useable aperture than a 4x5.65/4.5 Rota-Pol and in fact it has a larger useable aperture than a 4x5.65 inch rectangular polarizer when rotated.

What Makes a Good 4x5.65 RotaPol?

A large 5.1 inch clear aperture, giving a 16x9 rectangle of about 112.8mm x 63.69mm.

The large 5.1 inch round filter must be masked top and bottom to keep stray light entering the filter above the matte box.

There must be at least one thumbwheel, preferably two, for manual operation of the Rotapol. Those thumbwheels should be geared, preferably with a standard 0.8 gear pitch to couple with the geared motor for wireless motorized control of the RotaPola.

The geared thumbwheels rotate a larger main gear, which should be carefully machined from aluminum, Delrin or PTFE. Molded plastic gears work, however they are not as precise.

The main gear should run on Delrin rollers, for smooth and sure no-slip rotation.

There should be a 1/4 20 threaded hole for mounting a motor.

The round polarizer should be lightly bonded into the main gear, usually with silicone RTV, to prevent the filter slipping if rapidly rotated.

The polarizer itself should be of high quality, confirmed by the manufacturer to be neutral grey in color and extremely flat.

The polarizing filter should have a multi-layer broadband anti-reflection coating (MC Coating), preferably with a top layer of hydrophobic easy-to-clean coating.

An Essential Filter for Every Scene

A high quality polarizer will be extremely flat, neutral in color and have a broadband anti-reflection coating. A standard polarizer will have a 38% transmission, providing 1.5 stops of neutral density in addition to the polarization effects.

Polarizers can control reflections, not only large reflections big surfaces such as windows, but also smaller reflections and even micro-reflections in the scene. This can increase color saturation and increase visual impact. Polarizers can darken blue skies and enhance the contrast between clouds and sky.

A RotaPola makes utilizing a polarizer quick easy and repeatable, offering increased visual impact and story telling opportunities. It is truly one of the most essential tools in the cinematography tool box.