Filters beyond infrared

This site is mostly about infrared, but a few ordinary filters come up often enough to be worth covering. The real question is usually how they behave once infrared is in play, so each entry notes that.

Neutral density (ND) and IRND

A neutral density filter is a gray filter that cuts how much light reaches the sensor without changing the picture, so you can use long shutter speeds or wide apertures in bright light.

The catch for us: ordinary ND filters are not neutral in infrared. They leak IR and throw a magenta or warm cast, worst at heavy strengths past about ten stops, because the sensor stays sensitive to infrared that the filter does not fully block. IR-control NDs (for example Hoya’s Solas IRND and the PROND EX and HD Mk II IRND lines) add an IR-cut layer so visible and infrared are reduced evenly and the cast goes away. On a full-spectrum body a normal ND will not behave predictably, so reach for an IRND.

Circular polarizer (CPL)

A polarizer works like adjustable polarized sunglasses. You rotate the front ring to cut reflected glare off water, glass, and foliage, which deepens blue skies and lifts color saturation. The effect is strongest at about 90 degrees to the sun (Hoya).

One open caveat for infrared: how well a polarizer still polarizes deep in the IR band is not well documented, so treat its effect past 720nm as uncertain until you test it.

Diffusion (mist) filters

Diffusion filters soften fine detail, lower contrast, and bloom the highlights into a gentle glow, the dreamy look common in portraits and cinema. Tiffen’s Pro-Mist is the classic, and Black Pro-Mist does the same while lowering contrast less and keeping blacks blacker. Glimmerglass and Hollywood Black Magic are siblings with the same idea.

Whether the glow survives behind an IR longpass filter is not something we have confirmed, so test it before relying on diffusion for infrared.

Split diopter

A split diopter is a half-frame close-up lens (commonly +4) that covers one half of the frame, so a near subject and a far subject can both look sharp at once, with a soft blurred seam where the halves meet. A double or center split, such as PrismLensFX’s Centerfield, is a rotating version that keeps the center sharp while blurring the edges.

How filters stack

When you put two filters together, the combined transmission at each wavelength is the product of the two, not the sum. A filter passing 50 percent over one passing 20 percent gives 0.5 times 0.2, which is 10 percent (Kolari Vision).

This is exactly how a longpass and a shortpass combine into a bandpass, and it is how anything you add interacts with an IR longpass: multiply the curves wavelength by wavelength, with a little extra light lost at each glass surface. It is also why stacking a strong ND or several filters on an IR setup eats light fast.

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For the infrared filters themselves, see the filter reference and infrared filter types.