To IR or not to IR?

April 11, 2012

To IR or not to IR?

The Arri ALEXA’s low-pass filter pack includes UV and IR-cutoff filters. According to Arri’s promo literature, you don’t need additional filters:

The IR and UV cut-off filters let only those light frequencies reach the sensor which can be converted into a meaningful image. As with the D-21, the IR filter is specifically designed for the spectral response of the camera’s sensor, so no additional IR filter in front of the lens is required.(emphasis added)

According to Arri’s FAQ (under “Exposure Questions / Can I use traditional ND filters with ALEXA?”), the story is a little different:

While the close match between ALEXA’s custom designed IR filter and the sensor’s spectral response makes this issue less critical in contrast to some other digital camera’s, it is in general a good idea to only use traditional film ND filters up to an ND 0.9.

So according to the promo materials Alexa doesn’t need IR-Cut ND filters. According to the FAQ however, you need ND/IR-Cut at 1.2 and higher.

IR-Cut ND filters should be in the first stage of the mattebox. I.e. you want to cut the infra-red spectrum before it goes bouncing around between your filters and lens.

Lenses see everything. They don’t particularly care about wavelength. Those suckers are aiming and focusing light all the way down past UV into the microwave spectrum and all the way up, past UV, into the X-ray spectrum. An experiment for bored / unemployed cinematographers: Set a wide-angle lens to focus on infinity. Put the back of the lens approximately 50mm away from your radio antenna. Improve your reception. Someone try this and get back to me.

Alexei Berteig

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