ARRI Alexa XT cameras will be delivered with the Internal Filter Module (IFM) filter holder installed — except the Alexa Studio which has its own motorized ND filter system and cannot use of the Internal Filter Module.
The filters that go into the IFM are called FSNDs. They are not included with the Alexa XT cameras, and have to be purchased separately.
The Alexa has an EI of 800. So you need ND filters when shooting in bright sunlight or when a shallow depth of field is desired. An ND filter inside the lens mount some advantages over external ND filters:
• Internal ND filters are smaller and lighter.
• They can avoid reflections you can get with multiple filters in the mattebox.
• Internal ND filters can reduce flares and narcissism (double image from reflections between the rear element of the lens and the sensor). An ND filter between the rear element and the sensor can reduce these reflections.
• An internal ND (or clear filter) protects the sensor from dirt contamination during lens changes.
The new filters used for the IFM are not the same as current external ND filters.
• An anti-reflective multi-coating, which is absent in most external filters, maintains high image contrast.
• Water white optical glass is precision polished.
• A broadband absorptive coating ensures an even attenuation of the light over the full spectrum, which is why they are called Full Spectrum Neutral Density (FSND) filters. There is no color shift at any density.
The Alexa XT Internal Filter Module is compatible with all ARRI 35 format lenses. ARRI has yet to find a lens from any manufacturer that it is not compatible. However, you never know — so the filter sets include a depth gauge to make sure a lens fits.
The FSND filters are relatively far away from the sensor, so if there are small specs of dirt on the filter, they will not show up in the image at most common T-stops.
When using FSND filters, be sure the rental house has installed the IFM shim in Alexa’s PL mount. Using any filter between the lens and the sensor increases the flange focal distance by 1/3 the thickness of the glass, and this shim accounts for that. Once the shim is installed, you will always need some kind of filter between lens and sensor: either an FSND or an optical clear filter, which ARRI also provides.