Switchable ND Filters
From apertus wiki
1 Why
Larger image sensor diameters combined with more and more light sensitivity in image sensors has lead to drastic overexposure in daylight situations. Reducing exposure time might be undesirable because of the loss of motion blur and reducing the aperture might not be an option as it reduces the shallow depth of field. So a neutral density filter is the easiest way to reduce the amount of light hitting the image sensor. Many cameras nowadays have filters built in already. Alternatively filters can be put into the slots of most matte boxes.
2 How
2.1 Filter Wheel/Revolver
Sony uses something similar to this:
2.2 Filter Slider
Canon uses this technique:
2.3 Notes
- If a ND filter is between lens and image sensor it affects the FFD and needs to be taken into account when designing the lens mount. The option with no ND can not be left without any glass but rather has to use a clear glass to keep the FFD consistent for all ND filter options