How to ND a window

What is ND gel used for? Why would you want to use it on a window? What are two key important factors in setting it up?

ND gels are used to cut the amount of light without altering the camera settings. These gels are useful for windows because the outside tends to be blown out when the subject is in the shadows. It is important to keep in mind light leaks when adding an ND filter to the windows so that it doesn't look artificial. Though gaff tape is preferred, in times of need, double sided tape works just as well. When working on a high-budget production, windex should be used to stick the filter onto the window.