When shooting aerial cinematography, controlling the amount of light that enters your lens can be accomplished simply with a ND (neutral density) filter or a variable ND.
Those of you who don’t know, a ND filter goes in front of your lens via a matte box or depending on your lens, you can screw the ND filter directly onto your lens.
A variable ND allows you to go with one filter and rotate the ND to increase or derease the amount of light into your lens. This is a big time saver when on set.
When we shoot aerials, we typically want to have a deep DOF (depth of field), instead of shallow depth of field. This allows more to be in focus at once. Our lenses usually live around T8-T11, or F8 – F11 aperture.
Once all of your camera settings are completed, ISO, frame rate, shutter speed, aperture, in most cases, you find your image is still over exposed when outside on a sunny day. This is when you would use a variable ND to cut down the extra light to get a proper exposure.
I’ve seen some guys close down their aperture or increase their shutter speed to overcome this. If you’re wanting a cinematic image, don’t do either of these. All lenses have a sweet spot for their aperture. Typically it’s not when the lens is wide open or closed way down. This can cause your image to be softened or become degraded in some way. When shooting cinematically, you want your shutter speed to be a 180 degree shutter. This is double your frame rate. So if you’re shooting 24 frames, shoot at 1/48th, if you’re shooting at 120 fps (overcranked), then shoot at 1/240th of a second shutter and so on.
The variable ND allows you to set all of your optimized settings, including what aperture you want to live at, and then use the variable to cut more or less light down to exposure your image correctly.
One last tip… over time variable ND filters can loosen. If this happens, the filter can adjust itself during the flight. So take some gaff tape and gently tape the filter to prevent it from rotating on its own.
Here’s the variable ND that we use on our 77mm Rokinons. Click to view.