"If I want to shoot great video in the future with my 4K, video enabled DSLR, what should I buy first?" This is a question a get a lot of the time from photographer friends. Given they already own a tripod, my first answer is always to start with a cage. Here is why.
What Is a Cage?
A cage is an open metal casing for your camera with multiple female screw threads for attaching a plethora of accessories. This effectively is the first step in creating a modular video rig setup depending on the demands for a specific shoot. Cages are often specific to camera bodies, so make sure your camera body is in the manufacturers’ compatibility list.
The obvious use is the ability to fix various accessories to your camera body such as monitors, lights, and microphones. Using the hotshoe for a shotgun mic may be sufficient, but there will be imbalance issues if you try to mount a monitor or a light on there, not to mention the increased chances of a monitor or light falling out of the hotshoe mount and breaking.
Attaching handles above or on either side of your camera body provides smooth camera operation. A cage gives you all the necessary connection points for these accessories, and you can select which ones are going to be most useful depending on your shoot. If you will be shooting mostly from a hip height, then an underarm grip will be the one to go for, whereas side on grips will be better for shooting from the eyeline.
Shooting creative video will mean you need to take control of focus manually. Moving the focus ring whilst recording will create camera shake, so to minimize this you can attach a follow focus to the bottom of the cage using a rail attachment. Whilst video lenses will have gear rings with teeth, it is straight forward to add teeth to a photography lens using a small accessory.
Matte Box and Filters
You might also consider adding a Matte box to your rails. A matte box usually has movable metal flaps, or French flags, that allow you to block sunlight and artificial light sources that can cause problematic glare and lens flare. Try to purchase a matte box that includes filter sliders to easily add filters. This is especially useful when you are looking to shoot wide open on a bright sunny day. You’ll want to keep your shutter speed at 1/50 s for shooting 24fps footage, so ND filters restrict the light entry onto the sensor without having to dial down the aperture.
The benefit of a cage is the added protection for your camera a metal shell will provide. Super useful if you have a reputation a dropper of things.
Cages are an inexpensive necessity for DSLR filmmaking. They are a great starting point to any camera rig, and provide a modular framework around your camera to great really great looking footage. It's rare you will need to use every accessory out there at the same time, but a cage gives you plenty of options and configuration depending on the demands of your video shoot.