Filters can have a big effect on the quality of the video you shoot because they help you get the right amount of light and can make the video "POP" too.
The DJI Spark is quite new so there are only a few options, and the manufacturers are sometimes changing the design so be sure to read the review in full.
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There are two main types of filter you will want to think about for your Spark:
A neutral density filter makes everything darker. Why is that useful for your Spark?
With video you want to have a shutter speed that is about double your frame rate.
A frame rate is 24 fps on your Spark, means ideally a 1/48 shutter (which does not exist, so 1/50 is what you would use)
A framerate of 30fps, then use 1/60 shutter speed.
The thing is, most of the time (except sunrise and sunset) you will have too much light to get these shutter speeds. So you need an ND filter to help you do it. Without it, you can never get picture perfect video.
The idea with circular polarizer is make the sky bluer or reduce reflections from windows or water.
There are lots of reasons a polarizer is a great idea.
Most of the Spark filter sets come with them (on top of the ND filter which is perfect). Some sets just have the ND.
POLARIZER TIP: Because the polarizer has to be correctly aligned in order to block out the light (block reflections or make the sky bluer) you usually need to add them after the drone is turned on so that you are sure that it is correctly aligned.
Another thing to consider is just protecting your lens. Having a filter on means if you crash or run into something, your filter is broken, not your camera.
Here are a few options you can use if this is your only goal (both are almost identical):
There are other simple stick-on and fibreglass square covers available, but none are quality enough for me to recommend here. You can find them on Amazon if you want to see them.
Freewell make a lot of DJI accessories so it is no surprise that they are one of the first to market with the Spark filters.
This set has a nice combination of 6 filters to give you a lot of flexibility:
This is a great choice of filters as it allows you to choose what your goals are for the shoot. If it is just getting the right settings, then you have 3 ND filters to choose from.
If you want to reduce reflections or make the sky pop, you can also include a Polarizer with ND8 or 16, or just use it alone.
The system uses a mount which you add to the outside of your Spark lens. The cool thing about this is that it has a protective cover of it's own, so before adding any filters you get lens protection!
Then, you just choose your lens for the flight and slide it on, super simple!
You can check out how the system works in the Freewell video on the right (or below on mobile) :
Polarpro make great gear for drones and are currently one of only two companies making the Spark filters.
At the time of writing (Aug 2017), according to one review on Amazon, they are changing the construction of their filters due to a small issue. So, if you are buying not long after this - check with them if the new version is available.
Like with most 6 packs of filters, Polarpro give you lots of great options with this set:
Unlike the Freewell set above, they don't have a pure Polarizer filter. So that is one big difference between the two sets.
The other is the mounting method. These guys use a simple clip-over system (see how it is done in the video to your right/below on mobile). So, no need to mount a semi-permanent clip around the camera to attach filters.
Of course, this is also a plus for the Freewells as it protects the lens at all times and provides an easier slide on and off system.