Inspire 1 ND Filters – DJI & Polar Pro reviews
The DJI Inspire 1’s camera seems to do a really good job, I’d say it’s certainly on par with the results from my GoPro Hero 4. One thing that soon becomes apparent though is that much like shooting with the GoPro obtaining correct exposure for video involves a lot of compromise with regards to shutter speed.
Unlike larger lenses these tiny devices don’t offer anything in the way of aperture control so the only way to reduce exposure in camera (once ISO is at 100) is by increasing the shutter speed. The general rule of thumb for video and film is that each frame of the video is exposed for half of the time it’s available. So for instance if shooting at 25p (25 frames a second) the shutter speed would normally to half that at 1/50th of a second.
It gets confusing when talking about shutter speeds because shorter amounts of time have larger numbers, 1/100th of a second for example is twice as long as 1/200th of a second. The easiest way to remember when talking shutter speeds is that the ideal shutter speed number is double the frame rate, so at 24p the ideal shutter speed is 1/48th of a second, and at 30p it’s 1/60th of a second.
Having correct shutter speed is important because it effects the amount of motion blur that’s allowed to occur on each frame of the video, too slow and the image becomes overly blurry, too fast and it becomes stuttery and feels unatural to watch. Having a shutter speed thats too high can also cause additional problems when a camera has a CMOS sensor as the lack of motion blur can cause rolling shutter problems or “Jello” to become more apparent in the recorded footage.
So without any aperture control available on the Inspires camera the best way to reduce exposure and bring the shutter speeds down in bright conditions is to use an external ND (Neutral Density) filter. ND Filters are usually referenced in either Stops or ND numbers. I don’t really want to get in to the whole theory behind f/stops here but basically f/stop’s are increments in which apertures are changed on traditional camera lenses, each f/stop reduces the amount of light passing through a lens by 50%.
ND filters effect light transmittance as follows…
ND2 = 1 Stop (50% light transmittance)
ND4 = 2 Stops (25% light transmittance)
ND8 = 3 Stops (12.5% light transmittance)
ND16 = 4 Stops (6.25% light transmittance)
ND32 = 5 Stops (3.12% light transmittance)
ND64 = 6 Stops (1.56% light transmittance)
The Inspire comes with an included ND filter so lets see how much effect that has. I’ve also been sent a set of three filters from Polar Pro which includes whats listed as ND2 and ND3 filters plus a Polariser.
For the test the Inspire was pointed in to my garden and all the filters were changed in quick succession, the light remained steady throughout so hopefully this will give us an idea of what effect each filter has.
To begin with here’s a correctly exposed shot using no filter at all…
In order to achieve correct exposure at ISO 100 the shutter was at 1/1250, that’s way too fast and will likely not provide enough motion blur once camera starts to move around. Lets fit the DJI ND filter supplied with the Inspire and see how that compares…
The supplied ND filter from DJI certainly helps, it reduces the shutter to 1/320th. Based on that the DJI filter is likely an ND4 which is worked out as follows…
Starting Shutter: 1/1250
ND2 would take that to: 1/640
ND4 would take it to: 1/320
Next up the ND2 filter from Polar Pro.
The Polar Pro Filter does have slight effect on colour and contrast which although not terrible is worth noting. At 1 Stop (ND2) I would expect a shutter of 1/640 but this one resulted in 1/500. This could just be a result of a slight change in the sunlight, I’d need to test under more controlled lighting to know if it was an inaccuracy of the filter itself. Lets try the ND3.
The ND3 from Polar Pro produced results I’d actually expect from an ND4 so again either it’s labelled incorrectly or the lighting conditions were changing. The resulting shutter speed of 1/320 is the same as the DJI ND4. Colour and contrast remain similar to the ND2 version.
The pack from Polar Pro also comes with a Polarising filter, lets try that…
While the polariser does have an effect on exposure and drops the shutter speed to a similar level as the ND2 version, the main advantage is the effect it has on reflections. Check how much easier it is to see the dustbin through the glass door on the right of the shot in comparison to the previous examples with ND filters. We do also see a slightly more obvious change in the colour compared to having no filter attached.
The filter pack from Polar Pro certainly adds some options, the polariser could be handy in situations when dealing with reflections and having a slightly less powerful ND2 could be useful as it offers something between no filtration at all and the supplied ND4 from DJI. I do have to question the labelling though as they show the ND2 as 2 Stop and ND3 as 3 Stop, I think this is just Polar Pro’s way of labelling the filters but it’s a little confusing when a standard exists using the same naming convention.
Another thing to consider with the Polar Pro set is that the filters themselves are a little heavier than the ones supplied by DJI. The camera is a little more front heavy with the filters attached although the gimbal itself seems to cope fine when active. It’s a shame Polar Pro don’t supply storage boxes, you do get soft bags but I wouldn’t want to trust those rattling around in my case!
The Inspire 1 really needs a filter set that include more powerful ND options. In order to have achieved a 1/50th shutter in the test carried out today either an ND16 or maybe even an ND 32 would have been required as it would have worked out as follows…
Starting Shutter: 1/1250
Baring in mind that while today was a nice day, it was far from the brightest conditions the Inspire is likely to face so an ND64 would likely be required as well, especially if dealing with snow or other bright subjects on a sunny day.
Ultimately I feel having more filtration options is going to be useful but as stated the Inspire needs a stronger set of ND filters than those supplied in the set from Polar Pro, I’m going to try some more in the coming weeks and will update this post accordingly as I do.
Update 16th June 2015
Polar Pro have recently updated their Inspire 1 filter set to include an ND4 and ND8 as well as the Polarising filter. The newer filter set also includes printed references not the filters. Polar Pro generously sent me a set to test and here’s the results, first of all a test without a filter…
So now with the new Polar Pro ND4 fitted…
The results with the new ND4 are exactly where you’d expect an ND4 to be at 2 Stops. Next the ND8…
You’d expect the results of an ND8 to be 1/80th so the the test isn’t far off at 1/100. I’d say the new set is an improvement as the ND8 will certainly be more useful than an ND2.