ND (Neutral Density) Filters are used to reduce the amount of light which enters the camera to enable a longer duration of exposure. Think of them as a pair of shades for your drone’s lens.
They are commonly used to highlight movement (i.e. taking shots of a waterfall) or to give busy images a softer more surrealist appearance. These filters can also be used to support larger apertures, producing sharper images or a shallower depth of field.
Easy to use and able to produce beautiful imagery, ND Filters are a popular addition to any aerial photographer or videographer’s kit.
ND FILTERS EXPLAINED
In the most basic terms, ND Filters are sheets of semi-transparent glass to be placed over a camera lens to limit exposure. However, they’re a bit smarter than they seem, uniformly obstructing only an exact designation of light which means that the contrast and sharpness remain unchanged. ND Filters have also been designed for equality across the visible spectrum which ensures, for the most part, that unnecessary colour cast isn’t introduced – retaining a neutral feel – hence their name.
Despite their almost opaque appearance to the naked eye, this isn’t an indication of how the photo or video will turn out. Working in tandem with the filters, your camera will compensate and let in more light to bring the image in line with the ND’s stated lighting level.
ND Filters operate on differing levels of ‘f-stop’ reduction which correlate to the amount of light they let into the sensor. This combats excessive exposure and allows you to frame the perfect shot, even in challenging lighting conditions.
“The ‘f-stop’ of an optical system is the ratio of the lens’s focal length to the diameter of the entrance pupil.” – Modern Optical Engineering, 4th Ed: The Design of Optical Systems
In basic terms, the larger the number of ‘f-stops’ the less light that will enter the lens allowing you greater control over the exposure of the image or footage you’re trying to capture.
To see how the various ND Filter models relate to the level of ‘stops’, see the table below.
When you’re getting up the table towards 16 stops – you’re really looking for something that captures artful long exposures and highly stylised imagery (think ethereal black and white structure portraits or desert vistas). For capturing the type of footage most associated with aerial cameras, the standard ND Filters you’ll encounter from the main manufacturers are ND4, ND8 and ND16, although there are further options available to up the f-stops which can be especially useful when flying top-line cameras such as the X5S for the Inspire 2.
WHY USE AN ND FILTER?
This section looks at the basic uses for ND Filters, giving you an idea of what benefits (aside from limiting the amount of harsh light to the sensor) they can bring to your aerial photography and videography.
We’ll start with the benefits of ND Filters on a longer exposure which is often used to create a variety of aesthetic effects such as softening, blurring or accentuating motion in your images. For the best results, multi-second exposures are the way to go.
Naturally, this is dependent on the level of magnification, the type of motion you want to highlight and, of course, the effect you’re looking to emphasise. The use of ND Filters will vary from image to image so you’ll want to spend some time getting to grips with the settings and finding the result that works for you.
ND Filters can also be used to improve shots with a shallow depth of field in a well-lit environment prone to overexposure. When used correctly you will see a massive improvement in subject isolation and also gain the ability to create a strikingly artistic background blur.
You’ll want to keep in mind that the higher the level of stops (think 9 or above), the better the ND Filter will perform and capture your desired effect in both bright daylight and well-lit areas.
Using An ND Filter
When using an ND filter, ensure that your drone’s camera is set to manual via the DJI GO app and that your desired specifications (i.e. ISO & Shutter Speed) have been applied. Setting the camera to manual ensures that it doesn’t digitally correct the exposure level and, in doing so, reduce the capture quality.
Once this is done, you should see a scale at the bottom of the options panel which displays the image’s exposure. As you can see, in the above example it is reading +2.0 which indicates that using an ND Filter (in this case, a 2 stop ND4) would help to level the image lighting.
From here it’s simply a case of choosing the correct filter and playing around with the aircraft’s camera settings until you’re happy with the image transmitted onto your device and ready to capture perfectly levelled footage.
The interface on DJI’s products makes it easy to choose the right ND Filter to minimise overexposure which is inextricably linked with shutter speed, frame rate and ISO. This means that you can use these filters to gauge the correct camera settings and ensure optimum quality for every shoot regardless of the lighting conditions.
It is worth mentioning that they aren’t foolproof solutions and you must take care not to use an ND with too many stops or you’ll end up with underexposed, dingy footage which is as much of an issue as too much light entering the sensor.
Consider taking some time getting to grips with the manual settings on your drone’s camera, with or without the ND (as high shutter speeds can benefit drone photography), and you will be rewarded with a superior end result.
Note: Always power down the aircraft before making any changes to the camera’s lens to avoid damaging the gimbal.
Aerial Filming with an ND Filter
Using ND Filters when filming on your drone essentially manipulates the shutter speed allowing you to retain the optimum shutter angle and in turn affect motion blur.
If you’re looking to get cinematic quality from your drone’s camera it’s worth maintaining a shutter angle of 180°. This means you’ll be getting a shutter speed of around 1/50 of a second at 25fps. This strikes a fine balance of footage quality, negating the common issues of jitter and overt blurring to provide truly impressive results.
If the angle is any larger (i.e. 360°), you’ll notice that a strong blur effect carries over between frames making footage of moving subjects appear smudged in places and if it is smaller (i.e. 45°), your video will turn out jerky and stuttery with no blur to smooth the transition of frames often giving the appearance of images stitched together like a flipbook.
ND Filters are popular with aerial filmmakers as they grant you greater control over these settings – allowing you to capture smooth, professional quality footage that makes the most of the unique perspectives offered by drones.
HOW TO GET HOLD OF THEM
Here at Heliguy, we stock a range of ND Filters from both DJI and PolarPro compatible with a wide range of drones and designed specifically for aerial photography and videography.
Our team are available from Monday to Friday 9am-5pm to help you with any queries you may have whether it’s regarding ND Filters, drones or the commercial potential of unmanned aerial systems.
Got a question? Don’t hesitate to contact us via the details below.
Keep checking back to Heliguy’s Insider blog for more information on aerial photography, useful peripherals and, of course, the latest news from the drone industry.
Full post available at – https://www.heliguy.com/blog/2017/02/02/heliguys-guide-to-drone-nd-filters/