- Reduce shutter speed for motion effects
- Reduce shutter speed for natural video
- Widen lens aperture for selective focus
- Reduce overall exposure level
Photography and videography benefit from our knowing proper procedures and making use of the right tools. One often underappreciated and sometimes overlooked tool is the lens filter. Neutral density filters, also called ND filters, are one of the more useful lens filters for digital photography and videography. Variable ND filters make using these tools very simple.
Uses for Variable ND Filters
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What is a variable ND filter? Neutral density filters add density to the light path with no color tinge, allowing the exposure triangle to be altered in order to give us the exposure setting options we want. Why you need a variable ND filter will depend on whether you shoot stills or video and what type of scene you are capturing.
Here are a few uses for variable ND filters:
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When photographers think about variable neutral density filters, waterfalls and waves will jump to the front of their minds. That’s because those fantastic looking blurred water images of those subjects are often made with neutral density filters in order to get the slower shutter speeds needed.
Variable ND filters are also useful for time lapse photography that gets shown as a video even though it is a series of individual exposures. The reason you would use a variable ND filter for time lapse is in order to have a natural feel to the finished video, you want the objects in motion to be slightly blurred from motion. Otherwise, it looks like a jittery example of stop motion animation.
Natural Looking Video
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Frame rates and shutter speeds are part of what makes a video look natural on playback or look somewhat odd and uncomfortable. It’s an effect that mimics what happened with rotary shutters in movie cameras that used film. In order to achieve a natural look the rotary shutter needed to open a full 180 degrees which was equivalent to 1/48th of a second at 24 frames per second.
At ISO 400, it is difficult to get a shutter speed that slow in daylight or bright indoor lighting. With digital video, faster shutter speeds and frame rates have the same type of result now as it did with small degrees of opening in rotary shutters, an odd, somewhat jittery motion appearance which most viewers find unnatural.
Neutral density filters are used in these situations to diminish the light intensity coming through the lens. Variable neutral density filters are very useful for video in that you can attenuate the light variably instead of in one density level only, making your options more versatile.
Selective Focus Lens Aperture
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Related to shutter speed adjustment in the exposure triangle is the lens aperture or f-stop. Another function of lens aperture is depth of field or depth of focus.
A large lens aperture will provide shallower depth of focus, giving you the ability to select only a small portion of the scene to be in focus. This is called selective focus, where the foreground and background are out of focus and the subject is sharp. Problem is, just as with trying to get slower shutter speeds, many scene illumination levels just won’t let you choose a wide enough aperture and get correct exposure.
In videography and still photography, a neutral density filter can tame your exposure triangle to give you the wide aperture you want. A variable ND filter will give you options of how wide to open up the lens aperture, allowing more creative choices.
Reduce Scene Exposure Value
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There are the specific reasons listed above for using variable ND filters, but there are other reasons we can use these lens filters. Simply to attenuate the light into whatever settings we want to use is a good reason why you need a variable ND filter.
For still photography and videography, we can easily think of how to add light with flash, strobes, continuous lights, and reflectors, but some scenes are just too bright for the settings we want to use. We may not be intending to blur motion or use selective focus, but having control over the aperture and shutter speed is always desirable.
Some types of scenes tend to be very bright, limiting our options in regards to exposure settings. A full daylight scene of a cityscape, a bright beach with white sands and lots of water, or a snow covered field or mountain will play havoc with your options in the exposure triangle.
What Filter Is Best?
Well, we have discussed the reasons for using ND filters and why variable ND filters are a great option because of the ability to change how much density is being introduced to the light path. Now you want to know what filter is a good fit for your needs.
I used the word fit as a bit of a pun since I have really enjoyed testing a new concept in the world of photographic filters, the RevoRing from H&Y Filters. H&Y Filters RevoRing is an idea that should have been here sooner, but it is definitely worth the wait.
RevoRing is a variable size mounting system for filters that eliminates the need for multiple filters or dozens of step up adapter rings since it can be adjusted to fit whatever lens filter size you need. A great benefit of this method of mounting filters is that it also acts like a quick release for lens filters.
H&Y Filters has paired a RevoRing with one of their premier filters, the combination circular polarizer (C-POL) and variable ND filter together. The variable ND filter operates within a range of ND3 to ND1000 and has a C-POL built into the filter so you can also adjust the reflection reducing, haze penetration, and sky contrast while controlling exposure levels with the variable ND filter.
There are many good reasons to want to use variable neutral filters, so try one out for yourself. Especially if you shoot a lot of video with your digital camera will you find a variable ND filter quite valuable.