Benefits of a Neutral Density Filter for Video
I’m starting to use my Canon 7D’s amazing video capabilities more and more. When I do, I often choose the “24 fps” mode as that frame rate is considered the universal standard for film.
I want to get the “film look” people are familiar with while also avoiding any similarities to home video camcorders. In my mind, the film look can be summed up as
- a) extremely shallow depth of field when needed and
- b) smooth motion blur (i.e. proper amount of blur in each frame).
Getting a shallow depth of field requires the use of prime lenses with fast maximum apertures like f/1.8 and f/1.4 or big telephoto lenses (e.g. 200mm @ f/4). The motion blur aspect, however, is determined solely by your shutter speed.
NOTE: Shutter speed and frame rate are not the same. A 24 fps video uses 24 photos a second but those frames could be shot anywhere between 1/8000th and 1/30th of a second each (just can’t be slower than the frame rate itself).
To match the motion blur you’ve come to expect in traditional film, the camera’s shutter speed should be double that of the frame rate (or as close as you can come to that value with your camera). I therefore set my shutter speed to be 1/50th of a second for all 24 fps videos. This doubling has to do with how shutters work on traditional film cameras and it’s known as the 180 degree shutter rule.
You can shoot with a faster shutter speed but if you go too far it starts to feel like the opening sequence of Saving Private Ryan (i.e. each frame is overly crisp that it becomes noticeable). Useful for effect but not what I’m looking for generally.
Regulating Exposure in Video Mode
One big obstacle with video is that you’ve got less control over exposure due the shutter speed restriction. This becomes a big problem in the bright outdoors. Your camera can’t go lower than ISO 100 and your shutter speed is now “locked” at 1/50th. The only variable remaining is your aperture, right?
UPDATE: You should actually set your ISO 160 instead of 100 for best results. Find out why multiples of 160 are the best ISOs for video.
Since part of the attraction of shooting video with a DSLR is to get a very shallow depth of field, closing your aperture down is a real compromise and may even ruin the entire aesthetic.
For example, if 1/50th sec. @ f/1.4 @ ISO 100 creates an overexposed video on a bright day, you have three ways to solve the problem:
- Increase your shutter speed (e.g. move up to 1/500th) but video becomes overly crisp or jagged
- Close your aperture (e.g. reduce to f/5.6) but shallow depth of field is lost
- Use a darkening lens filter to block how much light reaches your sensor!
Neutral Density (ND) Filters
You guessed it, the last option is the best option as it ensures you don’t need to make the first two compromises. This type of filter is called a “neutral density” (ND) filter and it acts like sunglasses for your camera lens. It’s neutral in that colours remain “true” (i.e. less light, same colours). Like most filters, these are purchased to match the front thread size of your lens (e.g. 58mm, 77mm and so on). They come in various “densities” which represents how much light they block.
Variable ND Filters
To ensure you can deal with any number of lighting situations, I highly recommend a neutral density filter that’s variable instead of buying multiple filters of fixed densities. It will save you time, money and space. These special filters are constructed with two pieces of glass that permit a twist of the front element to vary the darkening effect on-the-fly.
This category of ND filters is often referred to as “fader” ND filters and have they usually can block 1 to 8 stops of light with the single turn. They usually cost more than a single ND filter but have many advantages such as size, speed and ease of use.
NOTE: Remember that one “stop” of light just means half or twice as much light so it’s exponential (e.g. 2 stops less light is just 1/4 of the total amount you started with).
Phil’s Pick: Cameron Multi-Coat (MC) Fader ND
There are a variety of Fader ND filters on the market but I found one recently at my local camera store that seems to be a good balance of quality and price. At CAD$90, it certainly doesn’t break the bank and it’s even cheaper in the USA. The product is called the Cameron Multi-Coat (MC) Fader ND. Like the others of its kind, it’s referred to as having a range of “ND 2 to 400″ and that is the same as saying it varies from 1 through 8 stops.
What I like about this filter, other than its price tag, is that it’s quite thin. It also avoids vignetting by having the front element be larger than the back element. I haven’t yet seen any colour cast but even if there is, it’s something you can adjust in an video editing program.
ND Fader Video Demonstration
While my son Keane was playing outside on a bright day, I took the opportunity to create a test video clip to show this product worked in comparison to other two exposure regulating options (shutter and aperture). I shot and posted this video in 1080p so you can see the quality and colour in all its glory. My poor son has no idea who I’m talking to.
In this video, I don’t see any obvious colour cast so I’m pleased overall. I really like how easily adjustable it is compared to fumbling with separate filters (especially square filters with holding brackets!).
Beyond video, these filters are useful for achieving slow shutter speeds on a bright day. That’s often done to blur moving water or to set a slow shutter for panning shots.
Variable ND Filters Vendors
Here are may vendors of these filters. You’ll notice that they vary greatly in price as it ranges from $24 to $420. Note that part of the price is determined by the size of filter you need as well (e.g 82mm will cost more than a 58mm):
- Cameron – Fader ND Filter (my choice)
- Light Craft Workshop – Fader ND Mark II (similarly priced to Cameron)
- Tiffen – Variable ND Filter (new as of June 2011)
- Genus – Fader Filter (expensive and only ranges from 1-4 stops)
- Singh-Ray – Vari-ND (expensive)
- RainbowImaging (cost so low you have to question quality)
- Non-branded versions on eBay
You can be sure the more expensive vendors use better glass but I wouldn’t assume the highest priced option is necessarily best option overall. Remember that if this purchase is primarily for video, you can afford some loss of sharpness given 1080p is “low resolution” when compared to a still photo. 1080p video only uses 2 megapixels of your 18 megapixel camera.
If you buy or own of these, please post a comment below to say which one and how your experience has been.