Part 1: A must have item for shooting video with your DSLR! The variable ND filter

December 6, 2012 — Leave a comment

With a background in shooting with video cameras, I am used to having built in neutral density filters (ND filters) at my fingertips. Transitioning to a DSLR to shoot video, I suddenly realized that I didn’t have the range of exposure control I needed if I wanted to shoot with a frame rate of 24/25/30 frames per second (fps) to create a more filmic look to the footage. This is especially the case when wanting to maximize the classic shallow depth of field look that makes footage captured by a large sensor camera look so attractive. This requires using the a lens wide open, or at least not stopped down beyond an aperture of perhaps f4. This is just impossible on a bright sunny day if you want to also use a shutter speed of 1/50 to also ensure that filmic look we all crave for and that best simulates the good old days on film cameras like the Aaton and Arriflex on which I learned the trade.

To clarify, here are the parameters-

  • To capture a more filmic look use a frame rate of 24fps or 25 fps, determined by whether you are in a country with NTSC format  (USA etc) or PAL ( UK, Australia etc)
  • You can also use 30fps as a safe frame for web based projects that will be shown world wide.
  • To capture the correct motion blur in each frame, to keep that filmic look, you should select a shutter speed twice that of the frame rate. For 24 or 25 fps, that makes it 1/50 sec (think of a 180 degree shutter rotating in a film camera. To capture 25 fps, that equates to a shutter speed of 1/50 sec)
  • One of the major strengths of using a full frame DSLR for video is the wonderful shallow depth of field look that mimics large format movie cameras. To achieve this, you need a use a fast lens (f1.2, f1.8. f2.8 etc) and not stop down the aperture.

The result is that you end up with aiming to shoot at 1/50 sec at perhaps f2.8, fine in a dark space, but impossible in sunny conditions, even with your sensitivity set to something like ISO100. The image would be way over exposed. Fortunately there is a solution, neutral density filters (ND), but you don’t want to carry around a large selection of sizes and grades to match each lens to each new lighting situation. That is where the variable ND comes into play.

These filters are made of two elements of glass. The front element turns freely, and by combining the elements at different angles, you create a darkening effect that typically ranges from 2 to 8 stops of light. This brings you back into an acceptable exposure range, even with camera setting of 1/50sec @ f2.8 on a bright sunny day.

My suggestion is that you can save a lot of money by checking the maximum lens diameter in your camera bag and purchasing a variable ND of that size. To pick a number, say that is 67mm. At the same time order step-up rings so that your classic 52mm or 55mm diameter lens will accept this 67mm filter. The conversion rings are a lot cheaper than buying a separate filter for each lens….. The same trick applies to other expensive filters like a circular polarizer.

There are various options on the market. Beware, the least expensive will cause you a lot of frustration as they will degrade the sharpness of the image and as cause strange color shifts. I won’t even list those manufacturers. Of the options that I would recommend, there are the following

And my current choice as a good combination of optical quality and price point, they are new to the market-

  • Genus (Also known as Genustech)- We will make these available at our store as they become available

There are also another couple of advantages to having a variable ND filter in your camera bag. They are perfect for creating slow exposure for stills. This can give you the classic misty look to moving water. You can also takes shots of busy street scenes or buildings, and with a long exposure, the moving people either disappear or become a blur, making them less distracting in a cityscape or landscape.

PS- Under some conditions, even the best variable ND’s can show signs of a color shift. If that happen, back off on strength ND slightly and the problem is gone.

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