Monday, March 29, 2010
Make your own extreme ND filter in an instant—03/29/10
Create a variable ND filter from filters you already own
Commonly available in “graduated” form, neutral-density filters permit photographers to darken the frame as if the amount of available light were lower, or as if the ISO speed were slower. Graduated filters darken one part of the frame while not affecting the remainder—perfect for balancing bright areas with dark. For limiting the overall exposure, though, non-graduated neutral- density filters darken the entire scene evenly—making it possible to create long-exposure effects even in bright sunlight.
For example, at ISO 100 and a minimum aperture of ƒ/32, a basic daylight exposure might produce a shutter speed of 1/25th of a second. That’s not nearly long enough to create the motion blur that makes moving water fluffily blurred. Add a two-stop neutral-density filter, though, and the shutter speed drops down to about a fifth of a second. Unfortunately that’s probably still not slow enough to create the right look.
Four stops would get you to a full second of exposure, which is much more effective at creating the motion blur that turns moving water into the beautiful, pillowy, white blurs landscape photographers love. Stack a couple to produce even further light reductions. But if you don’t own neutral-density filters, which aren’t exactly the most commonly needed accessory, what can you do? Why not make a variable neutral density by utilizing filters you may already have at your disposal.
Slap a circular polarizer on your lens and chances are you lose about two stops of light. That’s a good start for achieving the density required for a longer exposure, but it’s not quite enough. Put a second polarizer on your lens though, and now you’re really talking about density. And not just four stops either.
Because polarizers work by aligning microscopic slits to align and limit the wavelengths of light allowed to pass through the filter, when you stack them together you can create an exponentially stronger filter by the way in which you align those slits. That means you can take two 2-stop polarizers and create the equivalent of perhaps six or even seven stops of light blocking power. And like all polarizers, you can see the effects in front of your eyes as you rotate the stacked filters in front of your lens. (Be sure to reverse the second polarizer. If both are pointing in the same direction, you’ll get strange color shifts and density, but it won’t work nearly as well as with one of the filters pointing in the wrong direction.)
The problem, though, is that this extreme isn't free—it comes with a significant color shift as well. In practice you can probably expect four or five stops of relatively neutral density by stacking two polarizers. That goes a long way toward lengthening shutter speeds for special effects, even if you have to fight with all the other costs that come from stacking polarizers. Any time you stack filters you increase your chances of optical aberrations and image imperfections, including lens flare and vignetting. And since you’re working with polarizers designed to minimize distracting glare and reflections, you’re not truly creating a neutral density. For all these reasons, stacking polarizers isn't an ideal solution when compared to high-quality neutral-density filters. But in a pinch, this makeshift technique can be a life saver.