You’ve all seen those photographs, usually of a waterfall or a stream where the water becomes a beautiful ribbon of silky smoothness. Those images are created using long exposures. I like to call the process painting with motion because the combination of a slow shutter speed and moving water creates a flowing, painterly effect to the water.
If you are shooting on overcast days, in deep shade, in the evening, or even at night, you can easily get your shutter speeds down to produce that coveted silky look. Use the lowest ISO your camera will allow and stop your lens down to f22 or so and you’re set. But what do you do on those bright sunny days where you can’t get a shutter speed slower than 1/60th of a second?
This is where neutral density filters (ND) come in handy. They are basically sunglasses for your lens, reducing the amount of light that hits your film or sensor. The neutral refers to the lack of color shift. ND filters can also be used to reduce the depth of field in very bright light allowing you to shoot at wide open apertures such as f1.4 to turn your background into a creamy smoothness.
There are two types of ND filters, circular and square. Circular filters are quick and convenient, however if you have a variety of lenses each with a different filter diameter you need to purchase multiple filters or purchase the largest diameter filter and buy step up rings. The 4 x 4 inch square filters can be used on most lenses as wide as 16mm and you only need one. The disadvantage is that you need to buy a filter holder, plus an adapter ring for each different filter diameter lens you own.
Neutral density filters come in a variety of light blocking strengths from 1 stop up to 10 or even more. If you were to purchase just one, I recommend a 6 stop. With it, even on a bright sunny day, you can bring your shutter speed down to as slow as 1 second or less. I like to carry a 3 stop, 6 stop and a 10 stop filter which allows me more shutter/aperture control in all types of light.
Determining exposure is easy. If your camera can see through your filter to auto focus, then you just focus & meter as normal. But if your ND filter is so strong that you cannot auto focus then you need to compose your image, focus, and take a meter reading at your chosen aperture before putting your ND filter on. You then simply reduce the shutter speed by whatever strength nd filter you want to use, remembering of course to first turn off auto focus.
For example, if you determine the exposure of your scene to be 1/60 sec at f22 without a filter and you want to use a 6 stop ND, you simple slow the shutter down by a factor of 6 which puts the correct shutter speed at 1 second at the same aperture, f22.
You can also purchase apps for your smart phone that will factor all of this for you. My favorite is called LongTime Exposure Calculator: http://itunes.apple.com/us/app/longtime-exposure-calculator/id362297743?mt=8 It is free, however works only on iOS 3.1 or later devices, but there are other apps available for android users. ND Calc looks to be a good one : http://play.google.com/store/apps/details?id=com.briandorey.ndcalc&hl=en
Here at Camera West, we stock Heliopan, Rodenstock , Lee and Hitech filters and accessories. Stop by and we can help you pick the right filter for you.
In a related blog post, see how I got rid of hundreds of cars and pedestrians on the Golden Gate Bridge in one very long exposure : http://www.camerawest.com/cwblog/2012/09/13/long-exposure-magic/
Also, note that I am leading a long exposure photo walk in February where we will photograph the waterfalls in Marin County. Details here : http://www.camerawest.com/cwblog/2014/12/05/february-photo-walk-cataract-falls-hike/
Michael Maloney is a sales associate, instructor and photo walk guide for Camera West Walnut Creek. After a 37 year career as a photojournalist, ( 30 years at the San Francisco Chronicle ) Michael still gets out to shoot most weekends concentrating primarily on landscapes.