Convergence-The use of Neutral Density Filters

Over the past few years, I have fielded a number of reader's questions regarding the motion-effect from water in many of my posted prints. Some have asked it if was a special Photoshop plug-in, while others have ventured that it was an "in-camera" effect. In actuality, these shots were exceptionally long exposures, due to the use of a Neutral Density Filter-a  Heliopan ND 10 stop filter, which as the number implies, creates a ten stop reduction in light reaching the sensor (or film in traditional cameras). When looking at the filter, it appears as black glass and does not allow a see-through view in an SLR camera. As such, using the Bulb (B) setting on your dial, first compose and focus sans filter, then carefully place the filter on the lens, being very careful not to move the camera. Exposures can range from one minute, to as long as fifteen minutes, and require a very sturdy tripod and cable release which will lock open. This process was not an option in my days of film, for it would have been a prohibitively expensive venture. Of course, with digital work, a wrong guess and the only thing you are out is your time. With 4x5 sheet film, hours could be spent in the darkroom to discover mostly clear negatives, at a great expense and the requirement of copious exposure notes.  A dear friend and fellow artist and I went through a period during which we worked with classic 120 film cameras such as the Agfa Isolette with the lovely Solinar lens. We found that after some months of experience and using the same film consistently, we could guess F Stop and shutter speed combinations quite accurately, without the use of an external light meter (as the Isolette does not have a meter). The same intuitive skill will develop for you over time when using the ND filter. You will learn to judge by light conditions and F-stop (typically F14-22), the length of time for the exposure. With practice, one becomes skilled enough to get close on the first exposure and then fine tune with a second shot if needed. A challenge with ND filters is waiting the time out.....ten minutes can seem like an hour when you are looking at the exposure timer on the top of your digital SLR. Take a walk, read, scout out your next shot, or practice some mindful meditation-the wait will be worth it. Any thing that moves, in particular, fast water or rapidly moving cloudscapes, turn into a creamy blur. Don't scrimp on buying a filter, for like in the rest of life, you get what you pay for. Heliopan and B&W, both fine German filters, are expensive, but give sharp results. Cheap filters degrade images. Depending on the filter size for your lens, you can expect to pay close to $190 for the Heliopan. Merry Christmas.