Bruce's Photo Blog
Bruce Foreman | October 2012

Using “Black Glass”

I remember being asked through the years, “What filters do I need?”. Well, before digital photography dominated our lives this was an important topic. We often needed a UV filter to take out the UV component of daylight, sometimes a “haze” filter to remove some of the bluish cast from atmospheric haze and the way some films reacted to such. But the most common need was for color correction and conversion filters. We could only load one roll of film at a time and that usually was daylight balanced for daylight use, tungsten balance for indoors and interiors lit by incandescant bulbs and we had to sometimes keep “conversion” filters handy in the bag if we had to work outdoors with the tungsten balance film we started using in the camera last night.

Or if we had to use the daylight balanced film we started this morning in lighting conditions other than daylight.

The need for such is nowhere as necessary (if at all) with digital, we sometimes use a multilayer coated UV filter to protect the lens but for the most part the sensor doesn't even “notice” UV. Lighting balance is done in camera with the “White Balance” options or sometimes in “post processing” in image editing software (PhotoShop for one).

But one type of filter that is seeing growing use is the Neutral Density (ND) filtration used for the purpose of reducing the light coming through the lens. Often in bright daylight you might want to use a larger lens aperture than conditions will allow and ND filters help out a lot here. Or you might want to be able to slow the shutter down for longer exposure times than lighting conditions will allow to photograph moving water as fluid motion rather than “frozen” in a slice of time.

If you're beginning to use your DSLR for some video, ND filters become even more necessary. It's recommended we use a shutter speed of 1/60th or 1/50th second to render the motion blur within each frame in much the same way we've become used to seeing in movie theaters or on the TV from a DVD player. This severely restricts the use of shutter speed as a component of exposure control, so even at the lowest ISO setting we still see apertures at f16 or f22 and smaller. This has two unwanted effects, depth of field may be MUCH deeper than we really want with too much in apparent focus, and we begin to experience a degradation of image quality due to “diffraction” as the image forming light rays begin to try to “scatter” some.

Images affected by “diffraction” will have a loss of contrast, some edge definition will be lost, and there will a slight diffuse look to image tones making the image overall a bit “soft”, so we use ND filters to get back down into the f11 - f5.6 range. I have the following strengths: 0.6ND (two f stop reduction in transmitted light), 0.9ND ( three stops), and 1.2ND (four stops). I have them in 52mm filter thread size which fits two of my zooms and I use 46-52mm step up rings to adapt them to 3 primes that have 46mm filter threads, and one 37-52mm ring so I can use the same filters on the one lens that has 37mm filter thread (the m4/3rds system lenses are often quite a bit smaller than the Canon or Nikon optics).

A current trend is the use of one “variable” ND filter instead of single strengths, the variable often covers an up to eight f stop range. The good ones such as Light Craft Workshops get pretty expensive with Singh Ray costing even more. The cheaper ones can very easily introduce an unwanted color shift, even with single strength filters I don't chance any lower quality than Tiffen for that reason.

But the variable ND filters are constructed with two pieces of glass, probably using Polaroid technology to achieve the variable darkening. I've seen complaints of their use on some zooms, including some Canon L series, resulting in some degree of “softening”. And the two pieces of glass introduce FOUR additional reflective surfaces that can cause problematic, or at the least, distracting flares and internal reflections.

So as convenient as it is to simply “dial in” the degree of added density required with variable ND filters, I've discontinued use of the ones I have (even though I saw no problems in my work) and keep the single strength ND filters described above in my “kit”.