MASTERING VARIABLE NEUTRAL DENSITY FILTERS

A TECHNICAL MEMO FOR PHOTOGRAPHERS

I have finally come to terms with my Singh-Ray variable neutral density filter.  I’ve had it for several years but only used it a few times, typically, I thought, with underwhelming results.  I view it as something to be used to smooth out the water of cascades or waterfalls or surf, and I bought it for that purpose.  The idea is that the neutral density filter reduces the amount of light which can enter the lens.  This requires a longer exposure for a given scene, and if that scene contains running water, it will be smoother in appearance in the final image.   I was first attracted to this kind of image from the work by David Muench or perhaps even his father, Josef, as seen in Arizona Highways magazine, and, notably the work of countless National Geographic photographers, and I sought to emulate it as have so many others.

Of course one doesn’t need such a filter, per se, to achieve the effect.  If the light level is low enough and with a small aperture and a low ISO, e.g. ISO 100 for a DSLR, one might obtain the desired effect.  If, however, it’s a bright, sunny day you might get there using a conventional circular polarizer which will “add” two stops of loss (Singh-Ray states that their design results in about 1-1/3 stops of loss).  Another alternative is to add a neutral density filter of sufficient darkness to enable you to get the desired result.  Rather than trying to guess which one you might need or carrying several grades you can buy a single device, the Variable Neutral Density Filter.  This consists of two polarizers in a single mount, one of which can be rotated.  As it’s rotated the amount of light loss increases to a maximum effect of eight stops.  It’s a pretty nifty device, but its cost ($130-$400) can reflect its niftiness.

Anyhow, I bit the bullet and bought the Singh-Ray.  I first used it on surf rushing around jetty rocks.  Here was the result:

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Early results for the scene were fuzzy until I realized that the tripod legs were sinking into the sand as the wavelets rushed by so I compensated for that.   This underscores the need for a good tripod placed on solid ground or with its legs dug well into the sand.  (Another lesson not mentioned in photography magazines.)  The image took a first place in a Cherry Hill municipal arts festival and the township bought it.  Fine.  Technique mastered; off to other adventures.

I experimented with it off and on in subsequent years, typically at waterfalls in the Catskills or the Adirondacks.  Then, last year, I became intrigued with the idea of blurred surf of which I had plenty in my back yard.  I set out to do that last summer but the results weren’t pleasing.  I found that when using my usual Av (aperture variable) mode, the shutter speed was still too fast to achieve decent lighting let along creamy surf.  So, I switched to Manual mode but most exposures were then washed out.  I saved face by calling them high key and moved on.

_MG_9653 640

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This season (2015) I determined to get it together somehow.  Once again, the exposures in Av mode were terrible.  Example:

_MG_1291 640

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I packed up, retreated to my cave and went on-line to learn.  Mirabile dictu, salvation was at hand.  Right there on the Singh-Ray page of tips (drum roll), right there as tip #2, it said “Ralph, be sure and close off your viewfinder window.”  What???  Could it really be that simple???

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Yes, it is and it’s necessary.  At first I couldn’t make it work in my head because I was thinking that the DSLR mirror flips up out of the light path so how could the light leakage from the viewfinder hit the sensor, and wouldn’t it make the image brighter?  Noooo.  It turns out that the light leakage is hitting the exposure-determining circuitry whose decisions are made as the shutter button is depressed.  Hence, the exposure sensor is seeing a brighter scene than the lens itself sees so it causes a lower exposure time.  Q.E.D., and what a relief.  It also brought back a memory of Richard Bernabe telling me years ago in the Charleston area’s Cypress Gardens that an image of mine of a camellia was dark because of light leakage through the viewfinder.

But, would it work on a foggy morning?

First, compose and darken the filter to taste (experiment with this setting a few times until you get what you’d like).  Second, cover the viewfinder eyepiece with something (my Canon has a rubber cover on  its strap just for this purpose).

Then shoot.

_MG_1305 640

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You be the judge but   ….   I love the effect on the water and I’m pleased.

10 Responses to “MASTERING VARIABLE NEUTRAL DENSITY FILTERS”

  1. Ken Curtis Says:

    Thank you for the explanation, Ralph. I am going to give it a try.

  2. Virginia Rice Says:

    Brilliant now to give it a try!

  3. Janet Hadley Says:

    I’ve been fighting against covering the viewfinder, too, Ralph. I have no idea why. Just lazy, I guess. But your explanation and images are so compelling, it seems like a good time now to use common sense and give it a try! Thanks, Ralph.

    • Ralph Berglund Says:

      Thanks, Janet. I’ve a friend who tells me that one should cover it all the time. I don’t and won’t for almost all of my work. I always check the histogram and reshoot if the exposure is off.

  4. Pat Worley Says:

    Thank you for doing so much homework. I know to cover the viewfinder for night photography. but didn’t know it would effect the image so much for a ND shot.

    • Ralph Berglund Says:

      Thanks, Pat. Most of my night sky shots are in Manual as, I suspect, are yours. Since we’re setting in the exposure and aperture I don’t think light leakage is a factor. For anything shot in other than Manual, yes, cover the viewfinder.

  5. Ken Klaus Says:

    Thank you, Ralph, for the thoughtful and thorough discussion on this topic of VNDF. I’ve always wondered how to accomplish this with all its subtleties, and now… voila! I too have worked with ISO and F-stops and have gotten some descent pictures, but the viewfinder leakage… brilliant!


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