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SRB Photographic - Variable ND Filter - part 4 of 1 2 3 4

by Mike McNamee Published 01/12/2012

olarising light can produce seemingly random effects depending upon lens focal length, construction, f-stop and the camera angle relative to the sun. The darkening of the sky might be uneven due to polarisation effects which are typical of a straight, single polariser. The Maltese Cross is something quite different. The patterns produced are known as isogyres and are actually used in diagnostic optical microscopy - for example the uric acid found in gout-affected joints produces the distinct pattern and this is the common test for gout. For photographers the isogyres are as much of a pain as gout (well perhaps not!). The intrusion of the effect is increased as the polarisers reach extinction and this limits the use of the variable ND as the strength of the filter is increased. Wide-angle lenses are more vulnerable to the 'defect' and presumably any lens with fluorite elements or strained glass (ie not stress free) will also produce unexpected results (polarising microscopes have specially made, stress-free lenses and lens mounting rings to overcome these issues). We also detected a significant red spot in the centre of our 105mm lens.

OVERALL

While the concept of a variable ND filter is excellent, the unexpected variables such as Maltese Crosses require some caution. However, the effect only occurs close to extinction and the cost of the filter is on a par with any normal fixed ND filter of the more usual 2x and 4x strengths.

On this basis you might find that the filter suits your needs but some experience will be required before you can safely predict what you will get away with in terms of lens selection, angle to the sun and ND setting. The change in transmission as extinction is approached is very rapid and metering 'in-camera' is lost and so test shots are required in almost all circumstances. You also have to be alert to the darkening of skies due to the 'normal' polarisation effect; it might not be quite what you want at the outset.

The only alternative to this filter is the more expensive Lee Filters Big Stopper. These are excellent and greatly sought after (currently the delivery situation has eased a little but it remains at two-three months).

The next feature by Paul Gallagher shows just what you can do with ND filters, the Big Stopper in particular.

One thing that we did conclusively prove is that shifts of colour using varaible ND are not at all a problem.

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1st Published 01/12/2012
last update 21/07/2022 08:46:26

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