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UNDERSTAND THE PROS AND CONS OF VARIABLE-STRENGTH ND FILTERS AND TAKE FULL CONTROL OF LONG EXPOSURES.

THE NEUTRAL DENSITY filter is a staple item in the landscape photographer’s kit bag and a firm favourite for creating images with energy and depth. An ND filter is a sheet of glass or plastic that is dyed with a pigment to reduce light transmission, thereby darkening the scene and allowing longer exposures. The standard filters come in strengths of between one and four stops, with ultra-long exposures requiring a ten-stop model, such as Lee Filters’ Big Stopper. The variable ND is a versatile alternative however. These circular filters attach directly to the lens thread and are constructed of two sheets of polarising material. By rotating the outer ring, light filtration can be adjusted, effectively providing multiple filter strengths in a single unit. This is both cost-effective and allows fine control over exposure.

There are certain disadvantages however. If used improperly, characteristic aberrations can be introduced, such as a tell-tale darkening of the image centre. In this tutorial we will explore the best method of implementing a variable ND filter and explain how to avoid common problems when using these models at full strength, most often experienced with wide-angle lenses.

ATTACH THE FILTER

01 Ensure your filter is the correct size for the filter thread of your lens. Some models are only available in one size, so use a stepping ring if needed to adapt the thread to the appropriate lens diameter.

COMPOSE THE SCENE

02 An advantage of variable NDs is that the image can be composed easily even with the filter in place, unlike high-density, fixed-strength filters. Use the optical viewfinder or LCD to arrange your framing.

SET FILTER DENSITY

03 While looking through the viewfinder or at the LCD, rotate the filter ring to reduce light transmission. Keep an eye on the shutter speed and set the filter when your intended exposure value is achieved.

TAKE A TEST SHOT

04 Use the self-timer or a remote shutter release to make an exposure that is free of vibration. If you are unhappy with the composition, reset the filter position for a clearer view, and reframe.

CHECK FOR UNEVEN DARKENING

05 At wider focal lengths, a dark cross shape can appear in the frame centre. Be sure to review the images and if present, slightly reduce the filter density from maximum until the darkening disappears.

ADJUST FOCAL LENGTH

06 Another strategy for reducing the aforementioned cross effect is to zoom in by a few millimetres. Set a 10mm optic to around 14mm for example and reshoot. This helps to reduce strong vignetting.