The KF Concept Variable ND4-64CPL (or KF Concept Nano-X Series Variable ND4-64CPL MRC, to give it its tongue-twisting full name) is one of a select few filters that provide separate control of both polarisation and neutral density. Polarising filters are extremely useful to photographers, as they can enhance or suppress reflections, saturate colours, or deepen blue skies. Meanwhile, variable neutral density (VND) filters enable the use of long shutter speeds for motion blur effects. They’re also invaluable for video, as they can balance the exposure to achieve a desired shutter speed and aperture. Combining both filters into one unit is potentially very handy.

　　KF Concept Variable ND4-64CPL filter at a glance: £64-£89 Circular polarising filter 2-6 stops variable neutral density 28-layer multi-coating Available in 37mm to 82mm sizes kentfaith.co.uk VND filters work by rotating two polarisers relative to each other, so this filter has two independently rotating sections. Firstly, twisting the front section relative to the rear via a pair of orange grips controls the density between 2 and 6 stops, with the values marked on a small scale on the rim. Then rotating the entire filter via a grip around its edge adjusts the polarisation.

　　Its crucial to understand that, as the polarisation is defined by the front part of the filter, adjusting the density will also change the effect. Those orange grips provide a handy indicator of how it’s set. But they also mean the filter can’t be used with lens hoods.

　　KF Concept Variable ND4-64CPL filter key features: Case: The slim, square case has a soft microfibre lining and a handy ribbon for removing the filter Grips: Two chunky metallic orange grips make it easy to adjust the degree of neutral density ND scale: The neutral density level is indicated by a small scale along the edge of the filter Front thread: This is one size up from the rear thread: for example, 72mm on a 67mm filter. As part of the firm’s Nano-X range, the glass has been treated with an array of coatings. This includes nano coating to combat any loss of contrast due to flare or reflections, plus a waterproof, oil-resistant and scratch-resistant outer covering. The latter doesn’t stop fingerprints adhering to the glass, but it means they clean off extremely easily.

　　The filter comes in a slim square protective pouch, with a lid that snaps shut magnetically. In a neat touch, pulling on an orange ribbon that protrudes from the side of the pouch extracts the filter. This reduces the risk of dropping the filter or getting fingerprints on the glass.

　　I’ve used the 67mm version for both photography and video on a range of cameras, and found It works pretty well. The density scale is reliably accurate, and there’s no obvious impact on sharpness and detail. It does add a slight but noticeable warm tinge, with a colour shift of about 350K. But this can be countered by using a custom white balance if necessary, either in-camera or during processing.

　　Comparison between no filter (left) and with KF Concept Variable ND4-64CPL filter (right) In the comparison above, shot using the Olympus OM-D E-M1 Mark III and M.Zuiko Digital 12-40mm F2.8 Pro, you can see how the polariser darkens the sky and suppresses reflections from the car windowscreen. But theres no unpleasant colour cast or any degradation of sharpness and detail. Full size images here: No filter and With KF Concept Variable ND4-64CPL filter.

　　KF Concept Variable ND4-64CPL filter: Our Verdict The KF Concept Variable ND4-64CPL is a useful filter for both photography and video. It’s nicely designed and built, and does its job without any unpleasant surprises.

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