Camera technique
Using ND filters
Overall plain grey neutral density filters come in varying strengths starting from2x (1EV) and available as strong as 32,000x (15EV) and more. An ND filter’s strength is indicated by a filter factor, such as 1000x, or by the amount of f/stops it absorbs, such as 10EV. Youmight see another figure though, and that is optical density and in the case of a 1000x (10EV) filter, that is 3.0, ie 0.3 per 1EV. The weaker ND filters, such as 2x, 4x and even 8x, are more useful in terms of general photography – allowing, for example, the use of wide lens apertures for blurred background effects even when it’s bright. If you want to get shutter speeds down to one second and longer, you need stronger ND filters, even if you set the camera’s lowest ISO speed and a small lens aperture. Let’s take a practical example. The camera indicates that correct exposure of a scene is 1/125sec at f/8 and ISO 200. Setting ISO 100 takes you to 1/60sec and stopping down to f/16 takes you down another 2EV to 1/15sec, but you’ll still need a 16x (4EV) ND filter to get to 1sec, a 500x ND (9EV) to allow 30secs and a 1000x for 60secs.
Most ND filters have a set light-absorbing value, but there’s a growing number of variable ND filters that cover a range of light- absorbing strengths in one filter, so are convenient and potentially money-saving, too. However, some work better than others. Variable NDs work by having two polarisers in the same filter mount and, as one is rotated, the amount of light transmission is altered. At the stronger settings on some filters, there can be an X-pattern, so check that with a test shot or two. Some filters have some sort of index to indicate which strength you are using, but many don’t so you might need to so some educated guesswork with exposures beyond the camera’s timed range. One final technical point: despite the termneutral density, not all NDs are neutral, especially when you get into the denser values. With your camera set to its auto white-balance, the ND filter might get close to neutrality, but often there is a cool or a warm colour cast. If you are shooting Raw, this is easily remedied in post-processing, but if you prefer a more accurate out-of-camera result, adjust the
camera’s kelvin setting until you are happy. If images are cool and blue, set a higher kelvin value, 8000K andmore; if too warm go for 5000K and lower. By the way, if you do this, remember to reset yourWB setting when back to normal shooting. Deciding which strength of ND filter to fit depends on you want to achieve and the subject itself – and experience helps. Apopular subject for long-exposure technique is water. If it’s fast-moving, an exposure of one or two seconds could easily be enough to blur and any longer might not make much difference, while a benign sea might need several minutes. It’s the same with that other popular subject: the sky. Fast-moving fluffy cumulus clouds on a windy day will give nice streaks with a 30sec exposure, while calmer days and high clouds might needmuch longer. And of course, somuch depends on your previsualised effect, because milky water and streak-filled skies are not to everyone’s taste, so it’s a matter of balance, and that’s when experience comes into play. See the ‘Using extreme ND filters’ section (in the box on the right) for more on super-long exposures.
ABOVE Dull days can mean you can try long exposures without needing ND filters. Here, a Canon EOS-1D X with 70-200mm f/2.8 zoomwas used and the combination was on a tripod. Setting an ISO of 50 enabled an exposure of 4secs at f/25. The lens was zoomed in for the last two seconds of the exposure so part of the image is sharp. Zoom later and the leaves would be even more defined, while zooming as soon as the shutter was opened would give more blur
Zoom the lens
Moving the camera is one way to give your shots a sense of movement, but how about keeping the camera still and zooming the lens? Or moving the camera and adjusting the lens at the same time? You need to alter your camera settings or use an ND filter to enable an exposure time of 2secs or more. This gives you time to open the shutter and zoom the lens without
being rushed and jogging the camera. In the example shown here, the exposure time was 4secs, with the lens zoomed for half that time. Zooming the lens in, ie zooming a 70-200mm lens from 70mm to 200mm, gives a different effect from zooming out, ie from 200mm to 70mm. In the example shown, the lens was zoomed in. It is always worth trying both.
ABOVE A growing number of mirrorless cameras are coming with a built-in neutral density filter or have a feature that replicates the effect, so you don’t even need to carry any filters with you to enjoy longer shutter speeds. The Olympus OM-D E-M1 Mark III and OM-D E-M1X both have Live ND modes. This works with the camera’s electronic shutter taking a sequence of shots and then merging them in-camera to give the effect of a longer exposure. It has a range of strengths fromND2 (1EV) to ND32 (5EV) and works in manual and shutter-priority AE modes within a 1/30sec to 60secs shutter speed range. The really useful thing is that in works in Raw as well as JPEG.What’s more, you can see the effect before taking the shot. This was taken on an Olympus OM-D E-M1 Mark III, with the 32x Live ND filter allowing an exposure time of 15secs
12 Photography News | Issue 77
photographynews.co.uk
Powered by FlippingBook