Photography News Issue 55

Photography News | Issue 55 | photographynews.co.uk Technique 47 Camera School Here we lift the lid on all things camera related, showing how to get better results from your CSC or DSLR, and providing all the info you don’t find in the manual. So, stick with us and you’ll soon be wielding your camera like a pro. This month, how to use ND filters to make long exposures

Using a long- exposure filter

1 Set up and meter Compose and focus, then switch to manual focus. Now, in aperture- priority mode (A or Av), dial in the

f/number you want to use, for instance f/11. Set the ISO to a low level, like 100 and take a look at the resulting shutter speed. Let’s say it’s 1/30sec. Now fit your ND or LE filter.

2 Calculate exposure Now take the metered shutter speed and adjust it by the strength of the filter. So, if you’re using a three-stop

filter (ND8 or 0.9), you can double the length of the shutter speed three times; 1/30sec falls to 1/4sec. If you’re using a ten-stop filter, it would be 30secs.

Above This two-minute exposure was taken in the middle of the day so required a ten-stop ND filter.

speeds of many seconds or minutes even in bright light. It’s more down to whether they’re ‘neutral’; an ND filter should only block the overall intensity of light, not blocking any individual wavelength and therefore not changing the colours. But this gets more difficult as filters become stronger. So very strong filters, like those of ten stops will often give a colour cast. In any case, for marketing reasons, most manufacturers label their long exposure filters as NDs whether they’re truly neutral or not. Any colour cast you get fromanLE filter can usually be corrected in Raw processing, but many photographers who shoot long exposures prefer to work in black & white as it also suits the simplicity of the images created. ND and LE filters can be bought in different strengths, typical examples being one, two, three, five and ten stops. When choosing, these may be referredtointermsofstops,ordensity. So, a one-stop filter is also called an ND2 (or ND 0.3), a two-stop filter an ND4 (or ND 0.6), a three-stop filter an ND8 (or ND 0.9) and so on. A ten- stop filter is also called an ND1024, or often an ND1000, as it looks better in adverts (also ND 3.0). These numbers come from the light-reducing factor, so anND8 is 2 to thepower of 3 (stops).

To increase the light stopping power of individual filters you can stack them. Thisworkswhether you’re using square filters in a holder or screw-in types, though the latter are more likely to vignette first, showing as a shadowor a hard line at the edges of the frame. This can be cropped out in processing, so it’s still an option if you need to generate more light- stopping power than one filter alone provides. There’s also the chance that image qualitywill fall due to stacking, due to the extra glass softening the details and potentially adding flare, but this depends on the quality of the filters you’re using. If you’reusing regularNDsyoucan rely on the camera’s metering when shooting, but with very dark filters you’ll need to work it out yourself. It’s not hard to do, and if shooting with very long shutter speeds, remember 15 to 30secs is still only a stop, so you can afford a few lapses in accuracy; shoot in Raw and you’ll have more control after the event, as well as helping to correct colour. Of course, it’s not just about hitting the slowest shutter speeds possible. Another benefit of using these filters is that youdon’t have touse the extended low ISO settings or very small apertures which can both affect image quality.

Words & pictures by Kingsley Singleton

When you want to create a long exposure, you’re limited in terms of shutter speed by the brightness of the scene. You can lower the camera’s ISO to its minimum, like 100 or 50, and close theapertureasmuchaspossible, say to f/22, but there will be a shutter speed beyond which the image will overexpose. So when you’ve reached the limits of what’s possible, you have two choices. You can either wait until the available light dims, or you can use a neutral density (ND) or long exposure (LE) filter. ND and LE filters reduce the amount of light entering the camera by a given amount, stated in stops, just like the ISO, aperture and shutter speed settings on the camera. In this way you can tell that a one-stop filter will reduce the amount of light entering the camera by half, so ¼sec becomes ½sec, while a three-stop ND will halve it three times, so ¼sec becomes 2secs. So what’s the difference between ND and LE filters? It’s not in terms of strength, although LE filters are typically much stronger than NDs, and designed to give you shutter

3 Shoot and start timing Switch to manual mode (M) and dial in the time required (if it’s over that

Long-exposure noise reduction

NEXTMONTH MORE ABOUT LONG- EXPOSURE TECHNIQUES, INCLUDING CREATIVE INTENTIONAL CAMERA MOVEMENT EFFECTS. allowed by the camera, switch to bulb mode (B) and a fit a remote release. Now, if the camera has one, cover the viewfinder eyepiece, and open the shutter. If in bulb, time the exposure yourself (unless your remote has a timer), then close the shutter when the calculated time is up.

Most cameras have a long-exposure noise-reduction function, which is turned on by default. This records a second, ‘dark’ exposure of equal length after the first to remove digital noise caused by keeping the sensor charged for an extended period; this noise isn’t the same as that found when using high ISOs. Long-exposure NR slows down your shooting, so if you’re not going over a fewminutes with your exposure, consider turning it off.

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