Photography News Issue 44

Photography News | Issue 44 | absolutephoto.com

Technique 63

PART9 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 using ND filters (neutral density) can give you more exposure options in bright light...

Words & pictures by Kingsley Singleton

The shutter speeds you can use are, to a great extent, governed by the intensity of the available light. If conditions are very dim, you won’t be able use the fastest shutter speeds, and if it is very bright, the slowest speeds will out of reach, too. The reason for this, as discussed last month, is that with the shutter speed and aperture working in concert, one has to adapt to the other, thereby letting in enough light to make a good exposure. So, if you want to use a fast shutter speed, the aperture usually needs to open to allow more light in. And if you want to use a slow shutter speed the aperture usually has to close to restrict the amount of light.

Above Neutral density filters are vital for controlling shutter speed and aperture. Mounted on the lens either using a holder, or screwing into the lens’s filter thread, these filters block a proportion of the light entering the camera. This is calculated in stops, just like the other exposure settings, so it’s easy to tell how strong a filter you need and what effect it will have on the light.

what the camera itself can do, and fitting a neutral density (ND) filter to your lens. There are two main forms of ND filter; those which are slotted into a filter holder, and those that screw directly to the lens. In either case, the ND filter will cut out a given amount of light, and thereby let you extend the shutter speed over what you could have done in the available light (or a wider aperture). The ‘neutral’ part of the name means that, despite stopping a proportion of the light, there should be no change in the hue of the colours in the image compared to when you’re shooting without a filter. To make it easy to know what you’re getting, the strength of ND filters ismeasured, like exposure settings, in stops. So you might have a one stop ND, a 1.5 stop, or a two stop version, and those will allow you to extend the shutter speed by the same amount. In practice, if your camera is topping out at 1/2sec with the maximum aperture and the lowest ISO reached, adding a two stop ND filter would let you use a shutter speed of 2secs at the same aperture and ISO. This means you can get the blurred water effect that you’re after.

The trouble is that there are physical limits to how much the aperture can open or close and, in terms of exposure stops, it only spans around a third of the range of the shutter speeds available. This mismatch in exposure latitude between the aperture and shutter speed means that, once you push the shutter speed beyond what the aperture can compensate for, you will either over- or underexpose the picture. When ISO can’t help If you want to shoot at faster shutter speeds than the available light and the aperture allow, you can raise the ISO setting, and if you want to use slower shutter speeds, you can lower it. But while you can keep raising the ISO, with the only trade-off being an increase in digital noise, if you want to lower it, you’ll soon hit the bottom end of the scale. Most cameras’ ISO ranges bottom out at 100 or even 200, so if you’ve set that and also reached the smallest aperture you’ll have to take additional measures.

NEXTMONTH Take an in-depth look at the third side of the exposure triangle, ISO. Which settings do you need, should you use auto ISO, and how do you get the best out of high ISO noise reduction? Which filter for you? When it comes to picking which filter you need, the strength of NDs is described in several ways, usually in terms of their optical density or their filter factor. For the former, you’ll find strengths of 0.3, 0.6, 0.9, 1.2 and so on; each of these equates to a one-stop (or EV) increase, so while a 0.3 ND will cut out one stop of light, a 0.9 ND will cut out three stops and a 3.0 ND ten stops. With filter factors. a ND2 cuts light by one stop, ND8 by three stops and an ND1000 by ten stops. One way to think about it is that NDs effectively take over where the lowest ISO setting ends, suppressing the light to allow longer shutter speeds than would otherwise be possible in the available light.

Above ND filters will be square, to fit into holders, or round, screwing onto the lens’s filter thread. They come in numerous strengths too.

Neutral density filters ‘Additional measures’ means going outside

Shooting in bright light

It’s not only slow shutter speed effects that using NDs allows – reducing the intensity of the light in the scene will also let you have more control over the aperture you set, both in natural light and when you’re using flash. For example, using an ND filter on a fast lens will let you shoot at, or close to, the maximum aperture even in very bright conditions, so you can get those very shallow depth-of-field effects without overexposing. When shooting with flash, even at lowest power, you often need to use apertures of f/8 and above to expose correctly, but NDs will offset this. And when combining flash with available light, they also let you keep below the camera’s maximum flash sync speed. Finally, the extra control that NDs allow in bright light means you can use the middling apertures for sharper pictures; so where you might have had to use an aperture of f/22 to restrict the light, which can soften shots through diffraction, a two stop ND would let you shoot at f/11.