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HOW TO USE ND GRADS?
Expose contrasts that your camera can't cope with are inevitable when shooting outdoors. When it happens you'll want a graduated neutral density filter to come to the rescue. Here's what they do...
How to use ND GRADS?
Can I tilt the filter?
Do ND grads come in different strengths?
What do I do if there are trees or tall buildings cutting across the horizon line?
Can I use more than one ND grad together?
How to use ND GRADS?
EVEN IF YOU class yourself as a casual landscape photographer, a neutral density(ND) graduated filter should be nestling somewhere in your kit bag. When you are photographing the countryside, more often than not you'll be faced with extremes of exposure that the dynamic range of your camera's sensor or film just can't cope with. It doesn't matter how you try to balance the exposure, you just can't capture what's in front of you as you see it. But using an ND grad, you can.
Grads come in different strengths to enable you to cope with different extremes of exposure. They are also available in both circular and slot-in formats - although the slot-in is more popular because they are much more adaptable, allowing you to vary the point at which the transition between the clear and grey (neutral) areas is placed within the frame.
You'll also find a choice between soft and hard-edged ND grads and the difference is pretty much as it sounds. With a soft-edged grad the transition between the clear and grey areas is over a longer part of the filter, while a hard-edged grad has a more abrupt transition. Both have their uses although, interestingly, a 2-stop hard ND grad is the most popular type currently.
Filter Tip:
Avoiding flare that can ruin your shots is particularly difficult when the sun is either in the shot or close to the edge of the frame. If the sun is in the shot the only way of preventing iT is to make sure your filters are spotlessly clean and free of scratches. If the sun is just outside the frame then using a lens hood can help, as can carefully shading the filter with one hand.
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Can I tilt the filter?
While most photographers think of an ND grad as a tool to hold back the exposure in the sky so the grey part is in the top half and the clear zone is in the bottom half, that's not the way it has to be. You can position the grad so the grey ares is holding back a brighter foreground - perhaps a snow-covered field or a large expanse of bright water. Here, using two grads, one for the sky and one in the 'upside down' position, would work very well. You could, of course, even use them in a side-on position if the scene in front of you is heavily shaded on one side and sunlit on the other. In fact, you can use an ND grad at any angle that's appropriate. You might even want to angle the filter at 45 degrees so that the transition zone falls across an especially bright corner of the frame. Again, this is useful if the sun is visible or just out of the frame on one side. However, if you do tilt the filter you may find your image suffer from vignetting. This is where edges of the filter/filter holder are actually visible at the edge of the frame so you need to be aware of this when you tilt your filter holder.
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Do ND grads come in different strengths?
Yes, they do. The most easily available and useful densities are those that reduce the brightness by 1, 2, or 3 stops. For example, Cokin very sensibly labels it's ND grads as ND1, ND2 or ND3, The top B+W Graduated Neutral Density filters are available in 2 grades; #501 (approx. 1 stop density at the top) and #502 (approx. 2 stops density at the top).
With some systems you can also get ND grad filters that reduce the light by 1½ stops(0.45) and 2½ stops(0.75). Obviously, the exposure difference you are wanting to control dictates which filter strength you should use. If the contrast between the bright and dark areas is high, then you will need a stronger density. Be aware, though, if you use a density that's too strong for the scene it can look unnatural. Below are three examples of when you might choose each different density of tilter.
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What do I do if there are trees or tall buildings cutting across the horizon line?
Well there's not much you can do other than work around them. It's easy if the land merges with the sky in a clear and uncluttered manner because a hard-edged ND grad can be placed on the point where the two areas meet. But, unless you only ever shoot coastal landscapes looking out to sea or in the flatter areas of the county, such as Cambridgeshire, Lincolnshire or Norfolk, then frankly you're going to have to contend with buildings, tress, etc jutting out above the horizon line. The problem is that a grad can darken down elements, such as the tops of mountains, buildings or trees, so they lose detail and look wrong. In these cases a carefully positioned soft-edged grad is certainly better than a hard-edged one. The more feathered transition of a soft-edged grad will mean any darkening effect on areas close to the centre of the filter are less obvious in the final image. It's possible that some objects cutting across the horizon are just too intrusive to even allow for subtle use of a soft-edged grad. In these circumstances you can either try your best o position the grad to give the least effect, or ditch the grad altogether and see whether a polariser on its own can help you to get the shot.
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Can I use more than one ND grad together?
Yes, although bear in mind that if you use the recommended slot-in style of ND grad then you must use a filter holder so that both rectangular filters can be held in place and moved to the right position with ease and precision. You can either choose to combine two ND grads so the neutral area is precisely aligned, for example a 3-stop filter on top of a 1-stop filter means you can hold back the exposure in a particularly bright area by 4 stops. Alternatively, you can carefully position the filters so the ND part of each is masking a different area within the scene. This could be particularly useful when shooting directly into the light, where the light intensity may vary between a bright visible sun, a band of darker cloud, then a brighter area of clear sky. However, be aware that using two or more filters can increase the chance of flare occurring. It's also common sense to consider that more than one filter in front of your lens could reduce the quality of the final image. In reality, two good quality, perfectly clean and scratch-free filters shouldn't downgrade an image to a point obvious to the naked eye.
Back to top Filter Tip:
It can sometimes be difficult to see(especially with a soft grad) where the transition line between the clear and grey area is positioned within your image when you look through the viewfinder of your camera. A simple trick to help with this is to frame your image as required, set your exposure and slot your filter into position where you think it should be. Now press the depth-of-field preview button on your D-SLR so the image darkens in the viewfinder. Although it is darker, the transition line should be easier to spot and you can slide the filter up or down as required.
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