ND-Grads

With a firm grasp of the whats and whys of the UV and ND filters, in this, the second part of our introduction to filters series, we take a closer look at what I believe to be one of the most useful of the garden variety filters; the graduated neutral density filter.

The graduated neutral density filter – or ND-Grad for short – operates using precisely the same physical phenomena exploited by the standard ND filter. Just like the ND filter, the ND-Grad is all about uniformly attenuating light before it reaches the lens and both types of filters use the ND Number Notation to indicate just how much stopping power they have*. In fact, the only difference between the two optics lies in how the light attenuating coatings are applied to the filter plate.

Unlike an ND filter, which has a light attenuating coating applied evenly across its entire surface, the ND-grad only has a coating applied to half of its surface. If you hold and ND-Grad in your hand, half of the filter will appear opaque – indicating the presence of the light attenuating medium – and the other half will be completely transparent. It almost goes without saying then that when an ND-Grad is placed in front of a lens, light passing through the opaque side of the filter will be attenuated while light traversing the transparent half will remain unchanged. This duality is remarkably useful to photographers as a tool for efficiently reducing the dynamic range of tones in a scene. At the risk of stating the obvious, when an ND-Grad is attached to a camera, half of the resulting image will be darker – less exposed – than the other.

 A Neutral Density Graduated Filter Kit. Top left: filter holder assembly and to its right, custom filter lens hoods. Bottom left: filters – soft, hard and medium edges.

A Neutral Density Graduated Filter Kit. Top left: filter holder assembly and to its right, custom filter lens hoods. Bottom left: filters – soft, hard and medium edges.

A good example of when this curious exposure imbalance can be beneficial is when trying to photograph scenes that include a skyline. For all its beauty, the sky is often a problem. Generally speaking, the sky is significantly brighter than any foreground element or terrain, and this is true whether shooting towards or away from the sun. It can sometimes be a challenge when trying to expose correctly for both the sky and foreground since adjusting the camera to account for the sky can leave the foreground considerably underexposed. The opposite is also true, adjusting for the foreground can result in a sky that is over-exposed.

Under these conditions, the dynamic range of the scene exceeds that of the camera; a situation that is often undesirable. There are several ways to cope with such scenarios and using an ND-Grad is one of them. By sliding an ND-Grad in front of the lens, the opaque portion of the filter can be aligned with the bright sky leaving the foreground aligned with the transparent area. The action of the filter then darkens the sky and leaves the foreground unchanged. This reduction in the difference between the brightest and darkest parts of the image effectively defines a decrease in the dynamic range. It is conceivable that an appropriately selected ND-Grad could reduce the dynamic range of a scene from one that falls outside the capabilities of the camera to one that falls within it.

And yet despite the enormous benefits attributable to managing the dynamic range of a scene, the ND-Grad has two weakness, and the first of these is its colour cast. For the ND-Grad, the issues of uniformity in its stopping power especially important. It is crucial to understand that by 'uniformity', I am referring to how evenly the filter attenuates light of differing colour. If a filter were to attenuate yellow more than blue, for example, the resulting image will have a colour cast base in that colour profile. In the case of the ND-Grad, this translates into a cast which only appears on the portion of the final image that is being influenced by the opaque section of the filter.

Ordinarily, colour casts are not a problem. Applications such as Lightroom and – to a lesser degree – Photoshop make dealing with a colour cast during post-processing a relatively straightforward affair. However, when a colour cast exhibits over only half an image, correcting it can be frustratingly fiddly. The reason for this ‘fiddlyness’ is that instead of merely adjusting colours until the entire image looks visually pleasing – as one would do with a simple ND filter – now editors are doing the same thing while trying to balance two halves of a photo which have completely different colour profiles. Furthermore, the human eye is particularly adept at discerning inconsistencies in colour making the margin for error with these localised adjustments surprisingly narrow.

The second weakness of the ND Grad is its transition zone, the area of the filter where the attenuating coating is faded out and finally becomes completely transparent. This region always lies precisely halfway along the length of the filter plate and is typically referred to as the ‘edge’. A soft-edge ND-Grad is a filter that has a gradual transition from opaque to transparent while a hard-edge grad features a substantially swifter transformation. Hard, soft and even medium-edge grads are all designed to produce the same level of dynamic range control but are each intended for use in different compositional situations.

Say, for instance, that you are attempting to take a photograph of a seascape. The scene is likely to present an abrupt transition from bright sky to the dark foreground at the point where the sea meets the sky on the horizon. Under these conditions, a hard-edge filter would be the most appropriate to use as it too features a rapid transition from opaque to transparent. By contrast, a mountainous scene that includes a lumpy horizon created by its peaks and valleys is less suited to a hard-edge. Were one to use a hard-edge, the mountains themselves would show the 'hard' transition from lighter to darker. Instead, such a situation calls for a soft-edge grad which – due to its gradual transition zone – allows the change from light to dark to be less perceivable.

It is worth my pointing out that despite the array of different ‘edges’, there is no perfect example of a transition zone. The fact is that ND-grads are manufactured with a complete straight ‘edges’, and the natural world is not graced with such perfect horizons. Even with a soft-edge grad, the mountains in the above example are likely to show exposure variations between their base and peak that match the transition of the filter from opaque to transparent. The best that a photographer can hope for is that the gradual transition between the two is not apparent to the casual observer.

It is easy to understand then that with all the limitation and inconveniences that I have mentioned here that some see the use of ND-Grads as less than appealing. This negative perception is likely to further reinforced when one considers that the effect of ND-Grads can be reproduced digitally and with far greater control. That said, there has to be something to them. After all, some of the best photographers on the planet choose to use them – despite the list of shortcomings – and while I cannot speak for all photographer, I believe there are two reasons for this.

First, there is left-brain logic. When you slide an ND-Grad into place, you are immediately presented with the results of your decision to do so. The little screen on the back of the camera will instantly display the results of the reduced dynamic range and, provided that an appropriate ND-Grad has been selected, the amount of detail visible in the photograph will have increased too. I contend that the clarity of thought resulting from the increased detail is of material benefit and can only help with the photographic decision-making process. I argue that the more detail you can see on your little screen; the more information you have to make choices that influence the quality of the final result. All the alternative methods of controlling dynamic range demand that you return to your desk before you are able to see what you have got to work with.

The second reason is rather right brained, using an ND-Grad is amazingly satisfying and, importantly, the gratification is immediate. There is something joyous about standing in front of a dramatic landscape and watching the little image on the back of the camera transform from flat and dull to vibrant and ‘contrasty’ as the filter is slotted into place. I would wager that most photographers wish that they could have their images looking perfect in-camera, without the need for spent hours in front of the computer bringing about the imagined perfection. To my knowledge, using an ND-Grad gets you as close you can get to this utopia, and this is unquestionably motivating. I defy a photographer to watch the image on the back of a camera come to life, and then to not want to take another photo. If only for this one reason alone, I suggest that ND-Grads find their way into every photographer's camera bag.

* Note: ND-Grads are not supplied as 'big stoppers'.


The Introduction to Filters Series

This series of articles takes a detailed look at the four most common filters available on the market today: the Ultraviolet, Neutral Density, Graduated Neutral Density and Polarising filters.