Rodney Campbell's Blog

BTS: Graduated Neutral Density Filters…

by on Jan.31, 2014, under Life, Photography

Graduated Neutral Density Filters are common pieces of kit in many a Seascape and Landscape photographers bag but just what are they and what are they used for

These filters are most commonly used by people who shoot seascape and landscape scenes at the extreme ends of the day (sunrise and sunset). At these times there is often an extreme variation in the brightness/exposure of the ground (which is usually much darker) and the sky (which is usually much lighter). This massive contrast difference usually makes it impossible for a camera (digital or otherwise) to optimally expose the scene such that you have details in the shadows and the highlights aren’t all totally blown out – i.e. often the ground is black and the sky white

Some photographers compensate for this large exposure difference by taking multiple bracketed shots (at least one to correctly expose the bright sky and another to correctly expose the dark ground) and then merging these exposures in post either by various layer blending techniques via manual masks or luminosity masking or via specialised high dynamic range software which may perform exposure fusion and/or HDR tone mapping to merge the images

It is however often possible to manipulate how the light enters the camera in the field reducing the dynamic range of the scene so that it’s possible to capture in a single well exposed image. It is for this purpose that we use Graduated Neutral Density Filters (or Grad’s or GND’s for short)

Essentially part (half) of the filter has a Neutral Density (ND) effect and the other half of the filter is clear with a level of transition between the two

The Neutral Density part of the filter evenly reduces the amount of light which passes through that part of the filter by a certain specific amount (measured in stops). It is designed to be neutral in that it evenly reduces the light for all visible wavelengths and thus should not cause any colour changes, casts or shifts – only a set specific reduction in exposure. In reality this (colour casting and uneven exposure) is one aspect which makes the difference between cheaper and more expensive quality filters – especially when used stacked (multiple filters layered on top of each other)

The transition between the two halves of the filter is generally described as either:
– Soft (where the transition is gradual) and is designed more for uneven horizon lines (or where objects go across the horizon boundary – like trees, buildings, rock or land formations, etc)
– or Hard (where the transition is much quicker – but is still not an abrupt line) and is ideal for straighter horizons (e.g. the ocean meeting the sky)

Neutral Density filters are measured in stops of light reduced – for Grads the options are generally 0.3 (1 stop), 0.6 (2 stops), 0.9 (3 stops) and 1.2 (4 stops)

Physically the filters most landscape photographers would use come as square (for straight ND’s) and rectangular (for GND’s) glass or resin plates and are available from a number of manufacturers including Cokin, Formatt/Hitech, Lee and Singray. Normally you attach some form of filter holder to the filter thread of your lens and the holder has a number of slots which allow you to both place and rotate the filters as desired. My Lee filter holder has two slots for the flat plate style filters along with a front 105mm filter ring which also allows me to attach a 105mm circular polariser on the front

Teaching Point:

Technically you could have a full set of soft and hard filters of various densities however that can get quite expensive – I’ve found from others that the 0.3 (1 stop) grad isn’t all that useful and that in general the hard grads are more useful than the soft. The transition on the hard grads is still quite “soft” and gradual but the transition on the soft grads is much too gradual for most seascape work. Most people find the 0.6 (2 stop) and 0.9 (3 stop) grads the most useful

The basic use of these filters is that you place the darker ND part of the filter over the sky in your frame with the transition area over the horizon and the clear portion over the ground/sea/etc. The strength filter you use is determined by the exposure differential between the sky and the ground and you can also stack the filters for even stronger effect. Compositional rules would say that you should always make it so that the sky is still a bit lighter than the ground and certainly not darker than the ground since that would “appear” strange – personally I’d recommend going with what you like the look of best so I sometimes do make the sky darker than the lower part of the image (often water)

There is also a specialty version of the GND which is designed specifically for sunrise and sunset where the brightest portion of the scene is usually right on the horizon. This filter is called a Reverse Grad and the major difference between it and a normal Grad is that with the reverse grad the darkest part of the filter is right near the transition point and then the filter gets gradually less dense as it moves through the ND area. This allows the maximum holding back of the light right on the horizon and allows the upper sky to be less darkened

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