Digital and film cameras are fantastic pieces of equipment, but they can never capture the full range of contrast and light the way our eyes can.
Our eyes can take in a huge range of contrast across a scene, from a brightly lit sky to the shadows on the side of a rock without struggling, while a camera can only record a small amount of that range before losing detail either in the shadows – the featureless slabs of black, or in the highlights – the bright white parts of a photo often referred to as “blown” or “clipped” highlights.
In fact most digital cameras can only record around 5-7 stops of contrast in a scene, and anything that falls outside of that latitude will be lost detail, either pure black or white with no detail.
This is a particular problem in landscape photography where we may want to shoot into the light at sunset to record vivid colours in the sky, but also want include plenty of detail in the foreground, perhaps around rocks on a beach, or grass in a field.
There are various solutions to this, but for me the simplest and most natural is to use neutral density graduated filters.
An ND graduated filter is a rectangular filter where the top half is a neutral grey (meaning it should have no colour in it) while the bottom half is clear. They work by placing the dark part of the filter over the sky, which reduces the light passing through to the sensor in that area, giving less exposure. The clear part of the filter, which is covering the foreground, remains unchanged. They are available in different strengths, or “graduations” most commonly from 1 stop to 3 stops, but there are 4 stop graduations available.
These filters also come in two kinds; with a “hard” graduation, where the transition from the dark part to the clear part is very abrupt and “soft” graduations, where the transition is more subtle. Hard graduations work better in scenes with a clear horizon line, like the ocean, while soft graduations are more suitable for less defined horizons, like when shooting mountains or rolling countryside like in the shot below.
OK, so that’s what they are, but how so we use them?
Well, first of all, it’s important that we understand a little about the way a camera meter works to use them effectively. Most modern camera meters work by reading the light from all over the scene and giving an average. As the camera’s range can’t cover the whole of the scene, this average is bound to mean some compromise resulting in either lost shadow or highlights.
The easiest way then to meter such a scene is to cut all the sky out by filling the viewfinder with the foreground. This will give us an average of the foreground which most of the time will be within the cameras exposure latitude. With the camera in Manual mode, set the exposure that the camera indicates.
Then, point the camera at the sky that you intend to photograph, and see how much brighter it is. Camera’s differ, but on my Nikon one of the bars in the viewfinder’s exposure indicator represents half a stop, so I can usually see how much brighter the sky is through the viewfinder.
Alternatively, just change the shutter speed until the sky is correctly exposed and work out how many stops there are between the sky exposure and the foreground reading you did first.
For example, in the shot above, taken in Tuscany the sky was very bright from the pre-dawn sun. The ground however was still in shadow and so the contrast between the two was quite high, which meant I’d need a graduated filter.
I set the camera’s aperture to f5.6 – it was windy, and even though I was using a tripod I wanted to ensure a fast exposure to reduce the chance of camera shake, and maximum depth of field wasn’t an issue in this shot.
Then I pointed the camera so the grassy hill in the foreground completely filled the viewfinder, and set the shutter speed that the camera’s meter suggested, which was 1/30.
I then pointed the camera at the sky, which gave me a reading of 1/1000, five stops brighter than the reading for the foreground.
I’ll try to keep everything in an image to a 4 or 5 stop range, so from the exposure I shoot at, I want the brightest parts to be no more than 2 or 3 stops brighter than that, and the darker shadows to be no more then 2 or 3 stops darker. In this particular case, shooting at 1/30 would mean the hill would be in the middle of the exposure range, the shadows of the trees should be within the shadow range, but the sky, at 5 stops brighter, would be too bright.
Placing a 3 stop ND graduated filter across the sky in this image pulled the sky back into the camera’s latitude and enabled me to record the entire scene and keep the rich colours of the sky, as well as the green of the grass in the foreground field.
In this case, a “soft” graduation was also useful as the horizon line was indistinct.
Of course, metering can be even more accurate using a spot meter and taking readings from various parts of a scene, from the darkest shadows and brightest highlights (for example sun reflecting off water) in the foreground.
For the shot above, I spot metered the the shadows in the rocks as well as the brighter parts of the water. None of these areas would be covered by the filter, so I wanted to ensure that all of it would be within the 5 stop range. As it happened, it was a little too much, so I had a choice between losing highlights in the water or shadows in the rocks. In these situations I’ll usually choose to lose shadow detail as I believe the eye more naturally accepts black shadows than bright white spots…as long as the shadows aren’t too big.
Sometimes, when shooting into the sun and the sky is brighter than usual, 4 stop filters can be useful. Singh Ray make 4 stop reverse grads, which are like normal graduated filters in that they are half dark, and half clear, but the dark part across the middle of the filter (where we place the horizon) is darker than the rest of the filter. So a 4 stop graduated reverse filter will cut out 4 stops of light on the part that touches the horizon, while everything above that (for the rest of the sky) is 3 stops. The image taken below was on an evening when the bright part of the sky was incredibly bright, and highlights were clipping even with a 3 stop filter, so the 4 stop reverse grad, with the dark part covering the sun on the horizon, brought everything back into the exposure latitude.
Filters are also extremely useful when shooting reflections. Reflections are usually around 2 stops darker then the sky they reflect, although this isn’t apparent to our eyes. Again, the same metering process can be used to establish the different exposure values.
In the shots above, taken in the Sibillini Mountains in Italy, and below taken in the Amazon, there weren’t any significant shadows in the foreground so the exposure was very straightforward. In both cases the sky was 2 stops brighter than it’s reflection in the water, so I placed a 2 stop graduated filter across the top half of the image. When shooting reflections it’s worth remembering not to over filter the sky. A sky which is actually darker than it’s reflection will always look a little strange to our eyes.
When it comes to buying filters, there is a wide choice available.
Cokin P series for example are very affordable and easy to get hold of. They are a great and relatively inexpensive option for starting and learning about filters. However, they are relatively small, just 85mm wide, and the filter holder will almost certainly vignette and appear in the corner of your images if you shoot wide angle shots.
Also, cheaper filters are prone to colour casts, adding a slightly different colour to your images, which can be extremely irritating.
It’s better to buy the best quality filters you can afford because, like your tripod and lenses, they are an investment that will improve your images and should last longer than your camera body.
For me, 100mm Lee filters are the best quality filters available. They have no colour cast, are wide enough to shoot even at 15mm (at 35mm equivalent) and are very solidly made.