One of the most useful pieces of kit that you can own is a set of Neutral Density Graduated Filters. They can rescue a landscape image which would otherwise be spoilt by washed out skies or dark foregrounds. A large percentage of landscape images will always have a contrast between these two areas and it is therefore essential to have, and to know how to use, these useful filters.
Neutral density graduated filters (ND Grads) can make a really dramatic difference to your landscape photography. Images captured on film or digitally cannot record the same range of brightness as the human eye, leading to disappointing results. By using ND Grads, you can control the contrast between a light sky and dark foreground, allowing the camera's sensor to record the detail in both these areas. They can rescue a landscape image which would otherwise be spoilt by washed out skies or dark foregrounds.
2. The Problem
If you think of light as a series of stops, digital cameras can only see about 7 stops in total from black to white, whereas the human eye can see approximately 20. On a bright day, to obtain the subtle detail in both the foreground and the sky requires a greater number of stops than the camera's sensor can capture. Without a filter to bring down the brightness of the sky, you have to choose whether you are going to meter for colour in the sky or to have detail in the foreground. If you choose to have colour in the sky, you will find that everything in the foreground becomes a silhouette. If you choose to have detail in the foreground, the sky will become bland and washed out. Neither of these outcomes is ideal.
Image 2: Image metered for sky - foreground dark and lacking in detail
Image 3: Image metered for foreground - sky burnt out
3. The Solution
An ND Grad is a filter that attaches to the front of your camera's lens. It is graduated 50/50 in colour starting from dark grey at one end to clear at the other. It is given a number which tells you exactly how much it is going to reduce the brightness by.
Image 4: Lee Hard Edge Neutral Density Graduated Filters from left to right - 1, 2 and 3 stop
ND Grads usually come in one-stop increments with the most popular being 1-stop, 2-stop and 3-stop. However, it is also possible to obtain 4-stop and 5-stop filters. These numbers tell you exactly how many stops of light they are going to reduce the brightness by. A 2-stop ND Grad will let one quarter of the light through, a 3-stop one eighth of the light through etc... To complicate matters a little, they are not named in stops but as shown in the list below.
|ND 0.3||1 stop|
|ND 0.6||2 stop|
|ND 0.9||3 stop|
By positioning the dark part of the ND Grad against the sky, you can reduce the amount of light that reaches the sensor and therefore the number of stops in the image. In the mid area, this filter gradually changes from dark to clear making the effect look natural. Because the dark part of the filter covers the bright sky, it helps to reduce it's brightness, bringing the number of stops in the image closer to the range that it is possible to capture with your digital camera.
The filters also come in hard and soft edged versions. I like to use the hard edge versions as there is always quite a distinct difference between the lighter skies and darker foregrounds in the Lake District. Generally speaking, I have noticed that the difference is always around 2 stops. The hard edged versions are also best when the horizon is perfectly straight such as a seascape with no interruptions. You can then align them perfectly with the horizon.
4. The Main Players
The main players in the field are Cokin and Lee. I started off with Cokin ND Grads which are roughly one-third the price of Lee. I used these quite happily for many years. When I turned professional and started using a fullframe Canon EOS 5D and 5D MK11, I moved to the Lee system because of their higher quality which I have found to give more consistent results.
Both Lee and Cokin filters are used with a filter holder that screws on to the front of your camera's lens via an adapter ring. The ring required depends on the thread of your lens so you may need more than one adaptor if you use several lenses.
Lee manufactures two sets of ND Grads. One set, for use with wide-angle lenses, has a soft transition area between the dark and clear areas of the filter. The other set, for use with standard and telephoto lenses, has a hard transition area. The Lee filters are also much larger than the Cokin ones (6" x 4" as against 2.5" x 2.5") and ideal if you own lenses that have a large front diameter. This reduces the chance of vignetting when you use them (i.e. corners of the image going dark). You can also purchase wide angle adapter rings for a wide angle lens. This wide angle adapter also helps to avoid the problem of vignetting.
5. How to Position Your ND Grad
ND Grads need to be positioned very carefully to get the best effect. They need to be slid up and down in the filter holder with the transition from clear to dark falling accurately on the horizon or the light sky where you want to obtain a correct exposure. This is easier to do if your camera is placed securely on a tripod. To help position your ND Grad, you can set your shooting aperture and hold down your camera's depth-of-field preview button if it has one. Although the viewfinder will be dark, it will show the transition more clearly. Where your image includes mountains etc, you still need to make sure that the light sky either side of the mountain is still covered by the lower part of the graduated filter. I know part of the summit of the mountain will also be covered by a part of the filter but this hardly affects the image.
Once in position, don't forget to rotate the holder if you change from shooting from landscape to portrait. It's easy to forget this and any shots taken like this will be ruined.
Image 6: Here you can see the Lee Filter positioned correctly in the holder
6. Metering Your Shot
Step 1: The first thing you need to do is to set your camera onto aperture priority (AV) at f11 which, as mentioned in my previous tutorial on hyperfocal distance, is the optimum f-setting for landscape photography to achieve maximum sharpness throughout the whole of the image.
Step 2: Without your ND Grad in place, take a spot-meter reading with the foreground filling the whole of the frame. Remember with spot metering around 3% of the middle of the sensor will be checked to calculate the shutter speed. This means you can be very accurate to meter over the dark foreground area.
Step 3: Repeat step 2 with the light sky filling the whole of the frame.
Step 4: The difference between these two readings indicates the strength of filter you need. Use a 0.3 ND grad if the difference is 1-stop, a 0.6 ND grad if its 2-stops and a 0.9 grad if it's a 3-stop difference. Most filter holders have two slots so if the difference is more than 3 stops then you can use both slots to increase the ND grad capacity.
Step 5: Attach the correct ND Grad and take your shot using the spot-meter reading that you took for the foreground. Because the foreground of the image will be shot through the clear part of the graduated filter, using the metered shutter speed will ensure a perfect exposure for that part of the image.
7. Taking My Image
This image was taken at Blea Tarn, Little Langdale in the English Lake District. It had rained heavily throughoutthe day but in the late afternoon some blue patches started to appear in amongst the grey and overcast skies. However, the light was still far from what I would have preferred it to be.
When taking my image, I spot-metered my shots as described above and found that there was about a 2-Stop difference between the light sky and the dark foreground. The metered details are as follows:-
- ISO 100 on Aperture Priority at F11
- Foreground was spot metered at 1/13th of a second
- Background was spot metered at 1/50th of a second
Image 7: 1-Stop ND Grad (0.3) used on Manual at 100 ISO, F11 and at 1/13th of a second.
As you can see, some definition has started to appear in the sky but this is still slightly over-exposed. The foreground is perfectly exposed as I used the shutter speed that I obtained for the foreground when the image was spot-metered.
Image 8: 2-Stop ND Grad (0.6) used on Manual at 100 ISO, F11 and at 1/13th of a second.
In this image, I used the correct strength ND Grad with the result that correct exposure has been obtained throughout the image.
Image 9: 3-Stop ND Grad (0.9) used on Manual at 100 ISO, F11 and at 1/13th of a second.
Here the sky has become slightly under-exposed but this is personal choice. Some photographers will prefer the more dramatic sky which using this 3-Stop ND Grad has given.
I think you can see that, for what is a modest outlay, a set of ND Graduated Filters is a 'must have' part of your photography kit. You would see a big improvement in your landscape images immediately, and the days when you thought that the sky was too flat for photography will be a thing of the past.