It’s a little late in coming, but I did promise a lesson in how to use graduated filters. It follows on nicely from what I spoke about in my post about the Zone System, so please familiarise yourself with that one before reading this if you haven’t already; this one will make more sense that way.
I mentioned that the range of brightness that a camera can ‘see’ is much narrower than that of the human eye. In most cases, you can isolate an interesting part of an image and bring out the detail where it’s relevant, however there are many situations where this simply doesn’t work. For example, if you are taking a landscape photograph that includes a lot of sky, especially late in the day, the foreground will generally be three or four stops darker than the sky. By metering for the foreground, the sky will be blown out, but if you stop down to increase detail in the sky, the foreground will usually become too dark.
Of course, with RAW files and sophisticated post-processing tools such as Adobe’s Photoshop Lightroom, it can be easy to fix such photographs afterwards, but this won’t always work, as often the sky is so overexposed that the highlights end up edged with grey as colour detail is lost. HDR is another option, but is time consuming, and the effect is not to everyone’s taste (myself included). It is far preferable, I believe, to achieve the effect you want in-camera. So how do you do this?
Take the following picture of the church at Prestbakki in northern Iceland as an example:
Original image
There is some great detail in the foreground, with the gravel and the weeds among the flagstones clearly discernible, but the sky is almost pure white. I could stop down by 2 stops to bring the clouds into detail, but then we lose the interesting textures on the ground:
Underexposed by 2 stops
However, by applying a 2 stop neutral density graduated filter (usually referred to as an ND grad) with the dark half placed so that it covers the sky, I can keep strong detail in both halves of the picture:
2-stop ND grad filter applied to sky
This is a particularly strong setting, mainly for illustration. The best use of ND grads is when you can’t tell it’s been used. Compare the next two photos of the river Lagan in Belfast:
Without filter
With 2-stop ND grad
The first was taken as is, and for the second I used an ND grad to drop the sky down 2 stops so that it was the same brightness as the river. The contrast between reflective surfaces (such as water) and sky will always be less than that between, say, grass and sky, so it’s possible to make the whole picture appear more uniform. This is another common use for ND grads where you won’t necessarily get the same loss of detail caused by excessive differences in brightness.
Finally, an example of where an ND grad really comes into its own:
2-stop ND grad used
This was taken at Thirlmere reservoir in the Lake District. The sky was turning some beautiful colours, but the bracken was a lovely shade of orange/brown and the drystone wall provided a nice way to draw the eye into the picture, so I placed my usual 2-stop ND grad on so that I was able to capture the lot.
Note that there are many types of ND grad filters. They vary from 1– to 4– stops, and in the degree of gradation, with a hard edge for clearly defined horizons, and softer edges for when foreground detail (e.g. trees, tall buildings) intrude on the sky and would be spoiled by excessive darkening. It’s a good idea to have more than one to hand, but you can vary the softening effect by using the aperture. A narrower aperture (higher ‘f’ number) will harden the boundary between dark and light halves, so open up as wide as you can if you want a softer edge without changing filters. You can also buy screw-on ND grads which rotate independently of the lens, but you can’t move the horizon up or down with these, so it’s better to get the rectangular acetate filters in the Cokin or Lee series.
I hope this lesson has also been helpful. Please use the comments below to let me know if I’m striking the right tone. I’d also like to see any photos you take using ND grads; you can link to flickr if you have any you’d like to share.
