Tag Archives: exposure

Graduated filters

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
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
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
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
Without filter
With 2-stop ND grad
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:


Thirlmere, Lake District
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.

The Zone System

What I’m going to try now is the first in a series of lessons, where I’ll show you some aspect of photography of which the average point-and-shoot snapper may not be aware. Hopefully if you put these lessons into practice you’ll be able to achieve the sort of photographs you’ve always wanted to. Let me know if I sound too patronising; that’s not my intention at all.

The zone system was developed by the American landscape photographer Ansel Adams, and the principle is simple. A camera can ‘see’ a more limited range of brightness than the human eye, so it needs to be adjusted to take into account lighting conditions. Most cameras try to render a scene so that the overall lightness is what’s known as 18% grey, or exactly half way between pure black and pure white. If we number these brightnesses, 0 for pure black, 10 for pure white and 5 for 18% grey, then by filling in the numbers in between we have the 11 zones of the zone system. Each zone is 1 stop brighter than the previous one.

The zones mapped to an exposure meter
The zones mapped to an exposure meter

Most modern SLR cameras, digital or film, allow you to over- or underexpose by up to 2 stops in auto or semi-automatic mode, which can be limiting, so the first thing to do when using the zone system is to switch to full manual mode. The next thing to do is to set your camera to spot metering mode; refer to your camera’s manual if you’re not sure how to do this. You are now equipped to try the zone system.

Each zone has a description of what it represents:

0 – Pure black
1 – Near black, with slight tonality but no texture
2 – Textured black; the darkest part of the image in which slight detail is recorded
3 – Average dark materials and low values showing adequate texture
4 – Average dark foliage, dark stone, or landscape shadows
5 – Middle gray: clear north sky; dark skin, average weathered wood, grass
6 – Average Caucasian skin; light stone; shadows on snow in sunlit landscapes
7 – Very light skin; shadows in snow with acute side lighting
8 – Lightest tone with texture: textured snow
9 – Slight tone without texture; glaring snow
10 – Pure white: light sources and specular reflections

What you need to do to apply it is to find a key part of your photograph that you can assign a zone to, then set the exposure accordingly. In the illustration above, I have mapped the camera’s exposure meter onto the middle 5 zones, but that does not mean you are limited to just these zones – by using full manual mode you can go beyond these zones but you need to count!

Now let’s take an example. Suppose you are in a forest on a sunny day, where the grass on the floor is partly lit by sun and partly in shade. The most effective way of exposing such a scene is to meter on the sunlit grass, and the rest will take care of itself. Point your camera’s spot metering zone at the grass so that the grass fills the metering zone, and adjust the exposure so that the camera says ‘0’ – i.e. perfectly exposed (or zone 5). You can then recompose, and because the camera is on manual, the exposure will not be affected. The result will be something like this:

Spot metered

A camera set to full auto would have seen the silhouetted trees in the background and overexposed by a stop or two, to look something like this:


Note how the vivid green in the foreground in the first picture has been bleached out, and the stump is not as isolated as a feature as it was.

Generally you will be able to find a part of your picture that fits within zones 3-7 so you can see your camera showing you how far over- or underexposed you have made it. However if you want to venture further into the more extreme zones, here’s where you need to count. To expose for zone 2, set the exposure to underexpose by 2 stops, then decrease the aperture or the shutter speed by 3 more clicks of the wheel (if your camera meters in 1/3 stop increments, as in the illustration above) or 2 more clicks (if 1/2 stop). For zone 1, use 6 or 4 clicks, and 9 or 6 for zone 0. To achieve a silhouette, for example, you should ensure the object you want to appear in silhouette is at zone 2 or below, but bear in mind that you may lose detail in the background if it is not bright enough, so take another reading from the background to ensure that you keep it in zone 5 or higher. If the contrast between the background and the object isn’t great enough, you probably won’t be able to achieve a perfect silhouette.

I hope this has all made sense; by all means look on the internet for more about the zone system and how a camera meters, but if you do get any good results using this system I’d love to hear from you.