Keeping image contrast down
Techniques to ensure your images contain no burned-out or black areas
Why would keeping image contrast down be important? Surely, images with lots of contrast have punch and the power to grab a viewer’s attention? That’s true: you really don’t want to have flat, lifeless images, devoid of energy, something typical of truly low-contrast pictures.
But there is a limit to how much contrast is good, and every photographer has experienced it. We’re all familiar with the scenario where the sky and the landscape look great to the eye, but it all goes wrong in the photo. The camera can expose correctly either for the landscape or for the sky, but not both simultaneously. The result is either a beautifully shot landscape against a sky that is burned out, or a nicely exposed sky in which all those white fluffy clouds are visible, but the land is lost in a dark brood.
The problem of dynamic range
This is not at all what the eye sees, so what’s the problem and how do we fix it? The problem lies with a mismatch between what our eyes can do and the capabilities of the camera’s sensor. While the human eye can capture detail across a whopping 20-24 stops (each stop being a halving or doubling of the amount of light), even the best DSLR and mirrorless cameras can currently manage only 12-15 stops. This is the dynamic range, and the difference between the eye and the sensor means that while a bright, high-contrast scene may be perfectly manageable for the eye, it will leave a camera’s sensor struggling.
What’s more, the 12-15 stop range is pretty much the best that a camera can manage. Smaller sensors with smaller pixels are able to handle a significantly smaller dynamic range than is possible for cameras with full-frame sensors and consequently larger pixels. This is particularly so for smartphone cameras, but it also applies to DSLRs and mirrorless cameras with smaller APS-C sensors. In addition, within the same camera, dynamic range will significantly decrease as ISO is increased, greatly reducing image quality.
The practical implications
So what does this all mean in practice? The result of this restricted dynamic range is that parts of a scene that look rather dark to the eye will come out completely black in an image. Those pixels trying to capture that part of the scene simply don’t receive enough light to register any image detail. They will be blank black. Conversely, areas that are too bright for the sensor will be ‘burned out’ in the final image: the pixels will go completely white, and again will capture no detail.
Does this really matter? If there are just minor parts of an image going black or burning out, such as a few highlights on sunlit water, no it probably doesn’t matter too much. But if whole blocks of the image are going blank – such as entire white clouds or shadows under a group of trees – then it matters a lot. Remember, the pixels covering such areas have captured no detail at all, and it wouldn’t matter what you did to the image in the computer post-photography you wouldn’t be able to rescue any detail. This could have a serious impact on the image’s visual effectiveness.
The conclusion to all this is that, in digital photography, the ‘ideal’ exposure is not necessarily one that gives us a ‘nice’ image as seen in the camera’s LCD screen. Instead, it is one in which we manage to produce an image in which there are no black shadows or white burned-out highlights. This is not always an easy task, especially on bright, sunny days.
Checking for black shadows and burned-out highlights
How can I tell if parts of my images are burning out or going black? One of the simplest ways to detect burn-out is to have the camera’s highlight alert switched on. When an image is viewed in the camera’s LCD screen it will create flashing alerts across those parts of the image that have burned out. Very simple and very effective.
To test for both black shadow and burned-out highlight areas, look at the image histogram. This lovely little graph comes with every image, and can be displayed alongside each image in the camera’s LCD screen. Make sure you have it switched on. Its curves record the tonal values from black through countless greys to white for every pixel on the sensor, with completely black pixels on the left, and white (ie burned-out) pixels on the right. If the graph is clipped at either outermost extreme then you have black shadows and/or burned-out highlights. Unfortunately, the graph won’t tell you where in the image that’s happening, though highlight alert will at least tell you that for the burned out areas.
Keeping image contrast down: the solution to the problem
Keeping image contrast down is the main way to overcome this problem of restricted dynamic range. In this way we reduce a scene’s contrast range down to something that more closely matches the camera sensor’s dynamic range. There are several ways to do this:
- Under- or over-expose the image;
- Use neutral density graduated (ND-grad) filters;
- Try the in-camera high dynamic range (HDR) facility;
- Avoid brightly lit high contrast scenes.
Let’s have a look at each of these in turn.
Under- or over-expose the image
OK, so I admit it, this method doesn’t actually reduce the contrast range, it simply shifts the exposure up or down a notch or two. When viewed via the histogram, the entire graph is shifted to the left for under-exposure and to the right for over-exposure.
It is usually used when the contrast range is only a little beyond what the sensor can handle, and only a minor tweak to the exposure is needed to bring the entire histogram – and hence the exposure – within the sensor’s limitations.
It is especially effective when the sky, for example, is just a little too bright, resulting in a relatively small amount of highlight burn-out. Under-exposing by just, say half a stop, shifts the histogram to the left and brings those previously burned out areas within the limits in which the pixels can capture detail.
The potential downside is that the entire exposure is reduced, meaning that shadow areas will get even darker, so you have to keep a close eye on this and be sure not to under-expose by too much.
This is a very simple technique. If you’re shooting in manual mode, just use a slightly narrower lens aperture to under-expose, or a slightly wider one to over-expose. If shooting in fully automatic or one of the semi-automatic modes (ie shutter priority or aperture priority) use the exposure compensation facility found in every digital camera. It’s usually easy to access, and you use it to tell the camera to under- or over-expose the image by a certain amount relative to what the in-camera exposure meter thinks ought to be the right exposure.
Neutral-density graduated filters
This is where we really enter the realm of keeping image contrast down. ND-grads are rectangular filters that fit onto the front of a camera lens. One half is completely clear, and the other half is darkened by a set amount, usually one, two or three stops. With the filter mounted on the front of the lens using a special holder, you put the darkened part over the bright area, and – as far as is possible – line up the filter’s dark-to-clear transition zone with whatever horizon you might have.
This has the remarkable effect of greatly reducing a scene’s contrast range down to something the sensor can handle, darkening those bright areas and rather lightening the dark places. The result is often something that much more closely aligns with what the eye is seeing, and totally overcomes the sensor’s dynamic range shortcomings.
You can also use ND-grads to exaggerate things, turning ‘modest’ grey skies into angry, stormy ones, for example, something that hugely increases mood and impact.
ND-grads are particularly effective in the classic scenarios of the rather dark landscape or seascape against a bright sky, and can work wonders to an image. ND-grads are a must-have inclusion in every landscape photographer’s kit bag.
In-camera HDR
The limitation of the ND-grad is that it only works when there is a clear division between a scene’s bright and dark regions, separated by a horizon. What do we do when the bright highlights and deep shadows are mixed together? This is quite a common scenario, and an example of this would include when you’re looking through strongly backlit vegetation.
This can be solved with in-camera HDR. The camera shoots several images at different exposures and then blends them together to create an image where everything is correctly exposed. I have to admit that I dismissed this technique for a number of years, because when it first came out it didn’t work very well. But of course, the software has improved and now it is quite a viable potential solution.
The facility is generally listed in the main menu, and by default is normally off. In switching it on, you usually have the option of having it on for just the one image or for all images until you turn it off again. Once activated, you then usually have a choice of blending style (‘normal’ or ‘vivid’ for example), and the exposure range to be used. When you press the shutter once, the camera will automatically take several images (usually three) and will then merge them together before showing you the final image. The whole process takes just a few seconds.
For many situations, this technique works quite well (including for those situations where I would normally use ND-grad filters), but there are occasions when artefacts creep in, so check the final images carefully.
HDR photography can of course also be done manually, though it does require time on the computer post-photography. Simply take a series of shots, successively under- or over-exposing the scene, and then merge them together using HDR software in-computer.
Avoid brightly lit high contrast scenes
It may seem like a strange thing to say ‘Just don’t take any photos’, but sometimes that’s the best course of action. Or perhaps, I should more accurately say ‘Tailor your photography to the conditions’. This is the ultimate solution to keeping image contrast down.
This means that if the sky is burning out in all your landscape images, for example, and you seem to be unable to solve it, then stop shooting views that contain sky. Frame your choice of images to scenes that consist only of land and/or water.
Two really big problem areas are woodland and fast-moving water under bright sunlight. The former in bright sunlight simple breaks up into a confusing mass of deep shadows and burned-out highlights in which the structure of the woodland and even of individual trees can be quite lost. When it comes to fast-moving water, this usually contains a significant amount of white water, which simply burns right out, especially when set against darker areas of deeper water. Both can be a disaster to shoot in sunlight!
So it’s much better to photograph such environments under soft lighting, usually quite cloudy conditions. Then the woodland becomes wonderfully exposed and visible, and the water a good mix of detail-containing white and dark water. This is particularly so for waterfalls, where white water is the dominant feature, making them very difficult to photograph successfully in bright sunlight. Under cloudy condition, rather than a trial it becomes a joy to simply put the camera on a tripod, use a long exposure and get images in which the water is a silken, well-exposed sheet of flowing, fully detailed white water.
Getting used to keeping image contrast down
So photography in brightly lit, high contrast situations is not necessarily a good thing, although you’d be forgiven for thinking otherwise when it comes to some forms of commonly-seen travel photography.
Essentially, you need to keep an eye on your image histograms and highlight alerts, and do whatever you can to ensure that the contrast range of a scene you’re shooting does not exceed the dynamic range capacity of your camera’s sensor.
This is a limitation that is universal to all digital cameras (and to film too), although not all cameras are equal when it comes to dynamic range. We can use the techniques outlined in this article to manage and limit this shortcoming, techniques that can have a dramatically positive impact on the quality of our photography.
There are people who believe that use of filters is in some way ‘cheating’, but in this scenario it can clearly be seen that ND-grads are there to overcome a problem, usually resulting in images that much more closely resemble what the eye sees.
Until the day comes when camera sensors have a dynamic range that matches the human eye – and that day is coming – we will need to continue to use all the techniques we have available in keeping image contrast down to a level that matches the camera’s dynamic range capacity. Doing this will help to maximise the quality of our photography.
