Focusing and exposure in low light levels

When you’re shooting in low light levels, you will often find the exposure process is not intuitive. Autofocusing can also be difficult and sometimes the lens fails to lock onto the subject and you end up with a blurred image – unless you have taken precautions to prevent this.

Setting focus and exposure can be tricky when light levels are low. In this scene, autofocusing is easy as the scene contains edges with different brightness levels that the AF system can lock onto. Exposure measurement is more difficult as the photographer must balance the relatively bright sky against the darker foreground while retaining enough details to make the shot interesting. (Dodging was used in post-production to bring out lighter areas in the grassy foreground.)

While fast lenses (with large maximum apertures) will allow more light into the camera than slower ones, they come with a few disadvantages. To capture more light, the glass elements used in fast lenses are usually large, which means fast lenses are always bigger and heavier than lenses that are just one f-stop slower. They are also significantly more expensive.

Choosing lenses for low light work will always involve trade-offs between lens speed, cost and the amount of space required in your camera bag – and how much weight you’re prepared to carry. These decisions must be made by each photographer for each situation they encounter.

Sometimes it’s better to use a slower, more versatile lens, especially if you want a reasonable depth of focus throughout the scene. In that case, stabilisation – either in the lens or the camera body – will be a big advantage as it can provide as much as four to six shutter speed stops of shake correction (depending on the lens and how well it integrates with in-camera stabilisation systems).

Fast lenses allow more light to reach the image sensor. But they tend to record images with a very shallow depth of field, which can be attractive in some close-up shots as long as the focus is correctly positioned.

But even when you benefit from stabilisation, successful autofocusing requires enough light for the camera’s lens to find an edge to lock onto. When it can’t, the lens will keep readjusting as it ‘hunts’ back and forth. It takes longer to focus and the lens may never lock on, leading to blurred results. In many cameras you can prevent this by setting the camera to Focus Priority (rather than Release Priority) which locks the shutter until focus is achieved.

Modern AF systems use sophisticated algorithms and fast stepping-motors that can move the focusing elements in microseconds.  Many cameras combine phase-detection pixels on the sensor which can quickly detect which direction and how much to move the lens to achieve optimal focus via contrast-detection, which stops the lens when maximum contrast is detected. These systems can focus very quickly and accurately.

Most modern cameras should be able to focus quickly and accurately with a scene like this as the AF sensors can easily find contrast boundaries along the edges of the Opera House sails to lock onto. Since they are in the centre of the frame, this is the area that will be prioritised with the default AF settings in the camera.

However, there are some occasions when the photographer must take over and either switch to manual focusing or ‘trick’ the system to force focusing. This can occur with subjects that have very low contrast and when you’re shooting through glass or wire grids or if the subject contains highly reflective (specular) surfaces.

Switching to Spot AF mode and positioning the focus point on an edge in the scene where contrast is visible will help you to force autofocusing. Make sure your depth of focus doesn’t cause important parts of the scene to become blurry. (This is important for close-ups but less so for general scenes – unless parts of the scene are close to the camera.)

Most cameras will allow users to magnify the image in playback mode to check focusing. This function can also be available in manual focus mode in some cameras for fine-tuning focus before the shot is taken.

Many cameras include a MF Assist function, which enlarges part of the displayed scene up to 20x to make manual fine-tuning easier. Panasonic provides additional help for low-light photographers with special functions like Live Boost, which brightens the LCD screen to make framing easier.

Most cameras also include an AF-Assist light, which switches on briefly to provide additional light to help with focusing. Some may rely on a brief burst of light from their built-in flash, which is also used to minimise red eyes in portraits lit by flash. While these aids can be useful, it’s important to remember that light falls off as the distance from the camera to the subject increases, so they only work with relatively close subjects.

Exposure measurements

It’s often difficult to estimate how much light is available to your camera because most human eyes can adjust very quickly to changing light levels. Moving between a brightly-lit exterior and dim indoor lighting will show you how quickly you can adapt.

Even entry-level cameras provide a selection of different metering patterns, which place emphasis on recording brightness levels from different sections of a scene. Understanding how they work enables you to decide which one to use in different situations.

The illustration above shows four commonly-available metering patterns.

Evaluative or multi-pattern metering is the default setting for most cameras because it can assess scenes containing widely differing brightness levels. The field of view is split into several segments and each is measured individually. The camera determines an exposure level based upon these readings. Some cameras include distance information from the autofocus system and/or colour data.

Centre-weighted average metering integrates readings from all over the frame but biases exposure settings towards the centre of the field. It’s best used for subjects with an average brightness range where the main subject is central.

Spot and Partial metering patterns take a single reading from a small section of the field of view. Partial metering uses around 6% of the frame, while spot metering measures only 2% to 3%. Both ignore anything outside the selected area so they’re best used when there are wide brightness differences in the scene.

While poorly-lit scenes are sometimes relatively evenly-lit, sometimes they contain an extremely wide brightness range, which makes recording detail in both highlights and shadows, and the tones between, quite difficult. Most cameras’ metering systems will average out the tonal values in a scene and set the exposure level mid-way between dark and bright tones. While this works well for many situations, such as the scene illustrated on this page, it’s not always successful.

Many cameras would find both autofocusing and auto exposure measurement difficult when faced with a scene like this, there are no easily discernible edges for the AF system to lock onto and the overall lightness (‘high key’) of the scene will normally cause the metering system to under-expose, which results in a dark-looking image.

Tweaking exposures

There are times when you want to retain the predominance of dark tones in the subject and this is where the camera’s histogram display can help you to decide on the correct exposure level. Unfortunately, low light levels may contain areas that are extremely bright, which could lead to highlight or shadow ‘clipping’. For example, if the scene is darker than average your camera’s meter will overexpose the shot and its ambience will be lost. You can ‘trick’ the camera into under-exposing, often by one stop (+1EV) or more to retain the natural-looking balance and the ambience you want to reproduce. This can be done with exposure compensation adjustments.

When you want to keep the dark tones predominant in the image, the histogram (shown in the bottom right hand corner) should be biased towards the left hand side of the graph.

Even simple cameras provide sensitivity adjustments in the form of ISO settings. When you select the Auto setting, the camera will estimate the slowest shutter speed at which it is programmed to give you the best chance of recording without camera shake. It will then adjust the sensitivity of the sensor accordingly.

Modern cameras provide a very wide range of ISO values and shutter speed settings to control how long the sensor is exposed to the light reflected from the scene. These settings will affect how closely the photograph matches the scene and how much of the resulting image appears sharp. They can also control how movement is recorded. Short exposures ‘freeze’ motion, while longer exposures make moving subjects blurred.

Article by Margaret Brown (see Margaret’s photography pocket guides)

Excerpt from Photo Review Issue 86

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See more low light tips in Low light Photography 2nd edition pocket guide (print and digital editions).