Spot Metering with a Digital Camera

All but the most basic of digital cameras offer a selection of metering modes, usually multi-pattern evaluative, centre-weighted and spot metering. Unfortunately, many photographers don’t know how to take advantage of these ‘advanced’ settings and leave their cameras set on the default multi-pattern mode. While this mode works well for a wide range of subjects, there are times when spot metering can deliver much better exposures because of the greater precision it provides for controlling which parts of a subject are captured with full detail.

Deciding which metering mode to use can be tricky, as you can see from the illustrations on this page. In some situations, switching to centre-weighted on multi-pattern metering will produce results that better satisfy your imaging requirements. This is largely a matter of taste, as only you can decide the kind of effect you wish to achieve.

If you’re after a silhouette-like effect, choosing centre-weighted average metering will produce a good balance between the main subject and the background, although the bright sky tends to cause the camel and the foreground to be reproduced with darker tones than they actually were in real life.

Choosing spot metering allows you to reproduce the camel and foreground accurately but makes the sky slightly brighter than it actually was when the picture was taken.

Because multi-pattern metering averages measurements of light values from between five to more than 50 discrete parts of the camera sensor’s field of view, it usually produces accurately-exposed pictures. With backlit subjects, the dark areas are often rendered correctly but the light areas, such as the sky, can be too bright. However, the overall impression is natural-looking.

However, since the main subject is the string of camels in the centre of the frame, many photographers may prefer switching to spot metering to record them with more detail. This bleaches out the sky but makes details in the leaves and bark of the trees more visible.

Spot Metering with a Digital Camera
The spot metering mode is usually identified by an icon showing a rectangular box with a black dot in the middle. In most cameras it’s located in the Function menu, although some cameras require you to use the main camera menu. The icon provides a guide as to how spot metering works. Unlike the other metering modes, which include the entire frame in their evaluations, spot metering takes a single exposure reading from the centre of the field and ignores everything else in the frame.

A typical spot metering circle can range from about 2.5mm to 3.5mm in diameter – or between 1% and 2% of the viewfinder frame with a ‘normal’ lens. The so-called ‘partial’ metering systems provided in some entry- and mid-level DSLR cameras have metering circles of between 5mm and 10mm and cover between 3% and 9% of the viewfinder frame.

The diagrams above show the difference in area between partial (top) and spot metering (above).

Regardless of whether your camera supports partial or spot metering, both controls can be used in the same way – although spot metering provides greater measurement precision. Simply locate the metering spot over the part of the subject that must be correctly exposed to measure the reflected light value (and set the camera’s aperture and shutter speed accordingly).

Spot metering is best used with the camera’s auto exposure lock (AE lock) and also with exposure compensation. On most cameras, half-pressing the shutter button takes a light reading which is retained until the button is released or the picture is taken. Most DSLRs have a separate AEL button, usually near the shutter release button. (AE lock may also be combined with AF lock.)

To measure the exposure from a selected area, simply centre the area in the camera’s viewfinder (or in the case of many digicams, on the LCD) and half-press the shutter button. Then either press the AEL button or keep the shutter button half pressed while re-composing the shot. Press the shutter button all the way down to take the picture.

On some cameras, the AEL will retain the exposure value for a second or two; on others you must press the AEL button a second time to clear the metered exposure. On most cameras, however, the locked exposure value is cleared once the photograph is taken, although some cameras provide alternative options via the custom or set-up menu.

What to Measure
Deciding which area(s) in a subject to measure can be tricky with some subjects, although very straightforward with others. For portrait shots, taking a reading from the subject’s cheek will usually produce the best results. However, you may need to adjust readings for different skin types.

For subjects with lighter Asian or Afro-Caribbean skin tones, no further adjustment is usually needed. Subjects with light Caucasian skin tones may look better with plus ½ to 1EV of exposure compensation, while a more natural rendition of darker Afro-Caribbean skin tones often requires minus ½ to 1EV adjustment. Use the monitor on your camera to gauge how much adjustment you need. (The histogram can help you.)

For landscape shots and cityscapes, the issue is more complex because these subjects often have a very wide brightness range. If you meter on the shadowed areas, the highlights may be blown out, while metering on lighter parts of the subject often produces blocked-up shadows. The best way to handle subjects of this type is to meter exposure values using a neutral grey card.

Grey cards are available through most specialist camera shops. Two options are common: heavy cardboard or fabric. Cardboard cards are the cheapest at just under $20 for an 18 x 23cm card. Fabric cards are more expensive (around $30 for a 30 x 30cm card) but easier to pack and store.

Spot Metering with a Grey Card
To use a grey card, simply position the card where it faces the camera in a place that is lit by the same lighting as illuminates the subject. Use the AE lock on the camera to measure the light reflected from the centre of the card. This exposure reading is used to take the shot. (Remove the card before actually taking the photograph!)

This technique works because the meter reading you take is an average of all the light illuminating the subject. It takes into account both highlights and shadows and sets exposure values to average out both extremes. For most subjects it should deliver correct exposures. However, with subjects that have an extended brightness range, such as beach and snow shots, detail in highlights and shadows may be lost.

In such cases, it is better to expose the shot to record detail in highlights because unless detail is actually recorded, the highlight will remain a flat, uniform white. It is usually possible to recover at least some shadow detail when you edit the picture. (Most of the better software applications support highlight and shadow adjustment.)

In-Camera Controls
Many digital cameras include additional controls that make it easier for photographers to measure exposure levels and compensate for subjects with wide dynamic ranges. The latest Olympus DSLRs provide two additional spot metering options: highlight control and shadow control. With the former, the camera uses the spot meter to measure the exposure but shifts into over-exposure to compensate for a very bright background. Shadow control does the opposite, shifting to under-exposure to allow accurate reproduction of dark tones.

Some cameras provide additional internal processing to pull the tonal range of extended brightness subjects down to a level that can be reproduced in prints or on screen displays. These controls are often used for correcting backlighting and may be linked to face detection systems.

Nikon’s D-Lighting system uses in-camera processing to adjust shadows, highlights, colours and, therefore, contrast levels. Newer Nikon DSLRs use an ‘Active’ version of this software that processes images on-the-fly to provide a wider tonal balance in both shadows and highlights.

All these systems are useful aids, particularly for novice photographers and photographers who shoot in a hurry. They provide a handy complement to a good understanding of how spot metering can be used.