Exposure Adjustments

Exposing correctly is the key to successful photography – both analogue and digital – and DSLR cameras provide photographers with all the controls they need to create correctly-exposed pictures.

Exposure consists of two components: lens aperture (the hole through which light enters the camera) and shutter speed (how long the light is allowed into the camera). A correct balance between them creates pictures in which all tones in the subject are recorded correctly.

Exposure determination in modern cameras is largely automated although more sophisticated models (like DSLRs) let photographers override the camera settings and also set exposures manually. All cameras include exposure meters, which measure the tones in the subject according to a selected pattern. Understanding how metering patterns work will help you to decide which one to use in different situations.

Exposure Metering
Three metering patterns are used in DSLR cameras: multi-pattern evaluative (or matrix), centre-weighted average and spot. Selecting the correct pattern for the subject makes it easier to obtain the correct exposure settings.

Multi-pattern metering divides the subject area into five or more segments and individually evaluates the light levels within each segment. A microprocessor in the camera takes the readings from each segment and biases them according to the difference in overall brightness (and often contrast) within each segment and between adjacent segments. It then calculates which aperture and shutter speed settings will deliver an optimum exposure. Some cameras also include distance information from the autofocus system and/or colour data. Multi-pattern systems are good all-rounders, providing optimal exposure settings for most types of scenes (including backlit subjects). However, because they deliver an ‘averaged’ exposure setting, they may not provide the best exposure for subjects with a wide dynamic range.

Multo-pattern metering takes the readings from each segment and calculates which aperture and shutter speed settings will deliver an optimum exposure. This pattern is ideal for landscape shots and other subjects with varying levels of brightness.

Centre-weighted average metering integrates readings from all over the field of view, placing more emphasis on the centre of the field. It’s effective for subjects with an average brightness range where the main area of interest is central. It’s not suitable for shooting bright, contrasty scenes with sand or snow or low-contrast subjects with a limited tonal range. In bright conditions, there’s a tendency towards under-exposure, while poorly-lit subjects with a reduced brightness range are often over-exposed.

Spot metering takes a single reading from a small section of the field of view. In most cases, the size of the spot is expressed as a percentage of the field of view, with typical spot sizes ranging from 1% to about 4%. (So-called ‘partial’ metering systems have slightly larger metering areas but work on the same principle.) With spot metering, areas outside the selected spot are ignored. This makes spot and partial metering patterns ideal for backlit subjects.

Centre-weighted average metering integrates readings from all over the field of view, placing more emphasis on the centre of the field. This pattern works best with subjects that have a fairly uniform brightness distribution.

Spot metering takes a single reading from a small section of the field of view. Ideal for backlit subjects, this pattern is also useful for other tricky lighting situations.

To use a spot meter, simply center the spot on the area you want to measure and press the AE lock button and/or press the shutter release half way down. This locks the exposure (and focus), allowing you to re-compose and take the shot by pressing the shutter all the way down. A spot meter can also be used to gauge the brightness range in the subject. Simply measure the brightest and darkest areas and calculate the number of stops between them. These factors make multiple spot metering the best option for digital photographers when shooting wide brightness range subjects.

AE Modes
Having selected the appropriate metering pattern, the next step is to choose the correct exposure mode and decide whether to rely on the camera’s auto exposure (AE) system or manual controls. Setting the mode dial to Auto puts the camera into ‘point-and-shoot’ mode, effectively handing over most controls to the camera’s microprocessor. The camera sets the lens aperture, shutter speed, ISO and white balance to match the requirements of the scene it’s pointed at. This mode is rarely found on professional cameras and, when present, is normally used for snapshots and situations where little time is available for adjusting camera settings.

In ‘P’ mode, the camera also determines optimal aperture and shutter speed settings for the subject. But whereas the auto mode limits the controls the photographer can access, the P mode in most cameras lets them change either aperture or shutter speed and will adjust the other parameter to produce the same exposure level. (This adjustment is known as ‘program shift’ or ‘flexible program’.) In P mode, the photographer can also adjust the ISO and white balance settings (which can be left on ‘auto’, if desired) and use manual focusing. Other adjustments normally blocked in full auto mode include selection of metering patterns, exposure bracketing, continuous shooting, custom functions, colour space selection and some flash settings.

In ‘A’ and ‘S’ modes the photographer sets the aperture (A) or shutter speed (S or Tv in some cameras) and the camera adjusts the other parameter accordingly. A mode is often used to control depth of field (see Focusing and Depth of Field) large apertures produce shallow depth of field, while small apertures make everything sharp from close to the camera to the horizon.

(Above) Shutter priority AE is used for selecting a fast shutter speed to ‘freeze’ action.  

The S mode is used to freeze action or produce intentional blurring in shots of moving subjects. Manual (‘M’) mode gives photographers full control over both aperture and shutter speed. The A, S and M modes provide full access to all camera settings (except exposure compensation in M mode).

Canon’s DSLRs have an additional shooting mode, known as ‘A-DEP’, which sets the camera to record a wide depth of field in the shot. The camera uses data from nine AF sensor points to determine the nearest and most distant parts of the subject then selects a lens aperture that ensures they will be sharply imaged. This mode is handy for landscapes and group shots.

Shutter priority AE can also be used to set long exposure times to record running water.

Dynamic Range
A DSLR can record a wider range of tones than a digicam because it has larger photosites on its sensor. With correct exposure techniques, a DLSR should also be able to record a wider range of brightness levels than a film SLR (especially when slide film is used in the latter). Digital camera sensors react differently to light from film and have a more linear response.

When a photosite receives more light than it can handle, the signal overflows into adjacent photosites, which may also saturate. This usually results in burnt-out highlights (e.g. white skies, snow scenes with no detail). Some cameras may also block up shadows, especially when shots are taken in JPEG format. To prevent over-exposure, most digital photographers set the exposure compensation (see below) on their cameras to -0.3EV, dropping it to -0.7EV or even -1.0EV in very bright conditions.

The histogram display is a useful guide for setting exposures because its shape reflects the tonal distribution in your subject. When the graph comes down to zero at or near each end of the scale, the photograph has recorded the full subject tonal range and highlight and shadow areas should contain detail. To avoid blown-out highlights, make sure the graph does not touch the right hand end of the scale.

Note: shooting raw files (see Raw vs JPEG Capture) allows you to adjust exposure levels selectively to restore shadow detail that may have been marginal at point of capture. However, you can never recover detail that was lost through over-exposure.

The histograms above show the differences between an exposure that records all subject tones (where the graph remains within scale) and one in which highlight and shadow details are not recorded (where the graph goes up at both ends).

Exposure Compensation
Exposure compensation allows photographers to alter the settings used by the camera to reduce or increase the overall exposure value. It can be used to make images look lighter or darker and also to ensure highlight or shadow detail are recorded. The control is indicated by a +/- icon, either on a button or in a menu. Pressing the button and turning the command dial adjusts the exposure level.

The above image show the typical exposure compensation range provided by a DSLR camera. Left to right: -2 to +2 EV.

In most cameras up to two exposure value (EV) steps of adjustment are provided for both over- and under-exposure; usually in 1/3EV increments, although sometimes also in 1/2EV steps. To cancel exposure compensation you must re-set the control to zero.

AE Bracketing
The technique of bracketing, which involves taking a series of shots with slightly different camera settings from those determined by the camera’s automatic measurements, is used to cover uncertainties in exposure and colour balance settings. In auto exposure bracketing (AEB), the camera produces between three and five shots, with the middle shot at the metered values and the others above and below them.

The photographer then chooses which of the shots in the series looks best and can delete the others.

The green lights indicate the span of the bracketing; in this case 1.0 EV below and above the metered exposure.

Most cameras allow photographers to adjust the size of the AEB steps between +/-2EV but you are usually required to apply the same adjustment to both over- and under-exposure. In burst or self-timer mode, the bracketed exposures are taken continuously; in single shot mode, the photographer may be required to press the shutter button for each shot. Some cameras prevent AEB with flash exposures and AEB is blocked when the shutter is set for bulb.

AE and AF Locks
Most DSLRs have an AE lock button that lets you lock the exposure on a different part of the subject from the point of focus. This control comes in handy with backlit subjects, particularly when the main subject is off-centre. In most cameras, pressing the shutter button halfway down locks both the AE (auto exposure) and AF (autofocus) settings. So, when you focus on a subject and half-press the shutter button, both focus and exposure are locked and an indicator in the viewfinder should show this to be the case.

To use AE or AF lock with off-centre subjects half-press the shutter button to capture the focus and exposure settings and hold down the AE Lock button. Then, without lifting your fingers, recompose the shot and capture it by pressing the shutter button all the way down. If you want to take more shots with the same setting, keep the AE Lock button down.

In some cameras, the position of the AE lock varies with different metering and focusing modes. With multi-pattern metering it is commonly linked with the selected focus point. With centre-weighted, partial and spot metering, it is applied at the central AF point.

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