Taking control of your DSLR

Closing the lens aperture down to f/16 ensured details in both the foreground and distant parts of the scene would be sharply focused.

In the A (aperture-priority auto exposure) mode, the photographer sets the lens aperture, leaving the camera to set the shutter speed. This mode is used to control how much of the scene is in sharp focus and which parts of the subject are sharply focused.

Wide apertures (denoted by low f-numbers such as f/1.8 to f/4) produce a shallow plane of focus, which is ideal for portraiture. Small apertures (such as f/11, f/16 or f/22) make everything sharp from close to the camera to the horizon, an advantage for landscape photography.

Nikon’s entry-level DSLRs provide on-screen guides to the effects produced by aperture adjustments. (Source: Nikon.)

When using this mode, be sure to monitor the shutter speed setting (shown in the viewfinder) as small apertures will usually be associated with slow shutter speeds. You may need to increase the ISO value to compensate and bring the shutter speed back to a level where hand-holding the camera is possible. Alternatively, mount the camera on a tripod to prevent camera shake from spoiling the shot.

The S mode (shutter-priority auto exposure) lets you select the shutter speed you need, leaving the camera to set the lens aperture. It’s ideal for action shots because you can set a fast shutter speed to ‘freeze’ movement and also useful when shooting with a long telephoto lens because you can set a fast enough shutter speed to minimise the effects of possible camera shake.

Another application for the S mode is when you want to create intentional blurring in shots of moving subjects, such as flowing water. Slow shutter speeds (one second or longer exposures) allow you to achieve this effect.

Shutter speeds of between 1/30 and 1/60 second are ideal for panning, when you move the camera in synchronisation with the subject’s movement. This blurs the background (and stationary objects in the foreground), effectively isolating the subject.

The M (manual) mode gives photographers independent control over both aperture and shutter speed. Check the exposure values in the viewfinder display when using this mode to ensure your shots aren’t over- or under-exposed.

The manual mode allows you to set the lens aperture and shutter speed separately. It’s useful in difficult lighting, as shown here, because you can match the controls to the subject requirements. In this case, a fast shutter speed was combined with a wide lens aperture and high ISO setting to capture action in low light levels

The manual mode provides total exposure control and is useful when you want to create special lighting effects. The manual control only applies to exposure values; autofocusing remains active in this mode when it is selected.

Canon adds an A-DEP shooting mode, which stands for Automatic Depth of Field. It’s designed to record a wide depth of field in the shot. All the autofocus (AF) sensors will be used to detect the subject and the lens aperture will be set to make the key subject elements appear sharp. If flash is used with this mode, the results will be the same as obtained with the P mode plus flash.

Pentax cameras include an Sv setting, which allows users to prioritise the ISO value, while the camera sets the aperture and shutter speed. It’s used when there’s a risk of noise interference at high sensitivity settings, although many cameras support ISO limitation (which achieves the same objective) in their normal or custom menus.

The TAv mode (shutter and aperture priority) is also unique to Pentax cameras. It allows users to specify both the aperture and shutter speed and have the camera adjust the ISO sensitivity.

A fast shutter speed of 1/1000 second plus a 300mm lens were used to ‘freeze’ action in this shot.

A few cameras include a Flash Disabled mode that prevents the flash from popping up and firing in poorly lit conditions. A tripod will be necessary when a slow shutter speed is selected.

The C series modes are Custom User modes that allow users to register a collection of preferred camera settings for different shooting situations. Parameters that can be registered include the shooting mode, menu settings and any custom function settings used. These modes are handy time-savers for photographers who shoot a lot of pictures, such as portraits or landscapes, in similar conditions because they can simply select the appropriate custom mode and know their camera will require only minimal re-configuration.

Exposure Metering

Aside from the M shooting mode, exposure determination is largely automated in modern cameras. Exposure meters are used to 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.

Three metering patterns are available 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 multiple segments and individually evaluates the light levels within each segment. A basic pattern contains between five and seven segments, as shown in the illustration on this page. More complex systems use hundreds of segments and some cameras can integrate colour and/or distance information from the image sensor to improve the accuracy of exposure measurements.

Multi-pattern metering divides the subject into a number of segments and individually measures light levels in each before integrating the data to provide a correct exposure.

The 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.

Multi-pattern systems are good all-rounders, providing optimal exposure settings for most types of scenes (including backlit subjects and landscapes with large areas of sky). However, because they deliver an ‘averaged’ exposure setting, they may not suit some subject types.

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.

Centre-weighted average metering biases exposure levels towards the centre of the frame but takes the remainder of the frame into account.

This system can fail when you’re 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 underexposure, 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. Because areas outside the selected spot are ignored, spot and partial metering are ideal for backlit subjects.

To use a spot meter, simply centre the spot on the area you want to measure and 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.

Spot metering takes a single reading from the centre of the frame. It is ideal for backlit subjects.

Chimping

This colloquial term refers to the practice of checking shots on the LCD monitor immediately after they have been taken. Some photographers use it in a derogatory sense but many experienced photographers know its value.

The practice has benefits and disadvantages, as outlined below. The main benefit is that it’s a quick and easy way to check focus and exposure in shots. This allows you to delete unsatisfactory shots and free up space on the camera’s memory card while you’re on location. But it has a few downsides.

Chimping takes your attention away from the action and interrupts your communication with your subject.

While you’re looking at the last shot you can miss photo opportunities, particularly when the action is fast-paced.

Chimping, or replaying the shot you’ve just taken to check focus and exposure, allows you to delete poor shots on the spot.

Even in relatively low light levels, LCD monitors rarely display the full depth of tones in images and you may miss blocked-up shadows or blown-out highlights. Colour fidelity may also be relatively poor, particularly with low-resolution displays.

In bright outdoor lighting, the view is often too poor to make useful evaluations.

As a general rule, chimping can be useful if you have time. But you need to use the histogram (see box) to evaluate exposures, rather than relying on what you see on screen. And you should NEVER chimp if it would interrupt a shooting sequence.

Exposure Compensation and Bracketing

All cameras provide an exposure compensation (+/-EV) setting that lets photographers adjust exposures when the metering system fails to provide correct exposure levels. (Flash exposure compensation is also available for adjusting flash output levels.)

Exposure compensation settings use a sliding scale that normally runs from three f-stops of under-exposure to three stops of over-exposure. More sophisticated cameras cover a wider range of settings.

When the exposure value (EV) is set at zero, the metered exposure value is used. Adjusting the slider makes each picture you take darker (minus values) or lighter (plus values).

To prevent highlights becoming overexposed in outdoor lighting, many digital photographers set the exposure compensation on their cameras to -0.3EV to increase the chance of capturing highlight details. This strategy works best when you shoot raw files because they can be processed to recover details from a wider tonal range than JPEGs provide (more on this topic in Chapter 9).

For beach and snow scenes with large, highly-reflective areas in the subject, the camera’s metering system can be overwhelmed by the excessive brightness and may under-expose shots, often quite severely. To compensate, set the exposure compensation to +0.3EV or as high as +1.0EV in very bright conditions.

The technique of bracketing involves taking a series of shots with slightly different camera settings from those determined by the camera’s automatic measurements. The most commonly used setting is auto exposure bracketing (AEB), which involves changing the exposure level across a pre-set exposure range (usually from one to two f-stops of under-exposure to the same amount of over-exposure).

The normal practice is to take between three and five shots, with the middle shot at the metered value and the others above and below it. This basic strategy is a kind of ‘insurance policy’ for obtaining a correct exposure because it ensures a chance at least one of the three shots will be correctly exposed.

It doesn’t matter which order you take the shots in; nor does it matter whether you take three or five ““ or even more exposures. The amount of over- and under-exposure can often be specified in the setup menu and the midpoint can be specified anywhere within the bracketing range. You can normally bracket shots across the same range of exposure levels as the camera’s exposure compensation range.

Other functions that can be bracketed include white balance, flash exposure and focus. White balance bracketing is used to obtain better colour accuracy in mixed lighting. It normally uses in-camera processing to change colour balance across two axes: amber/blue and magenta/green.

Flash bracketing is like exposure bracketing for flash but normally adjusts the flash output without changing the camera’s exposure settings. It’s limited with on-camera flashes because of the delays associated with power recycling. Focus bracketing involves making small changes to the focus position. Depth of field bracketing is similar but adjusts the lens aperture instead. Both are most useful for macro photography, where they can provide a useful way to ensure key parts of the subject appear sharp.

Other bracketing options include ISO bracketing, which is comparatively rare. More common in entry-level cameras is effects (picture style, digital filters) bracketing, which lets camera users select the best effect from a series created in the camera from a single JPEG shot.

Dynamic Range Adjustments

While human vision can see a very wide range of tones, all cameras have problems recording subjects with an extremely wide brightness range. This has led manufacturers to introduce dynamic range compensation, which compresses both the brightest and darkest tones in the scene until they fit within the sensor’s recording range.

Dynamic range adjustments are only applied to JPEGs; raw files record more image data and provide greater scope for ‘developing’ highlights and shadows in images (more on this topic in Chapter 9).

The in-camera adjustments use algorithms to ‘map’ tones differently in highlight and shadow areas, pulling them back into the range of tones the sensor can record ““ and a monitor can display. The result is more even exposure with increased detail in the highlights and shadows.

The success of these systems depends on how much the tones in the original scene are compressed and the quality of the compression algorithms. They work best on images with brightness ranges that don’t greatly exceed the sensor’s capabilities. Images with very wide brightness ranges can end up looking unnatural and over-processed, as shown in the illustration on this page.

Some photographers like the effects produced by dynamic range processing and most image editors include HDR (High Dynamic Range) blending modes that can combine sequences of shots taken with different exposure values into a single image with an expanded dynamic range. (AE bracketing can be used to capture the shots.)

These images show the effect of in-camera dynamic range adjustments. The image on the left is captured without adjustment, while the one on the right has received a high level of in-camera processing.

An example of an image with a wide brightness range that has been over-processed through use of a strong dynamic range adjustment setting.

USEFUL URLs

The following websites provide additional information on the topics covered in this chapter.

www.photoreview.com.au/tips/ editing/high-dynamic-range-imaging3 for detailed instructions on high dynamic range photography.
www.cambridgeincolour.com/ tutorials/camera-metering.htm for a tutorial on exposure metering.

 

This article is an excerpt from Digital SLR Pocket Guide 3rd Edition.