Burst Modes - and How They Work

The burst (or continuous shooting) mode on a digital camera is a new feature for those who have never used a film SLR but a familiar one for most SLR users. However, the way it works - and the options provided - are quite different on digital still cameras (DSCs). Whereas film cameras were dependent on how quickly and accurately the motor drive could jump the film through the film gate, digital cameras rely on the speed of the data processing system and the size of the internal 'buffer' memory where the image is stored while it awaits processing.

 

The burst (or continuous shooting) mode on a digital camera is a new feature for those who have never used a film SLR but a familiar one for most SLR users. However, the way it works - and the options provided - are quite different on digital still cameras (DSCs). Whereas film cameras were dependent on how quickly and accurately the motor drive could jump the film through the film gate, digital cameras rely on the speed of the data processing system and the size of the internal 'buffer' memory where the image is stored while it awaits processing.

Most buffer memories rely on DRAM (dynamic random access memory) chips, which are very fast, but relatively expensive. Consequently, cameras with generous buffer memories and fast burst modes usually cost more than those with modest specifications. Because DRAM is a short-term memory, images stored in the buffer must be processed immediately after they're captured. Once the buffer memory has filled, the image files must be downloaded to the camera's memory card, which is normally slower. While this is happening, the camera locks so you can't take more pictures until the buffer memory clears. This can take several seconds.

Continuous Shooting Modes

Compact DSCs typically support burst frame rates of between one and three frames/second (fps) for between three and 10 shots. In contrast, consumer DSLRs can usually support frame rates of between two and five fps for bursts of up to 10-15 frames and professional DSLRs have frame rates of five fps or more and can shoot dozens of frames in JPEG and RAW.

A few compact cameras allow an initial fast burst followed by a slower but continuous burst until the memory card is full. Whereas the burst settings on digital SLR cameras usually resemble those on film SLRs in that they simply capture a sequence of shots, many compact DSCs support several burst modes. The most common are:

- High-speed continuous, which uses a fast frame rate, typically 2-3 fps. Burst lengths are usually limited to 3-6 shots and resolution may be low.
- Low-speed (standard) continuous, which captures shots at 1-2 fps. Burst lengths are usually resolution limited. Most cameras start at 7-10 shots but some will record to the available memory capacity.
- Top/first continuous, which captures a burst of shots but only records the first four to six in the sequence.
- Bottom/last continuous, which captures a burst of shots but only records the last four to six in the sequence.
- Multi-shot continuous, which records a sequence of low-resolution shots (normally 16; hence the use of 'Multi-shot 16' by some manufacturers for this setting). The shots are captured at a fast frame rate and presented as a single image file.
- Interval ('time-lapse') recording sets the camera to take a sequence of pictures at specified intervals (normally between one and 99 seconds). Most cameras with this function allow users to set the number of images in the sequence.
- Konica Minolta's DiMAGE A200 has a novel ultra-high-speed setting that captures 40 VGA images at 10 fps, regardless of the image quality setting. Focus is locked with the first frame, regardless of the focusing mode. Digital zoom cannot be used and shutter speeds must be 1/30 second or faster. Shots can be viewed individually.

Auto bracketing is a variation on the continuous capture theme, which is offered in many of the more sophisticated recently-released DSCs. In most cases only exposure is covered, although some cameras also provide auto bracketing for white balance, focus, contrast, saturation and sharpness - although not necessarily all of these functions. In each case, the camera takes a sequence of three to five shots across the measured setting, allowing photographers to select the one that best matches their view of the subject.

Nikon's Best Shot Selector automates the bracketing process even further, capturing up to 10 shots while the shutter release is pressed and saving only the sharpest of them. It's useful in situations where camera shake is likely. However, it requires the camera to be set for single frame drive and does not work with either flash or the self-timer.

Restrictions

All continuous shooting modes limit camera functionality to some degree. Most manufacturers limit the number of shots that can be captured in a burst, although in many cases, this varies with the selected resolution and compression settings. In professional DSLRs the buffer memory is often generous and some cameras can capture 20 or more high resolution images in a burst. Most consumer cameras are restricted to bursts of fewer than 10 images at top resolution.

All cameras stop taking pictures when you release the shutter button or when the buffer memory is full. Some other common restrictions include:

1. Focus. Many burst modes lock the focus with the first shot in the burst. However, some slower burst settings allow the burst mode to be combined with the continuous AF mode so the camera can adjust its focus during the series. It is rare to find burst speeds greater than 2 frames/second where this combination is supported.

2. Flash and Zoom. Few burst modes can be used in conjunction with flash, particularly on a compact DSC. The same applies to zoom functions, particularly digital zoom, which is normally disabled, although some cameras allow optical zooming.

3. File Formats. Although many DSLR cameras allow the burst mode to be used with the RAW and RAW+JPEG resolution settings, in most compact digicams, burst capture is restricted to JPEG files.

4. Shutter Speeds. Some manufacturers require certain burst modes to be used with shutter speeds of 1/30 second or faster.

5. Power. Most cameras cannot switch to burst mode when a low battery warning is displayed.

BURST MODE TIPS

1. After shooting a series of pictures, all cameras require a few moments to process and save the shots. The camera usually locks while this takes place and no further pictures can be taken. Most cameras have a small light or indicator on the LCD screen that blinks until the buffer clears.

2. When purchasing a digital camera for sports photography, look for a model that can record at least 10 high-resolution images in a burst and check the supported frame rate. Many cameras have different capacities and frame rates for RAW and JPEG images. In some cases these are resolution dependent. Find out whether the camera requires a high-speed memory card to achieve its top burst rates.

3. When first and last burst modes are used, exposure, focus and white balance are normally locked throughout the burst sequence. Avoid moving forward or backward while the burst is being recorded. You can, however, move slightly either up or down or from side to side to vary shot compositions.

4. The first burst mode is best suited for action you can anticipate. Start shooting just before the peak of the action you wish to record and the camera should be able to capture it - as long as its buffer memory is adequate. The last burst mode should be used when you're uncertain of the timing of an event, for example when somebody blows out birthday candles. In this case, begin recording before the action starts and stop pressing the shutter button once it's over.
5. Use a tripod in dim lighting to minimise possible camera shake. A tripod will also let you use longer exposures (provided your camera supports them) and capture a related sequence of shots in time-lapse mode.