How to get perfect exposure
Despite the convenience of rescue tools found in image editing applications, there’s no substitute for getting your exposure right first time. Tim Daly reveals how
Correct exposure is achieved through the right combination of aperture and shutter speed, and makes a world of difference to your final image quality. Every digital camera has a built-in light sensitive meter, which is used to determine all auto exposure functions and, on more advanced cameras, the manual exposure readout in the viewfinder.
Light meters can only respond to the brightest values in your subject, regardless of their size, shape and colour, which means they can be fooled by everyday situations. A perfect exposure results when the photographer guides the meter into capturing a balance of highlight and shadow detail. Too much or too little light will have a profound effect on image detail, tone and colour reproduction.
Aperture, shutter speeds and ISO
These three independent variables are entirely interlinked and when one is changed, another needs to be changed to compensate. In addition to the creative consequences of using these scales, their primary function is to enable the photographer to shoot photographs in widely different lighting conditions. The ISO scale sets the sensitivity of the image sensor and works in an identical way to ISO speed in conventional film. At low light levels, a higher ISO value like 800 is best selected, so the sensor can operate with less light than normal.
At bright light levels, a smaller value like 200 is set. On basic digital compact cameras, the ISO value is fixed, but better models have a selection of different values such as 100, 200, 400 and 800.
Once your sensitivity has been set, then the right combination of aperture and shutter speed is sought to make a good exposure. The aperture is in your camera lens and is essentially a hole of varying size designed to let more or less light reach your sensor.
Apertures are a uniform size on all cameras and conform to an international scale described as f numbers like f2.8, f4, f5.6, f8, f11, f16 and f22. At the f2.8 end of the scale, an aperture is at its largest and lets in the most amount of light available. At the opposite end of the scale, such as f22, an aperture is at its smallest and lets in the least amount of light. To accompany the aperture scale is the shutter speed scale, again designed in a standardised range, but in fractions of a second such as 1/1000th, 1/500th, 1/250th, 1/125th, 1/60th, 1/30th, 1/15th, 1/8th, 1/4, 1/2 and 1s. At the 1/1000th end of the scale, the shutter remains open for short time, but at 1/2 second, the shutter remains open for longer.
Metering systems
There are three common metering systems used in digital cameras. Centre weighted on the left, matrix in the middle and spot metering on the right.
Most digital compacts use the functional rather than foolproof centre weighted metering. Centre weighted metering works by making an exposure judgement based on subjects that are placed in the centre of the viewfinder. This is perfectly adequate for centrally placed compositions, but can come unstuck if you intend to frame your subjects off-centre.
The much better matrix or segment metering system is designed to cope with the greater demands of more adventurous photographers. It works by taking individual brightness readings from the four quarters of your frame, plus an extra one from the centre. These five readings are then averaged out into a single exposure reading, resulting in a better balance.
The more complex spot metering system takes a reading from a much smaller area, typically the tiny centre circle superimposed in your viewfinder. Useful for getting accurate light readings from skin tones or other small and precise elements of a composition, a successful spot reading will emphasise this over other less important parts of your image.
Where to take your readings
The camera meter never knows which is the most important part of the image. It can only respond to variations in brightness so you have to trick the meter into behaving differently. With so many different levels of light reflecting off objects, the best exposure is a trade off between recording simultaneous detail in both highlights and shadow areas.
Most good digital compacts have an exposure lock button located close to your shooting hand, or accessible when the shutter is half-depressed. Exposure lock enables you to take meter readings from the important areas of your image, save the reading then recompose before shooting.
Camera histogram
All digital SLRs and quality compacts offer the benefit of a Levels histogram where an image is played back on the rear LCD preview monitor. If you find it hard to judge if your image file is overexposed, check its histogram. The histogram shows the quantity of pixels on the vertical scale together with their brightness values along the horizontal scale. At the left-hand end of the graph there is the shadow point with the highlight point set at the opposite right side. As each image is recorded, it’s possible to judge exposure by looking at the histogram. If this function is not available on your camera, it can be accessed via the Levels dialog in your image editor.
