How we expose a photograph for a given scene can make the difference between it heading for the trash and being mounted on a wall.  Modern day cameras can automate the process with a degree of success, but understanding the basic principles behind exposure can provide you with the knowledge required when it comes to trickier lighting conditions that built-in camera metering systems often can’t cope with.

When taking a photograph, there are three primary factors determining how it is exposed; aperture, shutter speed and ISO.   Aperture refers to the size of the opening through which light is transmitted through a lens, shutter speed is a misnomer referring to the length of time the digital imaging sensor in your camera is exposed to an image and ISO refers to amount of amplification applied to the light information received by the sensor.

In typical, well lit circumstances, a photograph is deemed well exposed when there’s a distribution of tones without any unintentional clipping of shadows or highlights which result in lost image detail.  This isn’t always possible to achieve especially when a scene contains a greater dynamic range than what an imaging sensor can record.

As with any rule in photography, there are exceptions as well as the bending of rules in order to achieve the final desired result, but in many cases the best exposure is one where all of the determining factors are balanced to achieve a spread of tones without any clipping.  It’s important to reiterate that this isn’t necessarily the exposure you would want for the image you wish to create, but is more of an average for averagely lit scenes.

There are three main factors under a photographer’s control which determine how a photograph is exposed; aperture, shutter speed and ISO.

An analogy for the concept of exposure is to think of taking photographs as filling a bucket.  Instead of light, our medium is water.  To substitute the lens, we have a hosepipe.  The size of the diameter of the hosepipe is our aperture and in place of the shutter, we have a tap.  Our bucket is therefore the ISO.

Rather than dealing with absolute values of litres and gallons, let’s suppose the non-clipped exposure for a given ISO is a bucket which is filled exactly to the brim without any spillage.  This is useful in that it immediately tells us that for a given size of bucket (ISO value) we require a specific amount of water (light) in order to fill it (achieve an average exposure).

One slight twist of logic is with our sizing of buckets.  The larger the bucket number, the smaller the bucket actually is.  Just as the larger the ISO value, the less light we require to achieve an average exposure.

So, exactly how much water is required to fill any particular bucket?  What diameter hosepipe do we require?  How long should we keep the tap open?  As with filling a real-life bucket, we can vary the diameter of the hosepipe as long as we vary the length of time we keep the tap open proportionally.  Likewise, we can fill a bucket by leaving the tap open for longer when we’re using a smaller diameter hosepipe or for a shorter period if we’re using a larger diameter hosepipe.

By balancing the diameter of the hosepipe with the length of time the tap remains open, we control the total volume of water that travels through it.  As long as that volume is enough to fill the bucket and no more then we will achieve our goal.

Going back to the world of photography, for a given ISO setting, the digital imaging sensor inside your camera requires a certain volume of light in order to achieve a non-clipped exposure.  In terms of exposure, it doesn’t matter whether you use a larger or smaller aperture as long as the length of time the shutter is kept open is long enough for the required volume of light to reach the sensor.  Too little light results in an underexposed image, just as too little water results in an under-filled bucket.  Too much water results in over filling, just as too much light results in overexposure.

There are considerations and consequences of using different apertures and shutter speeds, but it’s the balancing of these factors which determine exposure.  That’s why taking a photograph is just like filling a bucket.