Those of us who like to spend time in bars need to think about our lenses. If we are honest with ourselves - and over a beer or two, honesty can come fast, just ask Mel Gibson - many of us own one of these: either the Nikon 18-55mm VR or Canon 18-55mm IS. Some of us with a little more in the budget may have opted for the Nikon 16-85mm VR or 18-105mm or indeed the Canon 17-85mm IS if not the 15-85mm IS. Two of the hottest sellers are longer again: both the Nikon and Canon versions of the 18-200mm.
These are perfectly decent general purpose options except, like most zooms, when the available light begins to lessen. With minimal light the camera is forced to open the shutter for longer - imagine the mouth of Jason Akermanis - in order to correctly expose the picture. Here we encounter our problem: our hands shake, even minimally, and the picture blurs. This is because of the aperture range of these lenses, which, in photographic jargon, is “slow”. A “fast” lens can let in more light through the lens itself. This is a good thing.
Inside every lens is essentially a valve that can create a smaller or larger hole: the bigger that hole the more light let through the lens and the less depth of field (think portrait) or the smaller the hole the less light let through the lens and a greater depth of field (think landscape). Manipulating depth of field, also a function of the aperture - so often crucially blurring out a distracting background - gives us incredible creative potential as well. Aperture is measured in f-stops, which are fractions, and therefore the smaller the number, the larger the hole. So it follows that a lens with a fixed aperture of 1.8 will let in more light than our standard zooms mentioned above which generally begin at 3.5 at their widest point.
With the ability to command both the amount of light let into the camera and the degree of focus in our pictures, the aperture is one of our most essential tools in the quest for dynamic images and key consideration when considering buying lenses. Suffice to say, any discussion of aperture induces - I don’t know about you - a need for a drink. Hence the first sentence.