Having used a little point & shoot camera for years, you've finally decided to purchase that high resolution dSLR you've been lusting after – but you need a lens to go with it or it won't do you much good. Most dSLRs optionally come with a "kit" lens ... are these any good? Part of the SLR concept is to allow interchangeable lenses; and each manufacturer has a bewildering selection of lenses to choose from. So what to do?

A modern camera lens is actually an assemblage of multiple lenses called elements and housed in a barrel. It will also contain a diaphragm mechanism (see below) and often an internal auto focus motor and other electronics, not normally visible to the eye.

We won't be going into optical theory, but there's a lot more to know even about practical matters than meets the eye, so let's dig right in:

Note: No optical theory but we won't be able to avoid some elementary school arithmetic. Also: lenses are measured in millimeters (mm); and 1 inch = 25.4 millimeters, just to give you a sense of scale.

Focal lengths 1: the 35mm standard

Before digital most of us lived in a very simple lens world. We all used 35mm film and understood the significance of the 35mm focal length designations. We knew that:

• 50mm is the "normal" focal length, meaning it's neither wide angle nor telephoto.
• Any focal length shorter than 50mm is wide angle
• Any focal length shorter than about 25mm is ultra wide angle;
• Any focal length longer than 50mm is technically telephoto;
• 85mm to 135mm is considered a portrait lens;
• And anything longer than 135mm is what most people think of when they hear the term telephoto.