As a rule, the larger the maximum aperture of your lens, the better. A lens with a large maximum aperture, say f1.4 or f2.8 is often referred to as “fast”, a turn of phrase implying it will allow you shoot with faster shutter speeds/less ISO – it doesn’t mean how fast it focuses!. A lens that will open up to f1.4 is more useful than one which only opens to f4.
The benefits are that a wider maximum aperture will let more light in – you’ll be able to shoot in low light with a faster shutter shutter speed, or a lower ISO, and it will be easier for the camera to focus, and for you to look through the viewfinder. It will also allow you to shoot at a wider aperture, and therefore get a shallower depth of field, if that’s what you’re after.
It also has an effect on quality – pretty much all lenses don’t perform at their best either wide open or fully stopped down, and reach a sweet spot of sharpness and contrast about 3 stops down from their maximum aperture. In practice this means that a lens with an f4 max aperture will be hitting it’s best quality at about f11, whereas an f1.4 lens will hit it’s sweet spot at f4. If you want to see just how much this applies, go and look up the “MTF” graph for any lens you fancy, and you’ll see what I mean.
There’s a catch – isn’t there always. Building lenses with large maximum apertures is complicated, and complicated things get expensive. The same focal length lens may cost £100 for an f1.8 version, and £300 for an f1.4 version, and if they make an f1.2 or f1.0 well over £1000. The engineering problems inherent in making a zoom lens means that they’re much harder to build with fast apertures – the fastest zooms you’ll find will be f2.8, and often slower than that.
Variable and Constant Aperture
Some zooms have what’s called a variable aperture. In this case you’ll see 2 numbers quoted, the smaller number usually corresponds to the wider end of the zoom, and the larger to the longer end, so a zoom may be an 18-200mm f3.5-f5.6. What this means is that the maximum aperture of the lens is f3.5 at the 18mm end, and then it stops down to f5.6 as you zoom out to 200mm. On the plus side, this keeps the lens quite small, and reasonably priced, but the drawbacks to having a variable aperture are many. For one thing the lens as a whole tends to be pretty slow – f5.6 as a maximum aperture is going to limit your shutter speed/ISO combinations a bit, and the fact that your exposure settings may shift as you change focal lengths can be very inconvenient if you’re in manual mode and shooting with fixed lighting. If you’re using an automatic exposure mode of some kind, the camera will compensate for this.
There are zooms that don’t change aperture, known conveniently as constant aperture zooms. These tend to be faster – often around f2.8 – and obviously don’t change exposure during changes in focal length. The catch is they’re bigger and heavier, and certainly more expensive. My 18-200mm f3.5-5.6 costs £350 and my 70-200mm f2.8 (which obviously only covers a smaller focal range as well) costs £1500. However, I’d always rather work with constant and faster apertures, and only use the 18-200 when size and weight are a real issue.