“You pays your money, you takes your chances,” as the old adage goes. It perhaps perfectly sums up the end result of buying a cheap lens. Whilst you might get conceivably get a great lens, the chances are that by paying less you are going to end up paying more in the long run, by having to upgrade because the lens you bought really doesn't stand up to close scrutinization. There is an increasing tendency for people to be obsessed with getting the very best camera, only to put on a cheap and nasty lens and hence negating any advantages they have gained. So what are the things to be aware of in cheap lenses.
Optics: Optics are what a lens is all about, cheaper lenses are going to use cheaper glass. This can manifest itself as lower grade optical glass, less tolerant manufacturing processes, little or no lens coatings and an increased chance of blemishes in the lens elements.
All or any of these defects will show up in your images as softness, low contrast and flare. A good lens will remain sharp from the very center to the very edges at all apertures, in cheaper lenses, the edges will often be significantly softer than the centre, particularly at widest apertures, great for that soft focus portrait look but pretty much useless for anything else.
Distortion: Manufacturers also keep the cost of their budget lenses down by reducing their complexity. Camera lenses are made up of a number of different optics combined together in groups. On a cheaper the lens, there will be less groups of elements, which in practice leads not only to a greater possibility of internal flare but more importantly distortion. Distortion becomes most apparent when photographing straight lines such as in architectural images. The two main types are pin cushioning and barrel distortion. Barrel distortion manifests itself as the image appearing to bulge outwards whilst pin cushioning shows up as lines curving inwards on themselves. Both can make a picture look odd or even ugly.
Maximum Aperture: Cheaper lenses are always a compromise, and one area that is perhaps comprised the most is the maximum aperture. A fast lens, say 2.8 or above requires a large front element and all its associated costs. Cheaper lenses will use much smaller front elements meaning smaller maximum apertures thus an increased likelihood of image noise or camera shake. This is particularly relevant to sports and wildlife photographers where the use of a tripod may not be feasible, so the only option is to increase ISO and hence the image noise.
Variable Aperture: Cheap zooms generally have a variable aperture. This means that whilst the wider end, say 18mm might start at f4, as you zoom in, the maximum aperture decreases. On the super zooms this can be quite a significant change, some going down as far as f6.3. This again can be problematic, especially when working in low light at the limits of your camera’s capabilities.
Build Quality: Pick up a professional lens and you may be surprised at its weight. This is not only due to the increased number of optics inside, but also due to the quality of the materials making up the housing. Cheaper lenses are invariably made of plastic, which is much more susceptible to damage if dropped or mishandled. The internals screws can easily work loose or even crack the surrounding plastic if overused, jamming the focus or zoom rings. They will not be weather sealed to any great extent, meaning not only that rain and moisture can enter the inside of the lens but also, more commonly, dust or sand. This again can have the effect of at least degrading the image and at worst, jamming the lens. Another symptom of cheap build quality is lens creep. This is where the zoom creeps in or out if the lens is pointed up or down.
More than any other piece of photographic equipment, lenses should be the one thing you spend some good money on. Ironically, whilst camera technology is changing almost monthly, lens technology is reasonably static. You should see the purchase of a good quality lens as an investment for the future rather than an unnecessary expense. A good lens will almost certainly outlast several camera bodies and yet it’s quality should match the camera technology for a long time to come.