The thinking goes like this: “if only I had the Nikanonmax 6-800 f/1.2 LD VR DO, retailed at half my monthly salary, I’d take better shots than Arbus, Ansel Adams and Cartier-Bresson”. If you’ve ever experienced a thought like this, you are suffering from a potentially dangerous and expensive disease called “lens lust”. I’ve got it. I don’t know any photography enthusiast who hasn’t. Lenses are rather like cars; they serve all kinds of purposes and it’s hard to tell similar ones apart without learning the technical details. It must be just as much fun doing the weekly shopping in Porsche as in a Ferrari – and just as impractical, but when has that stopped anyone? Also like cars, lenses are only *just* beyond our budget, so we yearn and strive and, inevitably, make mistakes.
I’ve made quite a few mistakes and thought I’d share them here. This is intended to be a general guide without insulting anyone’s intelligence. If I do, I apologise in advance. I use Canon equipment and know almost nothing about other makes. Please feel free to round out my ignorance, but don’t plug a specific lens. The purpose of this initial post is to help potential buyers make up their own minds based on some basic information. I have included some internet pointers for further help at the end.

Different types of lenses
The company that makes your camera body also makes lenses which are usually incompatible with other makes and often with previous technologies under the same name. So-called ‘third party’ lens manufacturers such as Tamron, Tokina and Sigma offer lenses for other camera makes (Canon, Nikon, etc.). Your first task is to make sure the lens you want to buy fits your camera body.
You then have to decide between zoom and ‘prime’ lenses. Zoom lenses have a variable focal length, say 70-200 mm, whereas primes have a fixed one, e.g. 30mm. If you have a full-frame dSLR, these values are as stated. If your camera’s sensor is smaller than a 35 mm exposure, multiply the lens focal length by your camera’s crop factor. The sensor in a Canon d40, for example, is 1.6x smaller than a 35 mm exposure, which means that my 200 mm telephoto actually works like a 320 mm. This is great for telephoto lenses, not so good for wide-angles.
Although zooms are more versatile, prime lenses are reputed to offer better image quality (IQ). They suffer less from chromatic aberration, vignetting, flare and barrel distortion, whilst being sharper from wide open to their minimum aperture (see below for definitions). Recent improvements in design and manufacturing, however, should overcome the basic snob appeal of prime lenses, although even I would avoid a zoom lens offering more than a 10x factor because quality inevitably suffers. Basically, if you see something amazingly cheap and amazingly versatile, there’s something wrong somewhere.

Definitions
The minimum focussing distance is exactly that: how close you can get to your subject and stay in focus. The lens gasket or manual usually provides this useful information. You can also purchase a multiplier (see under “add-ons”) to shorten that distance.
Focal length is the (sometimes nominal) length measured in millimetres between the lens diaphragm and the sensor and changes the angle of view.
Aperture is the width of the hole through which light enters the lens. The wider the aperture, the more light enters when you take a shot, but also the narrower the depth of focus. Lens sharpness varies with the aperture value. There is always a ‘sweet spot’ (usually in the middle, around f/8 – f/16) where your lens is sharpest. Remember also that only high-end (i.e., expensive) zoom lenses feature a constant aperture value throughout their focal length. On cheaper lenses the widest aperture value narrows as the zoom lengthens.
Chromatic aberration introduces colour fringes on edges of subjects, especially when using wide-angle lenses. CA can be corrected with image editing software and rarely shows up in smaller prints.
Vignetting darkens the corners of your shots. Sometimes this can be put to good effect, but it is better to have the choice. Some lenses are designed specifically for smaller sensors and will vignette badly on full-frame cameras.
Flare introduces rainbow-coloured halos and highlights when shooting towards a strong light source (usually the sun). It can be reduced using the lens hood or your hand, but is best avoided.
Barrel distortion ‘bends’ straight lines in shots, especially at wide apertures as a result of the shape of the lens elements. This can also be corrected using image editing software but is best avoided at the outset. There are also claims that some lens manufacturers introduce a specific colour cast. In the unlikely event that this is true, the cast can be corrected.
Bokeh is a term frequently bandied around which describes the quality of the out-of-focus background behind your subject. Photographers usually prefer a smooth bokeh.
Vibration Reduction (Nikon), Image Stabilisation (Canon), Optical IS (others) is a recent and expensive feature on lenses which is designed to prevent blur from our frail and shaking hands. Some camera bodies (Sony and Pentax, to name two) have in-built anti-shake devices. Obviously, it depends on your photography, but don’t expect it to replace a tripod or perform miracles.
Fast / noisy / full-time manual (FTM) focussing: autofocus is now a given. Some reviewers complain about lenses ‘hunting’ in low-contrast/light conditions but this really has to do with the autofocus chip on your camera. How noisy the lens focussing motors are may be important if you are shooting wildlife with a telephoto. FTM means you don’t have to activate the manual focus option on the lens to make manual focus adjustments, a feature not usually found on cheap lenses.
Lens creep: I first thought this was how we described ourselves, but it refers to zoom lenses’ unfortunate tendency not to stay at given length. Some makers include a convenient lock at the shortest focal length for travelling.

Different types of lens and uses
In order, lenses are available in the following formats:
* Macro: ideal for close-up photography. They can also double up as portrait lenses. They require some skill in manual focussing.
* Fish-eye: extreme wide-angle lenses sometimes offering a 180° field of vision (which occasionally includes the photographer’s feet, as I have discovered) and huge distortion, which is part of the fun. Fish-eyes are the most prone to dust (the glass protrudes from the lens barrel), CA and flare.
* Wide-angle: any lens with a focal length of less than 30 mm capturing a wide, panoramic shot although the edges will be highly distorted. They also suffer from CA, flare and distortion although not as much as fish-eyes.
* Tilt/shift: very much a niche lens which alters the focal plane, but a godsend to people interested in architecture and interiors because converging parallels can be reduced or avoided. Never cheap.
* General purpose: anything that covers a 50 mm effective focal length, usually considered the closest approximation to what the human eye sees.
* Telephoto: anything from 70 mm to 600 mm, which ‘bring remote objects closer’ and can also be used for macro and, surprisingly, landscape photography. One interesting phenomenon in telephotos is ‘foreshortening’, i.e., the distances between objects in the shot appear shorter than with other lenses. There is one basic rule I religiously apply: I know that my shutter speed must be the reciprocal of my zoom length to avoid blur if my lens does not have IS, i.e., a zoom with a 300 mm focal length will produce blurry pictures at anything slower than 1/300th of a second. This is where you add the tripod, mirror lock-up and shutter release cable or timer to your telephoto shot considerations and budget. Even so, I sometimes wonder how anyone can a) afford and b) physically use the hulking monsters available for thousands of US dollars.
* Zoom: any lens offering a variable focal length. The kit lens shipped with dSLR’s is a zoom usually covering the 17-85 mm range. I would personally avoid any zoom whose telephoto extreme length is more than 10 times its wide-angle length (e.g. 28-300 mm) because the image quality is bound to be poor throughout the focal length. Zoom lenses tend to be bigger, bulkier and more obtrusive than the others. The barrel of lenses with a high zoom factor tends to extend significantly and collect dust on the extended barrel which may also rotate about its central axis when zooming in or out and thus alter your filter adjustment. You have been warned.

Add-ons
Lens hoods: if you own a Canon camera, you know how bloody frustrating it is that only L-series lenses are sold with a lens hood. This is simply a vital component, especially for wide-angle and telephoto lenses. It helps prevent flare and, more importantly, protects the business end of your lens.
Filters: there are so many filters that there is little point discussing them here, however one is always useful in every circumstance: a UV circular lens. It also protects the lens from accidents. There are also macro filters which serve as a poor cousin to macro lenses.
Extenders: usually available in 1.4x and 2x formats, extenders multiply the focal length and, unfortunately, the maximum aperture. They sometimes worsen image quality. Check that your lens is compatible with the extender.
Multipliers: basically a ring which increases the focal length and thus the reduces the minimum focussing distance.

Other resources
Google your prospective lens and add the word “review” for a host of opinions, arguments and death-threats. Amateur photography magazines are, in my opinion, thinly disguised sales catalogues, so be wary (when was the last time you read a magazine review saying “this lens is crap, don’t buy it”?). The reputable ones do, however, use standardised testing methods.
One consistently high-quality resource is www.dpreview.com, which is about as objective as it gets on the web. Unfortunately, it can’t cover every lens on the market.

Where I live, Adobe Photoshop CS3 retails for the cost of a professional grade walkabout lens. If you take the time to learn it (and therefore improve all your shots), it will emulate / improve on the performance of any niche lens with a lot of patience and some lateral thinking. I’d choose a PS CS3 license over any two ‘pro-sumer’ lenses without hesitation.

In the end, it boils down to your personal preferences and budget. Before putting down your hard-earned cash, take some test shots using your own camera and check for the basic defects listed above (CA, vignetting, etc.). Chances are, if we buy cheap and nasty, that’s exactly what we’ll get. But that doesn’t mean we have to buy dumb.