The key to choosing lenses is to put a system together that serves your needs. If you're a photographer who likes to shoot a bit of everything - from sports to wildlife, portraits to landscapes - you'll want a selection of lenses covering 15mm to 300mm on a full-frame DSLR (or 12mm to 200mm for an APS-C camera). And for a portrait photographer, it's not much different; they generally face a range of situations that ideally call for a range of focal lengths: close-ups, groups, distant candids as well as your ‘standard' portraits. So, as you can tell, kitting yourself out can be pricey, which is why looking on eBay for bargains can be the best place to start.
The lenses you choose not only depend on your budget but also personal style and your brand of camera. While for the most part you're restricted to lenses from the same manufacturer as your camera, like Nikon or Canon, you should also consider independent manufacturers such as Sigma and Tamron as optically their lenses can be just as good but at more attractive prices. One thing is for sure though, in the digital age, lens quality has never been more important; high-resolution sensors are so good that they can also easily highlight any optical flaws in a lens. So investing in the best glass you can afford now will be worth it as they're built to outlive many a DSLR upgrade.
Whether you opt for a zoom or a prime lens really depends on your budget. Primes (fixed focal lengths) tend to have the edge in terms of image quality, but zooms are more versatile. The great advantage of prime lenses are they often have a fast maximum aperture (f/1.2 - f/2.8) and can be cheaper than fast zoom lenses that cover the same focal length. Wide maximum apertures are an asset if you shoot with shallow depth-of-field and if you're often faced with low light; it lets you use faster shutter speeds with lower ISOs than you could with a zoom with a smaller maximum aperture.
Another benefit is image quality. Prime lenses tend to have fewer elements than zooms, giving them the edge in terms of image quality, and can provide sharper images for less money. However, the difference in image quality is more noticeable between expensive and cheap zoom lenses than it is between expensive and inexpensive prime lenses.
The sensor is at the heart of the camera and plays a fundamental role in digital photography. Among its many characteristics, the size of your camera's sensor determines the effective focal length of the lenses used with the camera.
There are several sensor sizes, but the most popular found in the vast majority of DSLRs are APS-C and full-frame. The relationship between your lenses and sensor can seem confusing, but with a little experience you'll get used to it. If you take two identical lenses and attach one to an APS-C camera and another to a full-frame DSLR, you'll find the former provides a magnified version of the scene.
In effect the smaller APS-C sensor crops the image and is said to increase the effective focal length of the lens, compared to the full-frame camera. Each sensor has a crop factor, which must be multiplied with the focal length for example: an APS-C sensor has a crop factor of x1.5 (x1.6 with Canon) making a 50mm lens on a full-frame camera equivalent to a 75mm on an APS-C camera. In basic terms, an APS-C camera can give your lenses more pulling power - great for filling the frame with far-away subjects, not so great if you want to shoot wide-angle views.
Two other factors you should consider are whether your camera has a built-in autofocus motor or not; if it doesn't you'll need lenses that do. And do you need a form of lens-based image stabilisation? Often older models will not have this feature (illustrated as VR, IS, VC or OS depending on the brand), and they're cheaper as a result, but often it lets you shoot 2-4 stops slower and is an asset you cannot do without for longer focal lengths.
Most focal lengths are versatile enough to take all sorts of images, with the help of some creativity. For instance, you can take great portraits with a 70-300mm lens, a 50mm lens or a macro 100mm lens, even a wide-angle lens has its place if you like quirky portraits. They key is to understand the effect difference focal lengths have on perspective, field-of-view and depth-of-field.
Kit lenses that come with a camera are fine when you're starting out, but their general build quality and optical construction leaves much to be desired. If you're looking for a step up in quality, look towards a faster standard zoom that covers similar focal lengths but with a fixed f/2.8 aperture at 50mm. These lenses are perfect for portraits and lend themselves well to shooting small groups, too. Marquee brands can be quite pricey, so look for third-party lenses by Sigma and Tamron, and expect these lenses to be much heavier than a kit lens.
Every portrait photographer should have one or both of these focal lengths in their camera bag. At f/2.8, they give brilliant corner-to-corner sharpness and at their widest apertures they allow you to isolate facial details like eyes and lips. You can pick up a used 50mm f/1.8 lens for as little at £50 on eBay, but there's also the more expensive f/1.4 and Canon does an f/1.2 version. The 85mm f/1.4 is a beauty of a portrait lens, especially on a full-frame camera, providing unmistakeable bokeh. For a smaller budget, there's an f/1.8 version. Look for first generation lenses if you cannot afford the current models, as well as Sigma and Tamron, which show arguably marginal difference compared to Nikon and Canon except for the price.
If you're not sure what type of portraits you want to do and need a good all-rounder that won't blow the budget, consider this focal range. It's good for travel and outdoor portraits, even the odd landscape. It's ideal for full-frame cameras, but if you've an APS-C DSLR remember you'll lose that wide-angle view. Canon and Sigma do the 24-105mm f/4 but Nikon does offer a good AF-S 24-85mm f/3.5-4.5G VR alternative. For the aspiring professionals, you'll want to look for the superb 24-70mm f/2.8 that lets you switch from shooting standard portraits to wider views.
Nikon, Tamron, Canon and Tokina do similar focal lengths but the Sigma is highly recommended. While for conventional close-range portraits a wide-angle isn't usually the tool of choice, unless you want to distort features, it comes into its own if you need to include more of your surroundings or you're in a location that's tight on space.
Every brand has a version of this lens, each performing marginally different from one another. The 70-200mm f/2.8 is a masterpiece and a staple for many pro portrait photographers for its highly flattering compressed perspective. The added benefit of a fast, constant aperture in a telephoto lens means you're also able to capture very shallow depth-of-field; great for separating a subject from their background. However, these lenses are very heavy so it's advisable to opt for one with image stabilisation.