Digital SLRs buying guide

Stepping up from a compact snapper to a semi-pro dSLR (digital Single Lens Reflex) is like trading in your city car for an all-terrain vehicle.

The city car is great for taking the family out for a day, and the compact camera -- discreet, simple, easily slipped in a pocket -- is perfectly suited to capturing such an excursion.

The all-terrain vehicle, on the other hand, will take a family out on safari, across the desert or through a swamp, adjusting its power and torque to make it safely through. In the same way, the digital SLR can be built and rebuilt time and time again, with lenses, filters and hoods (its answer to power, torque and snow chains) changing the way each frame is exposed to best record such an epic adventure.

dSLRs offer unrivalled flexibility, but they do demand a little more input from their users to get the best results. If you're just setting out to buy your first one, we'll guide you through the most important considerations, starting with the element on which most manufacturers like to sell their wares: resolution.

Resolution

Compare a rack of dSLRs with an equivalent number of similarly-priced compacts and you'll notice that resolution is becoming less and less of a defining factor. Even the smallest compacts now boast resolutions that match those of their professional counterparts, so why not save some cash -- and a bit of excess baggage -- and stick with a compact? It all comes down to the way that resolution is composed.

A dSLR gives those multi-million pixels room to breathe, by positioning them on a physically larger sensor (right), which in turn means each pixel can be bigger and more sensitive to a greater variance in incoming light. The upshot is cleaner pictures and a better overall result.

The second of our reasons for buying a dSLR, flexibility, is delivered in most part by the interchangeable lens system.

Lens

If you're buying your first dSLR, you're in an enviable position. You have a wider choice than anyone contemplating an upgrade, as you won't already have bought yourself a collection of lenses that you'll want to re-use on your next model. You're not locked into a single brand, so can pick and choose the perfect package for your needs.

And when we say 'package', that's precisely what we mean. All manufacturers offer a choice of body-only or body and lens kits, the latter being by far the best choice for beginners. These kits bundle a relatively inexpensive, mid-range lens with the choice of fair to good zoom, depending on how much you want to spend.

The classic workhorse is an 18-55mm lens, which gives a respectable wide angle at the 18mm end, and a moderate zoom at 55mm. More ambitious users should look towards an 18-135mm lens, which pins the wide angle but offers a significantly improved zoom, although you'll pay slightly more for the privilege.

You should also expect to pay more for brighter lenses with wider physical apertures. The aperture is the opening inside the lens that allows the light to pass through to the sensor. The wider the aperture, the more light passes through. This allows for shorter exposures in darker conditions, which freezes action and reduces unwanted noise (the variation in brightness caused by the camera's sensor or circuits).

The aperture size is expressed in terms of f-stops, where a smaller number denotes a wider aperture. An aperture of f/1.4 is therefore wider (and brighter) than f/8, f/12, f/16, f/22 and so on. A lens will be described with reference to its focal length (in mm) and maximum aperture (in f-stops), so while a portrait lens might be sold as a 50mm f/1.8, a general purpose zoom would likely be a 18-55mm f/3.3 - f/5.8.

Notice that the zoom lens has two aperture measurements attached to its name. These denote the maximum apertures for the wide angle (in this case 18mm) and telephoto (here 55mm) focal lengths. The lower the number in each case, the better.

Aperture size isn't solely related to light control, though. A useful side effect of opening up the aperture is that you also shorten the depth of field, which keeps your subject in focus while blurring the background (and foreground, if you choose). This is why wide aperture lenses are traditionally considered the best option for portrait photography.

Retro lenses

Most manufacturers produce older lenses which are designed for use with their film cameras but also work perfectly with digital SLRs. This gives you the opportunity to buy into an existing library of lenses that in most cases stretches back decades. Sony doesn't have a long history in the dSLR market, but its cameras are derived from a long line of former models produced by Konica Minolta, whose camera business Sony snapped up in 2006, so check out older A-mount lenses for compatibility. This significantly enhances your camera's flexibility -- and with it your potential creativity.

Depending on the size of your sensor, however, they may not perform quite as you expect...

Sensor

Professional-level dSLRs feature what's known as a full-frame sensor. This is physically the same size as a traditional frame of 35mm film (which is actually 36x24mm, as the 35mm measurement relates to the width of the film including the perforations or 'sprockets', rather than its usable width).

Most consumer-grade dSLRs feature smaller and cheaper APS-C size sensors which, as the name suggests, match the consumer APS classic film format. The exact interpretation of these dimensions differs slightly between manufacturers, but whatever metric they use, the outcome is the same: it throws off the traditional measurements used to describe a lens' focal length.

So, if you attach a 50mm portrait lens from a film-based SLR to a Canon consumer dSLR, you'll need to multiply that measurement by 1.6 to work out how it appears on the smaller sensor. In this case, that 50mm lens would give similar results to a significantly tighter 80mm lens on a film camera. To achieve the 50mm result, you'd instead need to opt for a film lens around the 30mm mark, which would multiply up to 48mm on the smaller sensor. On most Nikon and Sony cameras, you'd multiply the lens' native focal length by around 1.5, giving that 50mm film lens an effective focal length equivalent to 75mm.

For this reason, all of the leading manufacturers create their own line of lenses tailored to their specific interpretation of the APS-C format, which effectively cancels out the difference.

There is, of course, one significant upside to this optical algebra, which is the saving you can make on longer zoom lenses by sticking with optics designed for film use. Any camera with a crop factor of 1.6x already has a head start on its film-using ancestors, as well as any digital siblings with a full frame sensor. At this level of magnification, a cheaper traditional 250mm lens will give the same zoom as a dedicated 400mm tailored to an APS-C sensor.

Framing choices

Take a dSLR to pieces and apart from killing it stone dead, you'll find that it centres on an angled mirror that both bounces the incoming light up to the viewfinder and snaps out of the way when you fire off a shot -- hence the momentary blanking of your view every time you take a picture.

Until very recently this made it impossible to use a dSLR's rear screen for anything other than navigating menus and reviewing your shots. That's starting to change as more manufacturers implement live framing by holding open the mirror to pass a live view straight through to the screen. In this way they can implement features traditionally only seen on compact digital cameras, such as face recognition.

Allied to this are the digital viewfinders, which appear on some superzooms in dSLR-style bodies, and in Sony's SLT camera line, which drops the traditional opaque mirror in favour of a semi-transparent alternative. This allows light through to the sensor at all times so that it can be rendered within the eyepiece using a very high resolution screen. This gives you all the convenience of digital compacts' framing features without the hassle of holding a bulky dSLR-style body at arms' length.

Other considerations

Beyond these features, everything else is a bonus, rather than a must-have. All dSLRs have a choice of aperture and shutter priority modes, plus full manual mode for complete control of your finished product. All dSLRs will come with both viewfinder and screen. And all dSLRs will have the interchangeable lens system, despite what some salespeople may tell you. If it looks like a dSLR but you can't swap out the lenses, it's a superzoom.

Think of a dSLR as a kit of parts, where the body, lens and filters can be swapped in and out to build whatever kind of camera you need to best capture whatever you want to shoot. Their versatility -- and flexibility -- is endless.