Why a digital SLR?

So you've decided to invest in a new digital camera and have made your mind up that you want to step up to a digital SLR, but the huge range of models on offer and endless flow of technical jargon have left you more confused than when you started? Fear not, this page will take the pain out of choosing the perfect digital SLR for you, whether you're a seasoned shooter or a total novice.

Before we get down to business it's worth stopping for a moment to ask the question: why would anyone want a digital SLR when compact digital cameras are so much smaller, lighter and more affordable? The answer can be summed up in two words: versatility and image quality.

The versatility isn't just the fact you can change lenses and add a wide range of accessories - from basics such as flashguns and remote controls to the more specialized equipment that allow SLRs to capture anything from the tiniest bug to the most distant stars. It's also about the creative versatility offered by the more advanced controls and higher quality components.

And this leads on to the second factor; image quality. In broad daylight the quality difference between a good compact and a digital SLR is minimal; both will produce sharp, colorful results with little effort. But when you start to push the boundaries a bit more; shooting in low light, attempting to capture fast moving sports action or wildlife, or when you want to experiment with shallow depth of field (to add a soft background to a portrait for example), the advantage of a digital SLR's larger sensor and higher sensitivity start to make a big difference. A digital SLR can't beat a compact camera for 'pop it in the purse or pocket' convenience but for serious photography the SLR wins hands down. With prices lower than ever it's not that surprising to discover that many people own one of each.

What do you need?

With so many digital SLRs on the market at so many different price points it's a good idea to narrow down your options by thinking about what 's important to you, and what you want to do with the camera. Do you want the same kind of 'point and shoot' simplicity as you get with a compact camera or are you the type of photographer who likes to roll up his or her sleeves and take control of every function? Many entry-level DSLRs offer a range of 'point and shoot' subject / scene modes, whereas more 'professional' models don't (though all have a basic 'auto' program mode). Do you shoot a lot of sport or wildlife? If so you'll need a fast camera with a high frame rate, and will probably want one of the smaller sensor formats.

If you shoot in very low light you'll be looking for a camera with the best possible high ISO performance and possibly in-body image stabilization, if you do a lot of studio, portrait or macro work you may well decide that a 'live view' function is a high priority.

Finally there are practical considerations; do you need a particularly rugged (or weatherproof) body? Does the size and weight of the camera play an important role in your choice? Do you have a particular application in mind that requires a specialist lens or other accessory? Not all camera systems offer the same range of lenses and not all cameras are compatible with the more specialized add-ons. And don't forget that most DSLRs are compatible with many of the lenses and accessories originally designed for film SLR cameras (from the same manufacturer), so if you're already heavily invested in a film system you may want to stick to the same system when you move to digital.

Armed with some answers to these questions you can use the information on the rest of this page to produce a shortlist of cameras that match your needs perfectly.

Sensor size

Let's look first at the physical size of CCD or CMOS sensor used to capture your photographs. Although there are slight variations, virtually all DSLR sensors fit into one of three size categories (starting with the largest); Full Frame, APS-C and Four-Thirds. Sensor size isn't as important as some people would have you think, but there are some key differences. The first is what is known as the 'crop factor'. As the diagram below shows, as the sensor gets smaller it captures a smaller area of the scene, resulting in a photograph that looks like it was taken at a longer focal length (1.5x or 1.6x longer for APS-C, 2x for Four-Thirds).

The crop factor isn't a major issue for most users, but it does have important consequences for some. If you're buying a digital SLR to replace a film model because you've got a kit bag full of lenses you need to be aware that unless you buy a full frame model all your lenses will produce very different results on your new camera.

For telephoto shooters the result is quite a bonus, as all your lenses will effectively get even more powerful. On the other hand the crop factor means your wideangle lenses will no longer offer anything like a 'wide' field of view. Fortunately there is a wide range of specially designed 'digital only' lenses for smaller sensor DSLRs.

The three most common sensor sizes compared: full frame, APS-C and Four-thirds. Smaller sensors 'crop' the scene and make a lens appear to have a longer focal length.	For sports and wildlife shooters the smaller sensor has the effect of making their telephoto lenses and zooms even more powerful.

So which is right for you? Each has its own benefits and each has its limitations, and if you're building a DSLR system from scratch you needn't get too hung up on which is right for you.

The largest (and most expensive) DLSR format is full frame (so called because the sensor is the same size as a frame of 35mm film). DSLRs with full frame sensors have the biggest, brightest viewfinders and because there's no crop factor are often chosen by photographers who are upgrading from a film SLR and already own expensive wideangle lenses. The larger sensor also means that - all other things being equal - full frame cameras will produce the best results in very low light and at higher sensitivities. On the downside, full frame cameras are big and expensive, and there is only a handful of models to choose from. You also lose the focal length 'boost' offered by smaller sensor cameras when shooting with telephotos.

APS-C is by far the most common format, used in virtually all Canon, Nikon, Pentax and Sony DLSR models. With a crop factor of 1.5x or 1.6x you need special digital lenses to get true wideangle results, but these are readily available and are usually less expensive than their 'full frame' counterparts. The 'kit lens' supplied with most APS-C cameras is a good starting point, offering a versatile zoom range from wideangle to short telephoto.

The difference in size and weight between a full frame professional level camera (Nikon D3, left) and a compact entry-level DSLR (Nikon D60, right) is considerable.	The Four Thirds system promises slightly more compact camera bodies and lenses. Four thirds is also the only DSLR system that uses the slightly squarer '4:3' format (as used on virtually all compact digital cameras).

Four Thirds is a new 'all digital' format developed by Olympus and currently used in Olympus and Panasonic DSLR models. Unlike the other systems on the market Four Thirds is not based on any existing film SLR system and uses a totally new lens mount, so all the lenses in the system are designed for digital, making the crop factor issues mentioned above less relevant. With the smallest sensor size Four Thirds offers slightly more compact camera bodies and lenses. Although the smaller sensor should in theory mean that these cameras produce noisier (grainier) results in low light and at higher sensitivities, for most purposes the difference isn't huge.

Anti shake systems

Photos taken in low light or with long telephoto lenses can easily be ruined by blur caused by the camera moving during the exposure (camera shake). Image stabilization (IS) systems are designed to counteract the motion of camera shake and reduce (or eliminate) the resultant blurring. Every manufacturer has a different name for it ('Super SteadyShot', 'Anti Shake', 'Vibration reduction', Mega OIS'), but all use one of two techniques. Optical IS works by moving a small element inside the lens and is totally separate from the camera. Sensor shift IS - as the name implies - moves the sensor itself. Note that the terms 'optical' and 'mechanical' IS are often used interchangeably.

	Image stabilization systems reduce the blur caused by camera shake, allowing sharp pictures to be taken even in low light or at long focal lengths.

The net result is the same, though obviously sensor IS is built in to the camera, so offers its benefits to every lens you buy. Lens IS has the advantage of stabilizing the view through the viewfinder as well as reducing blur in the final picture, but if you intend to buy several lenses it can end up being more expensive as stabilized lenses cost more than their non-stabilized equivalents. At the moment Sony, Pentax and Olympus offer in-body sensor stabilization, whereas Canon, Panasonic and Nikon offer stabilized lenses (including standard 'kit' lenses).

Speed

If you're used to using a compact digital camera picking up and taking pictures with a digital SLR will be a revelation; even entry-level models focus and shoot faster than any compact. As you move up the range to more expensive models focus speed will increase slightly, but the real difference is continuous shooting frame rate, something of vital importance to sports and wildlife photographers.

Most entry level DSLRs offer a continuous shooting rate of around 2.5 or 3 frames per second; plenty enough for most casual photographers trying to snap the kids or pets running around the back yard. The most basic models limit the number of shots you can take in a single 'burst' (especially if you're shooting in the highest quality RAW mode), but again this isn't likely to trouble the average user.

If you do have real 'need for speed' you'll need to leave the entry-level behind and move into the mid range or semi-professional sector. Here the starting point is 5 frames per second (though if money is no object the Canon EOS 1D Mk II and Nikon D3 can deliver 10 and 11 frames per second respectively), and larger 'buffer' memory means you can take a lot more frames in a single burst. To compare frame rates and other aspects of SLR performance for any model you're interested in check out the reviews here.

Size, weight and durability

As mentioned above digital SLRs come in a variety of shapes and sizes from the ultra compact entry-level models to the tank-like professional models. If you're likely to do a lot of shooting in damp, humid or dusty conditions you'll need to look for a DSLR with some kind of weatherproof sealing and a solid magnesium alloy chassis, and possibly one with a built in dust removal system to keep the sensor clean. Obviously this kind of durability doesn't come cheap, though there are plenty of mid-range models that offer some level of protection from the elements.

On the other hand, if you like to travel light and don't feel like lugging two or three pounds (1 to 1.3 Kg) of camera gear round your neck then look for one of the new generation of ultra compact lightweight DSLRs - some of which tip the scales at not much more than 20 oz / 0.5 Kg with a small lens. It doesn't matter how great a camera is if you can't be bothered carrying it around with you because it's too big or too heavy. You can check out camera weights and dimensions using the comparison tools here.

Screens and live view

One of the defining features of an SLR camera is that you frame the picture by looking through the same lens that takes the picture (by looking through the viewfinder). And many users moving from a compact to a DSLR in the past few years have been surprised to discover that you can't use the color screen on the back of the camera to frame the pictures in the same way you can with a compact. That's all changing as more and more models offer a 'Live View' option that does just that.

Live view isn't without its drawbacks; in most cases it slows down the picture taking process in use as the design of an SLR means the mirror inside the body has to be flipped up to enable live view and flipped back down to focus or take a shot. Some models allow auto focus in live view mode (without the mirror flips), but this is often a fairly slow process.

	LCD screens have come on in leaps and bounds in recent years and 2.5 or 3.0 inch LCDs are now common. Some models offer very high resolution screens for an ultra-clear picture, though of course this adds a premium to the price.

That's not to say live view isn't a useful feature; in the studio the ability to magnify a portion of the display to check focus is a real boon, and the fact you don't need to put your eye to the viewfinder can make shooting from very awkward positions (such as low on the ground or with the camera above your head) considerably easier.

Some DSLRs even have articulated screens that can be angled to offer a better view when getting behind the camera is difficult. You can check out the live view performance of any DSLR model you are interested using the in-depth reviews reviews here.

Advanced photography features

Although all digital SLRs share a common base feature set as you move up the ranges you'll find more and more advanced features which may or may not be of importance to you. You'll also find that higher level SLRs offer far more customization options, allowing you to fine tune every aspect of the camera's operation. Many of these functions and options will be of interest only to the more advanced photographer.

System addict...

One last word. Buying an SLR is, for many people, only the first step on what can become a lifelong relationship with a specific camera system, so it's important to look at the 'bigger picture' when making your choice. The camera is only part of the equation when it comes to image quality; the lens has an equally important role to play. Most of the major players have broadly similar lens options (and there's plenty of third-party alternatives for those that don't), but inevitably each has its relative strengths and weaknesses. If you have a specific application that needs specialized lenses (or other accessories) it's worth doing some research before committing to one system or another; dpreview's lens reviews and user forums are an excellent place to start.

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