What is an SLR?

SLR stands for ‘single lens reflex'. This acronym refers to the way light passes through the lens and out through the viewfinder. This happens through a series of mirrors and prisms within the camera which reflect the light coming through the lens up to and through the viewfinder to allow you to see exactly what the lens is seeing. When you take the photograph, the mirror flips out of the way to let the light pass through the shutter and onto the film or digital sensor (DSLR: digital single lens reflex)

This is different to how a compact camera works. With the compact cameras you have a separate lens on the front of the camera which is what you look through. The problem with this is that the image you are seeing is not exactly the image being projected onto the film or sensor by the lens.

With a very few exceptions, Single lens reflex cameras have interchangeable lenses. You can remove a lens and replace it with another with different properties (focal length, aperture etc).

What are the advantages of an SLR over compact cameras?

SLRs or DSLRs are usually physically larger than cameras with a fixed lens such as compact cameras and bridge cameras. This extra size allows for more powerful electronic components to be used in the cameras resulting in improved performance in functions such as Autofocus, metering and frame rates.

The sensors used in DSLRs are usually physically much larger than those found in compact cameras. This has a number of advantages in terms of image quality.

The sensor in a compact camera being so much smaller than that in a DSLR means that if the two sensors have the same pixel count, each pixel on the DSLR's sensor will be larger. This in turn means that each pixel receives more light and captures more information so is better able to produce a high quality image. The sensor will not only capture more detail, but be able to record a greater dynamic range, and produce lower noise levels at higher ISO settings.

The major advantage of SLRs is that the lenses are interchangeable. With a camera with a fixed lens, the only options you have are to attach wide angle and telephoto converters. These solutions are never very satisfactory as the images tend to suffer from distortions as a result of the converters. With an SLR, you have the option of choosing from a wide variety of lens designed to fit each camera. These lenses vary in their focal length, aperture, image stabilisation properties and macro focusing abilities amongst other things. Lenses designed for specific purposes will have an advantage over the fixed lenses found on compact and bridge cameras which are designed to cover a range of applications. With an SLR you can utilise these lenses to obtain the highest possible image quality.

The "System".

When you buy your first SLR you have to consider the fact that you are actually buying into a ‘system'. A manufacturer such as Canon or Nikon will produce SLRs all featuring a lens mount designed by them. This mount in many cases has remained unchanged for a number of years meaning that any lens that they have produced in that time will be compatible with their current cameras. The system also includes the flash metering and exposure systems that the manufactures have developed and are designed to work in conjunction with their own flashguns as well as all the other accessories they make such as cable releases, batteries and so on. It's therefore important to consider more than just which camera you are looking to buy, but at which system you think would provide you with the best options in the future. Some manufacturers are bigger than others and the strength of their system reflects this. These manufactures will have a greater range of lenses available and will also update their camera range more regularly meaning you have more options if you wished to upgrade your camera at any time.

There are also a few companies who make lenses which you can buy with a number of different lens mounts. If you had for example a Nikon camera, you could buy Nikon lenses, or ‘Nikon Fit' lenses from Sigma, Tamron and other lens manufactures. If you buy into one of the bigger selling systems then you will find it easier to obtain lenses from the 3rd party manufactures.

What are the main features to look for in an SLR?

• Pixel count

For most people, the first feature they notice when buying any camera is the number of pixels it has. This is understandable as it is the feature that manufactures push the hardest in their advertising campaigns because it is one of the few quantifiable features to boast about in a camera. However, pixel count is not as important as many people think.

Any Digital SLR with 6 million or more pixels can easily produce fantastic quality pictures at A3 size, so unless you want to make huge posters, or do a lot of cropping, any of the current DSLR cameras would be good enough.

• Frame Rate

The frame rate is the number of pictures the camera is able to capture per second. Most entry level DSLRs will be able to shoot at 3fps. The semi-pro/ high end amateur cameras are often capable of shooting at 5fps and there are a couple of Professional cameras designed for sports and journalism which are capable of firing at an incredible 10fps.

Most people will only use their cameras in single shot mode, but if you enjoy photographing sports or wildlife then a camera with a faster frame rate can be very useful. This is especially true if you enjoy the ultimate challenge of shooting kids running around the back garden!

• Build Quality + Ergonomics

Often overlooked, the handling can be one of the most important aspects of any camera, especially an SLR. It's hard to get an appreciation of this without actually physically trying the camera but most magazine or online reviews will mention this. The main aspects of the ergonomics of the camera to consider are the hand grip, the viewfinder, and the external button functions. The advantages of a good hand grip and a large bright viewfinder are easy to appreciate. The way in which you access certain features and functions on a camera may not seem terribly important at first but you soon start to realise how useful a good control layout can be. A well designed camera body will have all of the regularly accessed functions available on external buttons and dials whereas a camera of poorer design may have these features buried in menu systems. Again, read the reviews and the camera forums and you will see which cameras are praised for these abilities. Often the biggest difference between two cameras will simply be the way it handles.

• AF System

The autofocus systems employed by each manufacturer are usually very comparable at a given price point in terms of speed and accuracy. As you start to move up the manufacturers' lines of SLRs you will encounter improved autofocusing abilities. These improved AF systems usually go hand in hand with a faster frame rate. This is due to fast AF system being needed most when you are in the same situations that require a fast frame rate such as sports and wildlife photography.

• Anti Shake

Anti shake technology has been developed to counteract blurred pictures caused by camera or hand shake. This technology has been available in lenses and camcorders for a few years now. Nikon and Canon have a range of lenses with this technology which can be used on their SLRs. Recently we have seen other manufacturers build sensors into their camera bodies which work on the same principle as lenses, using gyros to detect movement and then shifting the sensor slightly to counteract that movement and produce a sharper image. This is most useful if you do a lot of work with longer focal lengths or in low light.

• Anti Dust Mechanism

This is another recent development in DSLRs. Every time you take the lens off the camera small particles of dust will inevitably enter the camera. When the shutter opens it is possible for the dust to settle on a filter across the front of the digital sensor. When you shoot with a narrow aperture it can be possible for small specs of dust to appear on your pictures. Whilst you can clean the sensor every few lens changes with a bellows blower or sensor brush, it is a great help if you have one of the integrated dust prevention systems. The best systems shake the dust off the sensor every time the camera is powered on or off and have adhesive pads which prevent the particles settling back onto the sensor. If you are worried about cleaning the camera's sensor yourself then these systems can be quite useful.

• Memory Card Type

One other main factor to consider is what type of memory card the cameras use. Most take Secure Digital or Compact Flash cards. The difference between them is insignificant, but you may have a collection of one type already so this may rule out certain cameras. If you are upgrading from a compact camera then you may well have SD cards already, but if this is your first digital camera then the type of media it uses is less important.