Single lens reflex cameras have been around for decades and have long been the top choice for professionals and enthusiast photographers.

Digital SLRs use the same tried and tested design as SLR film cameras. You compose your photos using an optical viewfinder on the top of the camera, and the big innovation when SLRs were first invented was that you were looking at the scene through the camera's own lens, not a separate, external viewfinder.

This is still how the SLR design works. A mirror inside the body reflects the image up into the viewfinder, right up until the moment you press the shutter button. At that point, the mirror flips up out of the way and the image passes straight through to the back of the camera, where the shutter opens to expose the sensor.

Nikon D810
Inside every D-SLR is a mirror which reflects the image up into the pentaprism and viewfinder.

It sounds complicated, but camera makers have had decades to perfect this design, and SLRs are fast, responsive and durable, and give you an excellent view of the scene you're photographing.

Digital SLRs use two sensor sizes. Cheaper models use APS-C sensors, which are about half the size of the full-frame (35mm negative size) sensors in professional models.

There are just three SLR makers still active in the camera market. The big names are Canon and Nikon, but there's another brand in the mix too – Pentax is often overlooked as an SLR maker, but produces some great cameras.

Canon and Nikon produce cameras to suit all budgets and levels of expertise, from low-cost entry-level cameras for novices and students, right up to professional powerhouses designed for press and sports photography.

Nikon D3200
D-SLRs aren't just for pros. An entry-level model like the Nikon D3200 is cheap and easy to use.

The range of new SLRs from Canon and Nikon shows little sign of slowing up, and you're almost certain to find an SLR to suit your particular needs and interests.

There's a huge range of lenses for these cameras, too, and across the whole price spectrum – from basic budget lenses cheap enough for beginners to expensive specialised lenses for professional use.

But one of the biggest advantages of the SLR design can also be a limitation. Because the sensor is not in the light path during viewing, SLRs have use separate 'phase-detection' autofocus sensors underneath the main mirror. These are extremely fast and responsive, but the moment the mirror is raised, they're out of action.

Digital SLRs do offer a live view mode, where you compose your picture on the rear screen, not in the viewfinder, but the focusing in this mode is swapped over to a slower contrast AF system.

Canon has found an interesting solution with Dual Pixel AF technology built into the sensor itself, but for the most part live view autofocus is not a D-SLR's strong point.

Canon EOS 70D live view
D-SLRs aren't usually as effective in live view, but Canon's Dual Pixel AF system is a step forward.

The SLR design is also quite bulky because there needs to be enough space in the body for the mirror mechanism.

The is why 'mirrorless' compact system cameras are catching on. You still get the advantages of interchangeable lenses and a big sensor, but you also get full-time live view, just like a compact camera. Indeed, many compact system cameras are aimed at people upgrading from a regular compact.

To find out more about these cameras and which would be best for you, read our Best D-SLR article.