Digital SLRs are the great survivors of the camera industry, a testament to a versatile and highly effective camera design that's stood the test of time.
They sound complicated compared to modern compact cameras and CSCs. It's because they use an optical viewing system rather than an electronic one formed by the sensor. While you're composing the shot, a mirror reflects the image up on to a focusing screen, and this image is flipped round by the pentaprism so that it's the right way up when you see it in the viewfinder.
The mirror flips up at the moment of exposure so that the sensor can be exposed, so this extra mechanical movement makes DSLRs a little noisier and – you'd think – more expensive and less reliable.
In fact, DSLRs are still the cheapest way into 'proper' photography, and camera makers have been building them for so long they're extremely reliable. At the top end, Nikon's D4s pro DSLR has a rated shutter life of 400,000 exposures.
One of the key selling points of DSLRs, of course, is that they take interchangeable lenses. The big two makers, Canon and Nikon, have huge lens ranges that make their cameras capable of practically any kind of photography. Pentax is a little way behind but still has a bigger lens range than any compact system camera maker, and third-party lens makers like Sigma and Tamron make lenses for all three.
When you buy a digital SLR, you're buying into a system that can expand with you, and take you anywhere you want to go in photography.
The other selling point of DSLRs is the sensor size. Broadly speaking, amateur models have APS-C sensors and pro models have full-frame sensors two times larger – though there is some crossover. Even the smaller APS-C sensor size is massively larger than the sensors in compact cameras and produces far better image quality, regardless of megapixels – this is even more obvious at higher ISOs.
DSLRs vs mirrorless CSCs
All these advantages apply to the new breed of mirrorless compact system cameras too. The key difference is that DSLRs have a mirror in the body reflecting the image up into an optical viewfinder, whereas mirrorless cameras use the sensor itself to generate an image which is fed to a screen on the back of the camera and sometimes an electronic viewfinder too.
So why choose a DSLR over a compact system camera?
Many people prefer the visual clarity of the optical viewfinder of a DSLR over the electronic viewfinder of a CSC. It's true that an EVF can show you the scene exactly as it will be captured, but some still have a slightly 'gritty' (pixellated) look and some lag in poor light.
- DSLRs use dedicated phase-detection AF sensors which deliver extremely fast and responsive autofocus. These aren't possible in compact system cameras because they'd block the light to the sensor. Some CSCs have faster hybrid AF systems which have closed the gap, but DSLRs still have the edge.
- The lens choice is much wider. Compact system camera makers and third-party lens companies are ramping up production, but if you want to open up your photography to the widest possible range of subjects, a DSLR still offers the most potential.