Single-lens reflex (SLR) cameras show through the viewfinder exactly what will be captured by the film or digital imaging system, as against pre-SLR cameras which sometimes showed the photographer something significantly different from what was ultimately captured on film. SLR is most popular in high-end and professional cameras.

A digital SLR (DSLR) is a digital camera that uses a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera. DSLRs generally refer to cameras that resemble 35 mm format cameras. Professional still photographers prefer them over compact digital cameras for their accurate preview of framing close to the moment of exposure, accurate preview of depth of field, and their larger sensors.

DSLRs are generally considerably more expensive than compact digital cameras, and require additional expenses for lenses, filters and other accessories. Traditionally, they are also bulky, and only meant for still photography. However, some DSLR manufacturers have brought out very compact models like the Olympus E-620 and the Sony Alpha A 380. Some, like the Canon EOS500D, Nikon D5000 and Pentax K20D have added video capability.

Some Specifications Used to Evaluate SLR Cameras:

ISO Setting: The ISO setting on a digital camera determines its sensitivity to light – that is, how much light it requires to capture a perfectly exposed image. Good light requires a lower ISO setting. Inadequate light necessitates either a higher ISO setting, or a lower shutter speed – the additional time allows more light to filter in. Lower shutter speeds, however, may result in blurry images, and higher ISO, unfortunately results in image noise

The ideal ISO settings for different light conditions are:

• ISO 50-100: outdoors on a sunny day.
• ISO 200: outdoors on overcast or cloudy days.
• ISO 400 and 800: in dim light, as in dawn or dusk.
• ISO 1600 and above:  indoors, or for night shots. This is also used to freeze action shots.
• Most cameras, however, come with an “Auto ISO” setting which automatically sets the ISO according to the conditions.

IS: Image Stabilisation refers to a set of techniques used to minimise the blurring that results from the normal motion of a camera or its subject. In still photography, IS allows for the use of lower shutter speeds, thereby reducing image noise. It does not, however prevent blurring resulting from violent jerks or movements.

Live View: A live view on a digital SLR camera is the continuous image display on the LCD at the back of the camera, which allows photographers to preview the photos they are about to take without having to use the viewfinder. A live view LCD is especially useful to capture spontaneous moments and when taking pictures from odd angles.

Kit Lenses: are "starter" lenses, usually inexpensive ones that come with interchangeable-lens camera kits. The kit usually consists of the camera body, and various accessories necessary to get started in SLR photography. Kit lenses may also be sold on their own. High-end SLR cameras do not usually come with kit lenses as the assumption is that the buyer will invest separately in good quality lenses.

Aperture: The aperture is an opening through which light passes into the camera through the lens. It lets in more light when it's wide open and less when it's small. Effectively, it plays the same role in the camera that the pupil plays in the eye. The aperture can be controlled by setting the aperture opening, also known as an F-Stop. Aperture diameters are listed in terms of f-numbers.

Shutter Speed:  In a camera, the function of the shutter is to block light from exposing the film until the button is pressed, upon which it quickly opens and closes, briefly letting in some light. Shutter speed is the length of time the shutter remains open, which affects the sharpness and quality of the picture taken.

Popular Entry And Mid-level DSLRs:

Canon EOS500D: This is part of the Rebel mid-range series, with the difference that it caters to relatively serious photographers as it comes with features such as a larger viewfinder and spot metering that may mean little to the amateur, price conscious camera buyer.

Main Features::

• 15.1-megapixel CMOS sensor; 4,752 x 3,168 resolution
• DIGIC 4 image processor
• 3.0-inch LCD monitor; live view mode
• Built-in flash
• Four metering modes, using 35-zones: spot, partial, centre-weighted average, and evaluative metering
• Auto lighting optimizer
• ISO 100–12,800
• Canon EF/EF-S lenses
• PAL/ NTSC full HD video output
• SD and SDHC memory card file storage
• RAW and large JPEG simultaneous recording
• USB 2.0 interface
• Box contents: EOS 500D Camera, EF-S 18-55mm IS f/3.5-5.6 (non USM) lens, Battery pack (LP-E5), Battery Charger (LC-E5E), Eyecup EF, Camera cover (RF3), Neck Strap (EW-100DB III), IFC-200U USB interface cable, AV Cable AVC-DC400 and EOS Solution Software CD ROM.

Nikon D5000:  is specifically designed to appeal to novice digital SLR users, with easy to use Menu layouts, simplified buttons and controls,  and features that were previously only available on Nikon's Coolpix point- and- shoot cameras, like 19 automatic scene shooting modes and the Vari-angle LCD monitor. This is probably the closest competitor to the Canon EOS 500D.

Main Features:

• 12.9 megapixel DX-format CMOS sensor
• 2.7" tilt and swivel LCD monitor
• Nikon AF-S DX 18-55mm f/3.5-5.6G VR entry level kit lens with AF-S motor
• Movie capture at up to 1280 x 720 (720p) 24 fps with mono sound
• Live View with contrast-detect AF, face detection and subject tracking
• IS0 200-3200 range (100-6400 expanded)
• Extensive in-camera retouching including raw development and straightening
• Image sensor cleaning (sensor shake)
• 11 AF points (with 3D tracking)
• 4 frames per second continuous shooting (buffer: 7 RAW, 25 JPEG fine, 100 JPEG Normal)
• Box contents: The 12.3 effective Megapixel Nikon D5000 camera body, F3.5 - 5.6, 18 - 55 mm AF-S Nikkor VR lens, EN-EL9a lithium-ion rechargeable battery, Battery charger w/power cable, Body cap, Eyepiece cap, Shoulder strap, USB cable, A/V cable, CD-ROMs featuring Nikon Software Suite and 234 page camera manual.

Olympus E-620: This is the world’s smallest and lightest image-stabilized digital SLR camera. It has in-built easy-to-use art filters and multiple exposures that make capturing and creating creative images fun and easy – without the need for a computer or editing software.

Main Features:

• 12.3 Megapixel Live MOS sensor;
• TruePic III+ image processor
• 4 stop image stabilisation
• Flip-out, rotating 2.7" LCD with 230,000 pixel resolution
• 7-point autofocus system
• 4 fps continuous shooting
• ISO range 100 – 3200
• Offers art filters and multi-exposure
• Dust reduction system
• Live view with contrast detect AF and face detection
• Illuminated buttons
• Choice of single lens (F3.5-5.6, 14 - 42 mm Olympus Zuiko ED lens) or dual lens (F4.0-5.6, 40 - 150 mm Olympus Zuiko ED lens) kit.
• No movie mode.
• Box contents: Olympus E-620 camera body, F3.5-5.6, 14 - 42 mm Olympus Zuiko ED lens [single lens kit only], F4.0-5.6, 40 - 150 mm Olympus Zuiko ED lens [dual lens kit only], BLS-1 lithium-ion rechargeable battery, battery charger, body cap, eyepiece cover, shoulder strap, USB cable, video cable, CD-ROM featuring Olympus Master and manual.
• Optional accessories available: lenses, OM lens adapter, external flash, wired/wireless remote control, battery grip, right angle finder, underwater case and semi-hard case.

Pentax K20D: This is the highest resolution camera in its category, with a host of features that make it a photo-enthusiast’s dream.

Main features:

• 14.6 megapixel CMOS sensor with maximum noise control.
• Live View function on a 2.7 inch LCD monitor.
• Three preview methods: Optical, Digital, and Live View
• 11-point SAFOX VIII autofocus system with 9 cross-type AF sensors
• Pentaprism optical viewfinder with 95% coverage, 0.95x magnification
• PENTAX-developed body-based shake reduction.
• Custom Image modes, with six preset options.
• 36 custom functions with 93 individual settings.
• Full backward lens compatibility for over 25 million lenses manufactured by Pentax in the last six decades. Compatible with K-, KA-, KAF- and KAF2 lenses, as well as screw-mount / 645-system / 67-system lenses with an adapter.
• Advanced 16-segment metering and 11-point AF with 9 cross sensors.
• Dust alert function.
• ISO from 100 to 3,200 equivalent, with expansion to 6,400; automatic ISO selection.
• Built-in five-mode pop-up flash; hot shoe for mounting external flash.
• NTSC/PAL video out
• Multiple exposure support (2 to 9 frames)
• Box contents: K20D body, USB Cable I-USB17, Video Cable I-VC28, Rechargeable Li-Ion Battery D-LI50, Battery Charge Cradle D-BC50, AC Plug Cord, Strap O-ST53, Hotshoe Cover FK, Eyecup FP, Body Mount Cover, Finder Cap for ME, Focusing Screen LF-80 Frame Matte, Bottom Cap K10D, Software CD-ROM S-SW74/75
• Recommended optional accessories: Camera case for protection, accessory lenses, accessory flash: AF-540FGZ, large capacity SDHC memory card.

Sony Alpha A380: This is an upper entry level DSLR camera, and successor to the Alpha A350, retaining many features from this earlier model. The most prominent changes are the new smaller and lighter body shape and an easier control system designed for beginners upgrading from point-and-shoot cameras.

Main Features:

• 14.2-megapixel APS-size CCD image sensor
• A large 2.7-inch Clear Photo LCD Plus display
• Live View with Quick Auto Focus
• BIONZ image processor
• SteadyShot INSIDE in-camera image stabilization
• Integrated on-screen Help Guide and Graphic Display
• ISO 100 to 3200
• Creative Style settings
• Auto Pop-Up Flash
• Smart Teleconverter 2x Digital Zoom without degradation in picture quality.
• Sony Alpha lenses, also accepts Konica and Konica Minolta glass.
• No video mode
• Box contents: DSLR-A380 body, DT18-55mm f/3.5-5.6 SAM zoom lens (if purchased as a kit), body cap, lens caps, lithium-ion battery (NP-FH50), battery charger, USB cable, eyepiece cover, shoulder strap, CD-ROM, instruction manual and warranty card.
• Optional accessories: 28 different top quality α–mount lenses made by Carl Zeiss and Sony G Lens, lighting and power units and stylish carry cases.