see also:

• Canon EOS camera system
• Nikon F dSLR system
• Olympus Four Thirds dSLR system
• Sony Alpha dSLR system
• timeline of development of digital cameras
• 35mm full frame digital SLR cameras compared
• photography main index
• New cameras compared – Olympus OM-D E-M5 vs Canon 5D Mark III vs Nikon D800

• a dSLR is named as it is based upon a single mirror and lens which allows through-the-lens viewing of the subject, while the mirror flips out of the way during the exposure as with film SLR cameras, but with a digital sensor.
• most modern dSLRs also allow for shooting with the mirror up using Live View mode in which the image from the sensor is displayed on the LCD screen prior tto taking the shot.
• the benefit of the mirror is that of potentially clear, bright, real-time viewing through an optical viewfinder with only a very brief blackout time between shots.
• the problems of the mirror are:
  • requires a bulky, weighty pentaprism viewfinder (or a smaller, cheaper, lower visibility pentamirror)
  • the mirror must flip up prior to the shot which can cause camera shake, particularly when using telephoto or macro lenses.
  • the movement of the mirror is noisy which can be an issue in quiet environments or when stealth is needed such as in nature work.
  • the mirror must be moved up to allow Live View or movie mode which is quite annoying and clunky.
  • the autofocus system is not at the sensor plane but within the viewfinder system - usually as a very fast phase contrast detection AF system - but this allows errors in calibration between lenses (hence many cameras allow the user to calibrate AF themselves), and does not function in Live View mode (except in the hybrid Sony SLT dSLRs), and the sensors are usually cramped in the centre of teh frame and useless for AF when using Rule of Thirds.
• entry level cropped sensor dSLR's are increasingly being replaced by mirrorless camera systems
• there are a range of sensor sizes available in dSLRs in decreasing size:
  • medium format (eg. Hasselblad, Phase One, Mamiya, Pentax)
  • Leica S2
  • 35mm full frame (eg. Canon, Nikon and Sony)
  • APS-H 1.3x crop (eg. Canon 1D sports now abandoned by Canon)
  • DX 1.5x crop (eg. Nikon DX, Sony)
  • APS-C 1.6x crop (eg. Canon, Pentax)
  • Olympus Four Thirds dSLR system 2x crop

There are some excellent reasons to buy a dSLR, but if none of these apply, perhaps you should be looking at mirrorless camera systems

1. you aspire to be a enthusiast or a professional photographer and wish to either:
   1. build up a kit of professional lenses
   2. use certain lenses only available in dSLRs using their native functionalities eg. Canon 17mm tilt shift, Canon 85mm f/1.2 L, Canon MP-E65mm f/2.8 1-5x macro, Nikon 14-22mm f/2.8G, Nikon 85mm f/1.4G, full frame 24mm f/1.4, Olympus ZD 50-200mm f/2.8-3.5, Olympus ZD 150mm f/2.0, etc
2. you want to use large lenses which would not be ergonomic on compact mirrorless camera systems
3. you aspire to be a strobist and want to use remote TTL using PocketWizard radio remote controls (only available in Canon and Nikon)
4. you need full camera control with 3rd party utilities (eg. astrophotography)
5. you already have gear for that dSLR
6. you want to capture fast moving subjects (mirrorless cameras and entry-level dSLRs are not great at this)
7. you need to buy a full frame dSLR for lowest noise at high ISO, extra large prints, very shallow depth of field (DOF) or use wide angle tilt-shift lenses.
8. you need equipment readily serviceable or available for rent nearly anywhere in the world (Canon or Nikon are particularly ubiquitous)
9. you want to look like a pro and you don't mind carrying big, heavy equipment, learning the complexities of a dSLR, and the expectations placed upon you which go with possessing one.

• you really only have 3 brand choices here and much will come down to which camera and lenses suits your needs the best within your budget.

• Canon have some nice unique lenses such as EF 50mm f/1.2L, EF 85mm f/1.2L and TSE 17mm f/4
• the Canon 5D Mark III dSLR and Canon 1D X sports dSLR offer sensible pixel counts and excellent feature sets although cannot AF with f/8 lenses (ie. f/4 with 2x TC) and flash sync on the 5D is a bit slow
• you may be able to pick up a cheap Canon5DII now although it is not weatherproof and it only has an entry level AF system.

• Nikon have some nice unique lenses such as 14-24mm f/2.8G, macro TSE lenses, 85mm f/1.4G
• options for bodies include the old 12mp NikonD700, the new 36mp Nikon D800 dSLR, the new affordable 24mp Nikon D600 full frame dSLR, and the pro level 24mp NikonD3x and 16mp sports Nikon D4 sports dSLR

• Sony is an outsider in the full frame world and has not developed market share anywhere close to its Canon and Nikon rivals and being new has a limited range of pro lenses and only 2 optical viewfinder camera bodies - the 24mp Sony alpha 900 and the budget version, the 24mp Sony alpha 850.
• whilst both cameras offer sensor based image stabiliser which is unique in the full frame world, neither offer Live View or movie modes nor radio wireless TTL.
• in late 2012, Sony remedied the situation by adding the 24mp SLT Alpha A99 which is optimised for video and sports with its full time live view and dual phase detect AF system

• with the advent of high quality very functional mirrorless cameras such as the Olympus OM-D E-M5 camera, cropped sensor dSLR cameras make much less sense to me unless users are wanting telephoto reach or AF for fast moving subjects, or wanting to tip their toes into the dSLR world and don't mind using big, heavy lenses and cameras.
• Canon and Nikon make excellent cropped sensor dSLRs such as the Canon 7D and the Nikon D7000 but the problem is that neither company have made high quality lenses specifically designed for these sensors, and most would be best saving their money for the pro level full frame lenses.
• Sony make some nice cropped sensor dSLRs and their new SLT fixed mirror models are interesting and may suit some.
• Olympus Four Thirds offer a very nice semi-pro solution in their Olympus E5 dSLR as it not only has sensor based image stabiliser, most of the features of the Canon 7D and Nikon D7000 (except lower quality HD video), but very importantly, it has access to the best lenses designed specifically for cropped sensor dSLRs, and I would expect the forthcoming Olympus E7 will address high ISO and dynamic range issues which is a weakness of the old sensor in the E5 by using the current excellent E-M5 sensor. See Olympus E5 dSLR for some arguments for and against it compared to a Canon 7D.
• Pentax also offer a sensor-based IS system and have some nice pancake lenses.