digital SLR cameras
introduction
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.
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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)
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Why buy a dSLR?
There are some excellent reasons to buy a dSLR, but if none of these apply, perhaps you should be looking at mirrorless camera systems
you aspire to be a enthusiast or a professional photographer and wish to either:
build up a kit of professional lenses
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
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you aspire to be a strobist and want to use remote TTL using PocketWizard radio remote controls (only available in Canon and Nikon)
you need full camera control with 3rd party utilities (eg. astrophotography)
you already have gear for that dSLR
you want to capture fast moving subjects (
mirrorless cameras and entry-level dSLRs are not great at this)
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.
you need equipment readily serviceable or available for rent nearly anywhere in the world (Canon or Nikon are particularly ubiquitous)
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.
which dSLR system?
full frame dSLRs
Canon
Canon have some nice unique lenses such as EF 50mm f/1.2L, EF 85mm f/1.2L and TSE 17mm f/4
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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
Nikon have some nice unique lenses such as 14-24mm f/2.8G, macro TSE lenses, 85mm f/1.4G
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Sony
cropped sensor dSLRs