Before you read this, perhaps read the previous post on “which camera is best“.
There is no perfect camera, so I have found I need 3 separate camera types and usually choose 2 of these to take with me on any photo trip:
1. a relatively small, light, walkaround camera that is relatively discrete but takes great quality photos and is versatile that it can do almost any shot I need as well as HD video if needed.
2. a wide aperture telephoto zoom for when I want to isolate a subject and have beautiful smooth out of focus backgrounds.
3. a pro level camera that will excel at sports/action as well as be able to do super-narrow depth of field or tilt-shift work.
What I have chosen for each of these categories may differ from you:
1. Panasonic GH-1 with 14-140mm HD lens – extremely versatile and can use almost any lens ever made for added fun such as a Panasonic 20mm f/1.7 pancake for street photography or low light conditions or the many macro lens options for manual focus macrophotography. The 20mm f/1.7 lens is almost a must for a walkaround camera-lens combo, it is super sharp, and produces superb images – consider mating it with an Olympus E-P1 for even more compact kit if you don’t need the video or EVF of the GH-1.
2. Olympus ZD 50-200mm f/2.8-3.5 lens with Olympus Four Thirds image stabilised body (in my case an old E510). Many people opt for the smaller, cheaper ZD 70-300mm zoom which gives a unique 140-600mm telephoto reach at f/5.6, again, this is just not possible on Canon or Nikon for this size, price and quality. Others with more money may opt for the superb ZD 150mm f/2.0 which gives an unprecedented 300mm reach at f/2.0 and the option of 600mm reach at f/4.0 hand held – again, not available on Canon or Nikon.
3. Canon 1DMIII with tilt shift lenses, 85mm f/1.8, 135mm f/2.0 + 1.4xTC – and if I was really interested in sports, perhaps either a 300mm f/4IS or 400mm f/5.6
THUS, what I am saying is, you need at least two cameras – a small high image quality camera such as a Micro Four Thirds, and a larger sensor camera such as a Canon 1DMIII or a full frame dSLR so the two systems are complimentary.
You could of course use a 70-200mm f/2.8 IS lens on your Canon or Nikon dSLR to replace the Olympus 50-200mm in item 2, but the 70-200mm lens is twice the price, 50% heavier and too long to fit in most shoulder bags and forces you to buy those cumbersome backpacks which need to be placed on the ground to unzip.
I have left out an ultra-wide option, so if you are a keen user of ultra wide, you could have an option to add this to eany of the above items such as:
1. Panasonic 7-14mm MFT lens (expensive) or Olympus ZD 9-18mm lens (much better priced but still a great lens but bigger).
2. Olympus ZD 7-14mm FT lens or Olympus ZD 9-18mm lens or Olympus ZD 11-22mm lens.
3. Depends on whether you use Canon APS-C camera such as a Canon 7D or Nikon DX camera such as a Nikon D300s (perhaps a 3rd party 10-24mm lens), a Canon 1DMIII (1.3x crop which limits ultra-wide to full frame lenses this a 16-35mm lens becomes 21-46mm – not that wide!), of full frame such as a Canon 5DMII or Nikon D700 – the best ultra wide lens is clearly the new and expensive Nikon 14-24mm f/2.8 – absolutely superb and even worth getting for the full frame Canon cameras and using in MF mode only.
Finally the differences between a Olympus Four Thirds and a cropped sensor Canon or Nikon are not sufficiently large for me to consider having both (I consider the 1DMIII closer to full frame than cropped sensor as it is only 1.3x crop), although the Canon 7D does offer functionality such as 8fps not currently available in a Four Thirds system unless Olympus manages to produce a competitive upgrade with their successor to their E-3.