Which camera to choose between Canon 550d, Canon 600d, Nikon D5100, Nikon D3100, Nikon D3200?

All of the Nikon models you list require using the more expensive Nikon AF-S lenses if you want auto focus. The Nikon D90 and D7000 (both more expensive than the models you listed) can use the less expensive Nikon AF lenses and they will auto focus on those models. Of course you can use the AF lenses on the D3100, D3200 and D5100 but you'll have to manually focus them.

Nikon lenses are designated as either "DX" or "FX". A "DX" lens is designed for the cropped-sensor models such as the D3100, D3200, D5100, D90 and D7000 while the "FX" lenses are designed for Nikon's full-frame DSLR cameras such as the D700, D800, D3X and D4. However, a Nikon "DX" lens can be used on an "FX" body and the camera will automatically crop the sensor to the same size as the sensor in the "DX" camera bodies.

Since Nikon still uses the lens mount first introduced in 1959 any Nikon lens ever made can, with limitations, be used on any of their DSLR cameras. When Canon introduced their first auto focus 35mm film camera around 1988 they abandoned their old "FD" lens mount in favor of the current "EF" lens mount. As they moved into the digital age and needed a lower-priced line of lenses, Canon introduced the "EF-S" lens line.

Unfortunately, due to the design of the "EF-S" lenses they can't be used on a Canon full-frame DSLR like the 6D, 5D and 1D or on the older auto focus 35mm film cameras. So anyone using a Canon 600D and decides to add a couple of "EF-S" lenses will have to buy all new "EF" lenses should they ever upgrade to a 6D or 5D or 1D. If your budget allows in my opinion you should seriously consider the Nikon D7000.

There are a lot of previously owned Nikon AF lenses available so you will save money buying lenses.

Don't listen to Pamela Nikon doesn't make a 1800, I've own Nikon's since 1980s never had a problem with them. I've seen a lots of complaints about them, but 98% of these are Canon user using the internet to bash Nikon's.

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