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Yes. Make sure you purchase your sigma lens with the F-bayonet. Here is their lens finder: see related link
Yes, the sigma is a good lens for the money. One cannot expect professional result with this chepo lens but for an enthusiast it will serve the purpose.
Sigma has a 70-300 zoom lens that is comparible with the Nikon F55. It costs around $150.
Yes, it will fit. However, you will still need to see if the lens you have has or does not have features like autofocus or CPU connectors that will carry all the info that your camera needs... If it is a Nikon Lens it will work fine. So will others like Sigma, Tamron and Tokina. Carl Zeiss may not give you all the functions but will work in manual mode.
No http://www.amazon.com/Sigma-100-300-F4-5-6-7-Nikon-Manual/dp/B002CJ03TU
and nikon camera can use a nikon lens, try a D3000, a D50, or a D9
Nikon or Nikor
There are many good Nikon lens for portrait photography. However, perhaps the best Nikon lens for this purpose is the Nikkor AF-S 50mm f/1.4G lens. The cost of this lens is approximately $450.00.
Af-s and Af-I lenses are compatible. Listing them would be hard, considering tamron, sigma and nikkor lens can fit on a d60.
The Nikon d2h camera has the ability to adapt to many different lenses. The lens that comes from the factory is the Nikon 70-200mm VR f/2.8 Digital Zoom lens.
Yes
nikon 50mm 1.4 af