Hi there. I got a sigma 500mm prime last year after speaking to a few people who use them professionally.
They all said good things and have to agree with them. It is an incredible sharp lens, half the weight and half the cost of the Nikons.
They have NO image stabilisation though so tripod only I would say!
Focusing is super quiet and v.fast! Also used it with a 1.4x converter which makes the lens an F6.3. It will autofocus (I'm using a D300) and found no problems.
Wouldn't really recommended a 2x converter though!
If you really can't stretch to a Nikon this is a fantastic lens.
Not sure what you're budget is. These were selling for £1700 second hand.
regards.
Stu.
