Nikon 400 mm f5 - 6.3 lens vs Sigma 500 lens?
Nikon 400 mm f5 - 6.3 lens vs Sigma 500 lens?
Nikon don't have a 400mm f5-6.3 lens and you only get variable apertures on a zoom.
What lenses exactly are you trying to compare?
Anyway, what about them? You ask 1 lens vs another but don't say what you want to compare.
Do you just care about image clarity? Weight? Build quality?
How about you stop being lazy and ask a complete question that actually makes sense?
Or even do a bit of research? There's a useful tool on the internet called google…
The Nikkor 400 mm lens is a f/2.8 lens
There's a Nikkor 200-400 mm f/4 zoom
Sigma's 500 mm is a f/4.5 and the Nikkor 500 mm is a f/4
The difference is going to be in the angle of view between the Nikkor 400 mm ( 6°10') and Sigma 500 mm (5º)
That is as much as my crystal ball has without more impute from you.
NOTE:
Nikon makes an 80-400 mm zoom:
http://www.nikonusa.com/...ED-VR.html
Sigma makes a 150-500 mm zoom
http://www.sigmaphoto.com/...-dg-os-hsm
Here is a link that shows you how each focal length sees the world