Hi Raghav, , >I was planning to shoot some >spherical panoramas for which I was considering this lens >since it is not too expensive.
>I guess even the Sigma 8mmn doesn't have the electrical contacts(as >the Sigma website says).
No! The Sigma 8mm f/3.5 EX DG is an AF lens and therefore has a CPU Chip and electrical contacts so it can communicate with the camera body. So it will meter when mounted on your D5100. It won't AF on the D5100 because it has a screwdriver type AF drive instead of an AF motor built into the lens. Keep in mind that the Sigma 8mm f/3.5 EX DG is a Circular fisheye designed for FX bodies and will likely be a rectangular fisheye on a DX body. The Sigma 4.5mm f/2.8EX DC HSM Circular fisheye was designed to be used on DX bodies like your D5100 and the D7000 for that matter. It has a Hyper Sonic (AF) Motor built into the lens so it will AF on your D5100.
>So that leaves me with Nikon 10.5mm. But that seems >like a slightly costly option for me as of now.
The Nikkor AF 10.5mm f/2.8G ED IF DX doesn't have a built in AF motor so it won't AF on your D5100 either. Like the Sigma 8mm, it will meter.
>Need another advice, assuming I have to budget maybe a couple >of months later, do you feel upgrading to D7000(to use with >Samyang 8mm) is better than spending on the Nikon 10.5?
The Nikkor 10.5mm is optically excellent. I have never used the Samyang?Rokinon though I suspect that it isn't as good as the Nikkor or either Sigma both optically or in build quality. The Rokinon that yop provided a link to seems to have a CPU Chip so it should meter on your D5100. Good Luck and Enjoy your Nikons!