Could you please tell more about your settings, ISO, Aperture, shutter speed, set of focus, did you use hyperfocal distance and did you make a lens test seri to check sharpness to focus settings at different apertures?
If you set the focus around 1m @ f8 your depth of field is something like 0.2m to beyond infinity.
One thing to watch with the Sigma 8mm fisheye is that it is possible to set the focus beyond infinity (i.e. so that white marking line is well to the right of the infinity mark on the lens), which will give you a soft focus.
I agree with the other comments of the experienced colleagues.
Most of the lenses do their best over all at f/8 or f/11. For me personally, the Sigma is a good lens, but compared to the nikkor 10.5, it is more sensible to sharpness. Using same settings with the two lenses, Sigma tends to be "soft, unsharp" earlier than the Nikkor lens using different apertures and focus settings.
To get you on the road, use the settings mentioned by Badders and Hugh.
To learn about basics of your lens camera combination, you have to go through a test. Take a road with cars parking along the street. Take different apertures. For each one take a set of focus settings from 0.5 to infinity. Then you will learn about the setting to use to get the right depth of field for your special setting.