Yes, on a full-frame camera at the 8mm setting, it produces a 180-degree circular image. At 15mm, it provides a full-frame diagonal fisheye view.
No, the lens has a bulbous front element. It uses a rear gelatin filter holder that accepts cut-to-size filters.
Yes, as an L-series lens, it features professional-grade dust and moisture resistance, though a filter is recommended for complete sealing.
Yes, it works on APS-C sensors, but the circular fisheye effect is cropped. A zoom limiter switch is included to help avoid vignetting on smaller sensors.