The most significant improvement to the specification is the newly developed 16.1-megapixel CMOS sensor. Unlike its full-frame sibling the EOS-1Ds or rival D3s, the EOD-1D Mk IV uses an APS-H sized sensor measuring 27.9mm x 18.6mm, giving a crop factor of 1.3x compared to a 35mm camera. By comparison, the APS-C sensors used in most consumer DSLRs measures approximately 22.2 x 14.8mm, with a crop factor of 1.6x (i.e. Canon) or 1.5x (i.e. Nikon).
The new sensor is coupled with the same dual DIGIC 4 processors that power the superb EOS 5D Mk II, and they give the 1D Mk IV truly astonishing performance. In continuous shooting mode it can rattle off full-resolution shots at 10 frames a second, with full continuous autofocus and a buffer large enough for a sequence of 121 shots. The Nikon D3s can shoot at 11fps, but only in its 5MP DX crop mode.
The combination of Canon-designed sensor and processor also gives the EOS-1D Mk IV another superlative to add to its growing collection. It has an ISO range of 100 to 12,800, which is expandable to an amazing 50 - 102,400 ISO, which as fas as I'm aware is the highest sensitivity of any current digital SLR camera. Unfortunately the pre-production sample that I was allowed to play with had its card hatch securely taped up, so I don't have ant sample shots to show you, but I'm assured that picture shot at this amazing speed are quite usable. I'm looking forward to finding out for myself.