The “D” means digital and the “1” means top-of-the-line, as-good-as-it-gets, #1. The “Mark IV” in the 1D product name indicates the maturity of this model – the fourth generation. While the Canon EOS-1D Mark IV may not have every DSLR bell and whistle available at its release, it delivers a combination of speed, performance, image quality and durability unmatched by any other Canon EOS DSLR as of this review date.
#1. The 1D IV’s predecessor, the Canon EOS-1D Mark III was geared to the same group
In light of some of the newer Canon EOS DSLRs (especially the Canon EOS 7D), the 1D III was showing signs of age – especially in sensor resolution. I have fielded a significant number of 7D vs. 1D III questions – and the decisions were not always easy. Now, with few exceptions, the 1D IV has the clear performance advantage over the 7D alternative.