To help you get some perspective on the decision and get an idea of what to expect from a given system and manufacturer, it helps to know a bit of the history of the brand and how current models within each lineup compare. While you generally can expect cameras in a given price range to have similar features, there are key differences between the camera families that may be the deciding factor for you.
|Canon's DSLRs can trace their roots to the company's EOS film cameras, the first of which was introduced in 1987. The EOS system was based around a new electronic lens mount, with an AF motor in each EF lens, while other manufacturers adapted their existing SLR systems to autofocus use, with AF motors in the camera body rather than the lens. Note that all major DSLR manufacturers now offer a number of lenses with onboard AF motors.
All EOS DSLRs feature the EOS electronic lens mount and can use all EOS EF lenses (but note that only APS-C-sensor EOS DSLRs can use EF-S lenses, which were designed specifically for the smaller APS-C-format sensors). Current EOS DSLRs feature 14-bit A/D conversion, which can reproduce 16,384 gradations of tone or color versus 4,096 gradations with 12-bit images. All current EOS DSLRs also feature Canon's self-cleaning sensor system to keep dust off the low-pass filter that covers the sensor assembly.
Canon DSLRs also feature CMOS sensors designed and produced by Canon, and all since the EOS 50D feature Canon's powerful DIGIC 4 image processor.
Canon EOS-1D Mark IVThe Mark IV is the newest member of Canon's EOS-1 series of all-out pro cameras. Its forte is speed—both shooting (10 fps at full 16.1-megapixel resolution) and ISO (normal range 100-12,800, expandable to 50-102,400). Image quality and AF performance are excellent.
All recent EOS DSLRs use the DIGIC 4 processor, but the Mark IV has two of them (as does the 7D). This provides operational speed and the processing power to employ highly effective noise reduction and image-processing algorithms. The Mark IV can do 1080 HD video at 24, 25 and 30 fps, and 720p HD video at 60 and 50 fps.