ISO SETTINGSThe newest flagship DSLRs—Canon's EOS-1D X and Nikon's D4—have ISO settings up to a remarkable 204,800, much higher than the entry-level and midrange APS-C DSLRs. This, in part, is due to their larger full-frame image sensors, but it's also due to utilizing the latest technology. Canon's previous top pro DSLR, the EOS-1Ds Mark III, also has a full-frame sensor, but topped out at ISO 3200; that's how much the technology has improved in the four-plus years since the Mark III was introduced.
AF SYSTEMThe top DSLRs have the most sophisticated AF systems, with more AF points and advanced algorithms, powered by the most powerful processors. As a result, they will focus more quickly and more accurately on moving subjects than lesser cameras, especially when paired with pro lenses.
Canon's EOS-1D X features an all-new AF system with 61 points, 41 of them being cross-types able to read both vertical and horizontal detail. But you also get this system in the EOS 5D Mark III for about half the price. The EOS-1D Mark IV has a 45-point AF system, with 39 cross-types (with lenses of ƒ/2.8 and faster and some ƒ/4 lenses), but no cross-types at ƒ/5.6 or slower. The EOS 5D Mark II has a 9-point AF system, as do all other current EOS models below it except the EOS 7D, which has a 19-point system.
In Nikon's lineup, the D4 and D800 have a 51-point AF system like their predecessors (and all recent Nikon DSLRs from the D300S up), but now include 15 cross-types, 9 of which work at ƒ/5.6 and one at ƒ/8 (these are the first Nikon DSLRs that will autofocus with ƒ/8 lenses of lens/converter combinations). The EXPEED 3 processing and new AF algorithms result in even better performance. Nikon DSLRs below the D300S have fewer total AF points: 39 for the D7000 and 11 for the D5100 and D3100.
The new higher-end models also have more sensitive AF systems. The Canon EOS-1D X and EOS 5D Mark III and Nikon D4 and D800 have AF systems that can function in light levels as dim as EV -2 (ISO 100). That beats the longtime pro standard of EV -1, and some lower-end DSLRs require even more light than that to function.
METERINGAgain, the pro cameras have more sophisticated metering systems, powered by their more powerful processors. Canon's pro EOS-1D X has a new 100,000-pixel RGB metering sensor that provides 252-zone evaluative metering (35-zone in low light) linked to the 61-point AF system. Previous 1-series EOS cameras (and current lower-end models) have 63-zone evaluative metering and less processing power. As previously mentioned, the EOS-1D X uses a dedicated DIGIC 4 processor just for the metering system.
Nikon's entry-level D3100 and D5100 DSLRs use 420-pixel metering. The D300S, D700, D3S and D3X use the company's long-proven 1005-pixel RGB metering. The newer D7000 introduced 2016-pixel metering. In all cases, metering was linked to the AF points for greater accuracy. The D4 and D800 feature a new 91,000-pixel 3D Color Metering III system linked to the AF system for the most accurate metering yet.