WEIGHT AND SIZEThe top-line pro cameras are much larger and heavier than middle and low-end models. Also, photographers with small hands may find the big pro cameras uncomfortable to hold (as large-handed photographers will find the smaller cameras).
A typical pro DSLR measures 6.2x6.3x3.5 inches and weighs 42 ounces. A midlevel model may come in around 5.7x4.5x3.0 inches and 27 ounces, while typical entry-level models measure around 5.0x3.8x3.0 inches and weigh less than 18 ounces.
POWERThe top DSLRs use larger, higher-capacity batteries. The Nikon D3X can do about 4,400 shots per charge per the CIPA test standard, and the new D4 about 2,600, while the midlevel models can do 950 to 1,600, and the entry-level models last for about 550 to 800 shots. There are similar results for Canon. The EOS-1DS Mark III and EOS-1D Mark IV can do about 1,500 shots per charge per the CIPA test standard (Canon hasn't issued results for the EOS-1D X), the midrange models last for about 950 to 10,000 shots (except the EOS 60D, which is rated at 1,600 shots per charge), and the entry-level models are in the range of 550 to 800. You can buy an accessory battery grip for many midrange and some lower-end DSLRs to increase shooting capacity, but these add cost and bulk.
Power Tip:Regardless of the camera you use, it's a good idea to carry at least one spare charged battery and swap it for the one in the camera when the low-battery warning first appears. You can always put the first battery back in if you need its remaining capacity, but you may miss a shot if the battery dies at an inopportune moment.
PROCESSORSAs you'd expect, the pro cameras have more powerful processors and can deliver faster shooting speeds, better autofocusing and metering performance, and better video. They can handle more megapixels on the sensor and execute more elaborate noise-reduction algorithms for better image quality at all ISO settings.
The EOS-1D X and EOS 5D Mark III feature Canon's best DIGIC 5+ processors—two of them in the flagship X, one in the 5D Mark III. This processor offers 17 times the power of the DIGIC 4. While Canon's other DSLRs feature the DIGIC 4 as their only processor, the EOS-1D X uses a DIGIC 4 just to power its metering system.
Nikon's D4 and D800 feature the company's latest EXPEED 3 processing versus the EXPEED 2 (D7000, D5100, D3100) or EXPEED in other current Nikon DSLR models.