Wildlife photography cameras have four major requirements for the best images. They must be able to shoot at distance, filling the image with the wildlife subject. They must produce high-resolution images so the details of the subject are sharp. They must shoot bright images in the reduced-light conditions of early morning or late evening. And, finally, they must have the high shutter speeds to freeze rapid action.
The ability to zoom in for a close-up is the characteristic of the lens, not the camera. No single lens will be adequate for all types of shots, so the best cameras are the ones that have many interchangeable lenses available. Canon and Nikon are the camera manufacturers with the largest assortment of lenses. From this point of view, top-of-the-line Canon and Nikon cameras will be the best choice.
Image quality is determined by the resolution of the camera and the sensor size. Resolutions above 10 megapixels are generally enough for good enlargement, if coupled with a large sensor size to reduce noise and give crisp images. The Canon EOS-1Ds Mark III and the Nikon D3s both have full frame sensor sizes of 24-by-36 millimetres, the full size of the images provided by the lenses. The Canon camera has 22 megapixels to the Nikon's 12, so it is the better choice from this point of view.
Shooting good pictures in lowlight conditions depends on the choice of lens and the speed of the camera in ISO. Both Canon and Nikon have lenses that will satisfy all light conditions. For low light, lenses with an aperture of at least f2.8 are required, and both companies have such lenses. The maximum ISO speed of the Canon EOS-1Ds Mark III is 3200, while that of the Nikon D3s is 12,800. The Nikon will be the better camera for very lowlight conditions, giving a greater flexibility to the choice of lenses.
The ability to freeze action shots is a key feature for wildlife photography. The camera must have a shutter speed fast enough to freeze the motion of an animal moving quickly. A fast shooting rate allowing multiple rapid shots, one after the other, is also an advantage. The Canon EOS 1Ds Mark III has a maximum shutter speed of 1/8000, as does the Nikon 3Ds. The Canon camera has a maximum shooting rate of 5 frames per second, while the Nikon can do 8 frames per second. If you take frequent action shots of animals moving at high speeds, the Nikon has the slightly better performance.