The new 5DIII will have the ability to shoot 7 at +/- 5EV which is a huge spread. The 1D series such as my 1DsII can shoot 7 at +/-3 which is a total spread of 18 stops.
There are actually three different ways multiple exposures can be shot with the 5DIII, two of which are merged in camera. The third method is exposure compensation and AEB which is the one most of us are probably the most interested in.
Here are the three ways of shooting multiple exposures using this camera:
The EOS 5D Mark III camera features a built-in HDR mode, merging three images at various exposure levels into a single image, in-camera, for stunning photographs of landscapes and architecture with enhanced tonal gradation beyond the range of the naked eye. The exposure levels in the camera’s HDR mode can be set to cover a range of up to ±3 stops, in a choice of five settings: Natural, Art Standard, Art Vivid, Art Bold and Art Embossed providing unique visual effects.
Multiple Exposure Mode
The EOS 5D Mark III is the second EOS Digital SLR after the EOS-1D X to feature Multiple Exposure capabilities with the ability to combine up to nine individual images into a single composite image, with no need for post-processing in a computer. Four different compositing methods are provided for maximum creative control, including Additive, Average, Bright and Dark.
Exposure compensation is possible with the EOS 5D Mark III from +/-5EV in 1/3 or 1/2 stop increments. This can be combined with auto exposure bracketing (AEB) in two, three, five or seven-shot sequences.