Implementation notes by Jan Brittenson (bson@rockgarden.net)

This is notes on the implementation and against
http://www.burren.cx/david/canon.html.

The Canon makenote implementation has been tested against S100 and G1.

The S100 doesn't output focal length information; it leaves the fields
zero.

Tag 0x8 (Image Number): the value 1000 should be 10000.

Tag 0x4 AF point isn't implemented since it's not really understood.
I'll take a stab at it.

Both the S100 and G1 output tags:

    TAG 0x2-3:2,672,285,214,
    TAG 0x10-4:67371008,

The latter appears to be a long broken into two shorts.  The high-end
short is a large number, the low-end short a small number or zero.

Tag 0xc (Camera Serial Number) has been implemented but is untested
since neither my S100 or G1 creates this.