The first M series camera with TTL metering was the M5 in 1971, it achieved this with a mechanical swing arm carrying the photo diode. In 1984 Leica introduced the M6 which achieved its metering by measuring the light reflected off a white dot on the shutter, with the photo diode mounted in the base of the lens mount / shutter chamber. The M8 had a single white painted shutter blade instead of a white dot, the M9 uses a development of this system optimised for the large sensor size.
The M9's shutter now has three painted blades, one bright blade in the middle and two darker ones above and below it. Because the metering sensor is aimed at the center of the blades the metering pattern is now virtually circular and very similar to center-weighted average found on other cameras.
Base / bottom cover
The base of the M9 is very clean, with a tripod thread and the locking toggle for the bottom cover on the left. The tripod mount does appear to be exactly central in the base (laterally), however that means it's offset slightly from the lens axis. The bottom cover itself is made from a piece of milled brass (just like the top) and is removed by turning the locking toggle counter-clockwise, hinging on a small protruding lip on the right side. Once removed you can access the battery and SD card compartments, which are in the same postions as they were in the M8.
Battery / SD card compartment
As mentioned above to access the compartments you must remove the bottom cover, this may seem eccentric (and it is) but it does mean that (a) the compartments are protected while the cover is in place and (b) the clean lines of the camera are not spoiled by such annoyances as doors. The M9 takes SD cards (up to 32 GB SDHC) and uses the same battery as the M8, a Lithium-Ion 1900 mAh, 3.7 V (7.03 Wh).
The M9 is provided with the (considerably better) compact charger that first made its appearance with the M8.2.
The M9 has just one connector, a USB 2.0 mini-B socket on the left rear side of the camera. This can be used for file transfer from the SD card as well as remote control shooting using the supplied Leica Digital Capture software. The M8 only supported PTP transfer (causing headaches for Mac OS X users) but the M9 now supports both PTP and Mass Storage Device.
Flash hot shoe
On the top of the camera is the 'flash shoe', this allows the mounting and use of compatible flash units, including the Leica SF 24D flash unit (best suited because of its compact size) and other flash units which comply to the SCA (System Camera Adaption system) * which allows guide number control. You can also fit basic third party flash units which are simply fired using the center X-contact.
Lens mount / 6-bit coding sensor
The M9 has a Leica M-bayonet lens mount with a 6-bit lens identification sensor. Lenses are attached to the body by matching the red dot on the lens to that by the body's release button, then just twist clockwise approximately thirty degrees and the locking pin will click satisfyingly into place (a simple but remarkably well executed mechanism). As the M9 doesn't have a mirror box the distance between the lens mount and sensor / film is much shorter, around 29 mm compared to about 45 mm on a typical SLR. This is very obvious when you first remove the body cap and does make it considerably easier to examine (and if needs be) clean the sensor. At the bottom right edge you can see the 6-bit lens coding 'sensor strip' which is used to identify the lens used (see below).
6-bit lens coding
The Leica 6-bit lens coding system was announced in June 2006, it enables the M9 to identify the lens in use. With no electrical connection between the lens and body the dots system was the only solution. All new M series lenses are coded and existing lenses can be updated for a small charge. The identification of the lens is used by the camera to apply additional vignetting correction (if required), you can also manually select lenses from an internal database.