Wondering if someone would be kind enough to offer advice on the following.

Im helping a friend who is an Iridologist (taking pictures of the eye for
analysis) who needs to setup a Digital camera with Macro lens to replace an
older video capture based system which utilized a video camera and capture
software to take still frame shots of the eye. The video camera works ok but
the capture card died and new capture cards are not compatible with the
Iridology software she used. So we thought to go to a digital camera based
system which should provide much better pictures.

The Digital Camera has the same mount as the video camera and can screw into
the platform which then has limited movement up, down, left and right. The
patient whose eye will be photographed puts their chin on a frame to keep
still such that the camera can then be aligned with their eye for placement
of the shot. The camera platform can move to within 2 cm of the patients eye
but only moves back a maximum of about 20cm from the front of the lens to
the eye of the patient. My question is, if I buy a Macro lens such as the
Canon EF 100mm  F2.8 which I beleive is a good lens or perhaps a Tamron AP
AF 90mm and attach it to a EOS 300D, is this going to be a problem with the
limited distance I can move the camera back from the patient?. The specs for
the Canon say the minimum focus length is 31cm and the Tamron 29cm. Does
this mean the lens cant focus if the camera is closer than 31cm/29cm to the
subject?.  If so I wont be able to use this camera mount rig and does anyone
have any solution for taking digital shots of the eyeball using the current
rig, any other lenses or adaptors I can use?. TIA for any help.