Canon efs 60mm f2.8 macro underexposure wide-open
 

Hi

I just got the above lens, and, while testing it in tungsten light with
my 350D on Av auto between f/2.8 and f/16, with the target at a
distance of about 2.5m, I noticed that at f/2.8, and only at this
aperture, I needed to add 2/3 stop of exposure to get the same result
as at other apertures. In fact the exposure with the lens set to f/2.8
was the same as for f/3.5. I know the max aperture varies with
distance, but I thought the camera would compensate for this.

Anyone know where I'm going wrong?

Cheers
Geoff