This site says
The Canon EF-S 60mm f/2.8 Macro USM Lens loses 1/2 stop at 1:5, 1 stop at 1:3, 1.5 stops at 1:1.5 and 2 stops at 1:1 (lifesize).
Can anyone confirm that at 1:5 magnification, the Canon EF-S 60mm f/2.8 USM lens has a maximum aperture of f/3.4, at 1:3 magnification the maximum aperture is f/4.0, at 1:1.5 it is f/4.8 and anything below that is f/5.6?
Answer
My Nikon 105mm drops from f/2.8 to f/4.5 at closest focus, so that sounds right.
A post at betterfamilyphotos has a post where they say (emphasis mine):
You would imagine that using a macro lens is the same as using a normal lens, and you would be right except that with a macro lens when you get close to 1x magnification, you start losing light. My 60mm for example starts losing light at close ranges until it reaches 2 stops of light loss at 1x magnification, this means that the effective aperture is f/5.6 instead of f/2.8 (regarding light quantity entering, not DoF). If you are using auto modes on the camera like aperture priority or using flash in TTL mode then the camera will auto compensate for the light loss, but if you're metering light manually you need to take it into account, Canon has included a table in the user manual with the light loss values at each magnification level.
So if you have access to the manual for this lens, or request one from Canon, it should verify the information. It's expected for a close focusing macro to lose 1-2 stops.
The above post is incorrect, however, in saying that the reduction in aperture doesn't affect DOF. It does. Two references for those interested in the physics of it:
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