Lots of Focus Points

[Andy Stringer]

I'm curious as to why the macro lenses take such a big hit on AF point activation.

It's rare for me to enter a discussion on technical matters for fear of embarrassing myself, but I think I can help with this. At macro distances, the effective focal length of a macro lens increases, so the effective f number also increases. The following tables show the effective f number for different magnifications for two of Canon's macro lenses. I suspect that this has something to do with the limitation on AF point activation.

EF 180mm f/3.5L Macro USM

Magnification	1:10	1:5	1:3	1:2	1:1.5	1:1.2	1:1
Effective f/No.	3.8	4.0	4.3	4.7	5.0	5.4	5.8

EF 100mm f/2.8L Macro IS USM

Magnification	1:5	1:3	1:2	1:1.5	1:1
Effective f/No.	3.6	4.1	4.6	5.0	5.9