« on: August 19, 2012, 03:59:03 AM »
Could be something like that, certainly you're losing around two stops at 1:1. I know some of that is due to light loss within the lens, due to the design and physics, as well as some light loss from being in your own light . Also, there is no way anyone would be daft enough to want to use zone AF for macro work, so they could have left them selectable for non-macro work, considering it also makes a useful portrait lens among other things.I'm intrigued though why the 100mm macro USM is the only f/2,8 lens in group E, when even the older non-USM macro is in group C. I notice it's even worse with the 180mm macro, as it is in group G with or without the 1.4x extender.
Yeah it is odd. Glad I upgraded to L.
Maybe it is to sell more L versions.
If not then maybe something about the internal baffles and how it heads into macro mode as a non-extending lens, maybe the larger elements in the L get around this. The older non-USM uses an extended barrel type macro design. It is interesting that even the 100L is one of the few lenses faster than f/4 that does not get the central point to work as a super precision double cross point. Gotta be something to do with the way macro lenses are designed. Once you are at 1:1 I think they act more like f/5.6 in how much light they let in even when set to f/2.8 too, not sure where they decided to class the AF rating for it, at normal distance or macro.