Canon 70-200 MK II

ok55 wrote:

The centre AF point (or points on the 1Dx and 5Diii) on just about every Canon DSLR in use today has ( -ve) significantly greater sensitivity with the f2.8 lens [see Canon DSLR manuals]. The difference is particularly marked when shooting down to progressively lower light levels. At a stage the slow f4 lens will fail to lock on while the f2.8 performs faultlessly.