Pagman wrote in post #17452954
Here is something i didn't know till i researched it - in the D7100 and possibly others ? the focus points that surround the center cross type f8 one, are only cross types at f5.6 or larger, smaller than this and they resort to line type, this would make a difference with focus accuracy in dynamic mode if the subject moves away even slightly from the center, esp with more complex subjects or poor lighting.
That is pretty normal, most cameras (certainly most, if not all, Canon ones) are f/5.6 or faster on all points but the centre point (and most still require f/5.6 on the centre as well). So, are you using a TC for these shots? Maybe there is an issue with that on the 55-300?
Also, are you sure about them resorting to line type with apertures smaller than f/5.6? Most cameras can't AF at all past f/5.6, especially away from centre point. Often, they are line type at f/5.6 or larger and the cross type may only kick in with f/2.8, or f/4 maybe.
[EDITED TO ADD]: I have just looked up the d7100 AF system and the cross type points are always cross type as far as I can see. The centre cross point works at f/8 or faster, the other cross points work at f/5.6 or faster, as cross types. Smaller than f/5.6 and they don't work at all, it isn't a matter of resorting to line type. The other outer points are always line type only, and are again f/5.6 or faster.