I found that during portrait full body shots, the IQ deteriorates due to incorrect AF.
I use a 6D and from Center AF point, I can AF my 135mmF2 on subject's eyes in a portrait shoot. At distances of 2-4 m, the subject's eyes get perfectly focused and image looks great. But when the subject distance increases, in full body shots, the Center point covers more than eyes and >50% of faces. At that time, the sharpness decreases and I blame AF for this.
In those cases where I see through viewfinders and focuses on subject's eye, and when the center point covers much part of the face, I see AF is not upto the mark.
Do I need to go liveview with zooming of view to correct AF.
Please help me find the best method to get correct AF when subject is at >15m.