Focusing with fast lenses
I recently used an 50mm f/1.4 lens and was blown away by the look and bokeh, as compared to my kit lens. But when I analysed my photos later, I realised that a lot of them were out of focus. Why? - Erica Gondola
An f/1.4 lens has extremely shallow DOF. As compared to a kit lens, is very unforgiving of any careless errors. Switch off Continuous AF. It tends to refocus at any movement and is not the best choice when you need accuracy. Also, each lens has a certain tolerance level, up to which there may be some focusing inaccuracies. Similarly, each body has an error tolerance level, as well. A certain body-lens combination, thus may give backfocusing or frontfocusing errors, something that can be solved by the use of the AF Finetune feature.
If none of this helps, it is best to visit the official service centre.