What is focus and recompose when taking photos? How should I do it? And do you risk getting it out of focus images when you use this technique out in the field.
The big majority of lenses are designed so that the center of focus lies on a plane parallel to the camera sensor plane. The distance between that plane of sharpest focus and the camera sensor plane will closely match the distance marked on the lens barrel. Testing your lens to see if it acts this way is as simply as point it directly at a wall with lines or boards so that it is perpendicular to the wall vertically and horizontally. Focus on the wall. If the entire wall is in focus, your lens has the standard planar field of focus. If a circular or doughnut shaped area of the wall is in focus, your lens has a curved field of focus and is not unusual shape.