Example of focus shift resulting from "focus and recompose".

Both images were originally focused on the rim of the glass candle, midway between the front and back of the rim, while pointing the camera straight at the candle. The left hand image was taken after panning the camera to the right, re-composing the shot with the candle near the left edge.

Note that the left image, the one which was re-composed after initial focus, shows the point of sharpest focus has moved toward the rear of the glass candle's rim. This is because the 'focus plane' of the lens is flat, not an arc. The edges of the focus plane are further from the camera than the centre of the focus plane. After panning to the right, the focus plane has moved backwards from the glass candle, leaving the candle forward of the focus plane.

This pair of images shows the hazard of focus and re-compose, especially when using wide angle lenses and shallow depth of field.

Both images were taken with camera on a tripod, using a 28mm focal length lens at aperture f/4.