If you look at this picture from Venice closely, the eyes of the model in purple are not tack sharp, nor is the fabric and decoration of her costume in the immediate foreground. This is not good at all. There are two lessons to be learned from this. When you have two subjects in a picture, in most cases both of them should be sharp. Second, when using a telephoto lens fairly close to the subjects, the depth of field will be shallow if the two subjects are not equidistant to the camera. In this picture, the model in purple was closer to me than the model in blue, and my shooting angle was oblique. That was a problem. The solution? Either i should have used more depth of field (such as f/11 or f/16 instead of f/7.1), or I should have changed the shooting angle so both models were the same distance from the camera. In that event, both of them would have been tack sharp at any lens aperture.