The further you get away from the plane of focus, the less sharp the image becomes. So, the
obvious solution is to make sure that as much of your subject as possible is contained within the
plane of focus!
Be sure that the plane of focus of your subject covers as much of the item as possible. By that I
mean that... if we are having a problem with shooting straight into an insect’s eyes and its rear
end is out of focus – try turning the insect sideways so that it is parallel to the lens, that way we
can get more of it in focus. (Actually, you’ll be moving the camera, not turning the insect.)
Pretty obvious, but in the heat of battle, it is easy to overlook these little things.