Depth of Field

Depth of field refers to the regions in front of and behind the object being photographed that appear to be in focus.  You should noticed that I said appear to be in focus.  If you focus on an object 20 feet from the lens, only objects at that distance are in focus.  Objects in front of or behind that distance are out of focus.  So why does it appear that trees 100 feet away and a mountain 10,000 feet away appear to be in focus?  The answer is in a term called the "circle of confusion".  In the examples right and below, all lenses are focused at a distance represented by the dotted line.  Objects reflect light in all directions.  The light from a small point landing on the lens forms a cone.  the light is then bent by the lens to form an inverted cone, producing a point on the film plane (top right).  If an object lies behind the focusing distance, the inverted cone will produce a point in front of the film plane, and continue diverging to form another (smaller) cone.  The base of this new cone is what is recorded on the film as a disk, not a point (middle right).  This circular disk is called the circle of confusion.  Likewise, if an object lies in front of the point of focus, it will produce an inverted cone that would form a point behind the film plane.  This cone is transected by the film, producing a disk, or circle of confusion (bottom right).  The three points at the far right of the figure represent the image recorded on the film and viewed straight on. 

If the circle of confusion is small, the human eye cannot distinguish between the point and the circle.  The only true measure of the limit of visual discrimination is the angle of arc.  Humans have a difficult time distinguishing less than 1 minute of arc.  To most of us this has little meaning.  So let's put this in terms to which we can relate.  When viewing a 5X7 print at a normal distance taken from 35mm film, it would be difficult for a human to distinguish between a point and a disk 0.15mm diameter.  That means that a disk 0.03mm (30 micrometers) recorded on film would appear to be a point (in focus).  These numbers refer to this one condition.  If you change the viewing distance or increase the size of the enlargement, this approximation will not apply.  To produce an image with objects at greatly different distances and in focus, we need to reduce the size of the cone.  We accomplish this by reducing the size of the hole through which light enters the camera (stopping down the lens).  The figure at the left demonstrates this.  Since the cone of light entering the camera is smaller, the inverted cone on the other side of the lens will also be smaller, producing a smaller circle of confusion.  The far point is still focused in front of the film plane and the near point is focused behind the film plane, but the circles produced are small enough that the eye may not distinguish these circles from a point.   The downside is that we are allowing much less light into the camera, and must leave the shutter open longer to expose the image, making long depth of field exposures in low light difficult.  This technique can also be used to reduce chromatic aberration. Carrying this argument further, if we could make the hole small enough, all objects regardless of distance would be in focus.  In fact because the light entering the camera is no longer a cone, there is no need for a lens because there is no need to bend the light.  Such a camera is called a pinhole camera.  Of course, the light entering this camera would be very small, requiring extremely long exposure times.

As a general rule of thumb, use a wide open lens for portraits, producing a sharp subject with an out of focus background, and a small aperture for landscape shots to get as much of the scene as possible into focus.  Also, if you want to freeze the action, use a wide open lens so that you can use a fast shutter speed.  If you want to give an impression of motion, stop down the lens and use a slower shutter speed, producing a motion blur to the moving object.

I discussed Tilt/Shift lenses in the section on perspective control.  However, I did not discuss the tilting function.  It is appropriate here because tilting the lens affects depth of field.  This description is meant to be simple and the theory goes way beyond the scope of this discussion.  (For more on the theory click here.)  When the lens is parallel to the film plane, the plane of sharp focus is also parallel to the film plane.  Tilting the lens relative to the film plane alters the plane of sharp focus as seen in the figure at the left, producing a long depth of field.  A small lens tilt produces a large increase in the depth of field.  The crude example at the bottom demonstrates how we can focus on the entire rail fence by a small tilt of the lens.  The angles in this example may not be accurate.  This is merely a schematic representation of the tilt effect, and there are formulas for calculating the intersection of all three planes.  Most photographers in the field, however, rely on their eye to adjust the lens tilt, producing a long depth of field.  One of the problems with a lens tilted is that you cannot use the focusing aid in the center of the eyepiece.  The focusing aid (usually a split screen) will only indicate that which is in focus at the center of the field of view.  When the lens is parallel with the film plane, anything in the field of view (top, bottom, left or right) will be in focus if it is at the same distance from the camera as the object in the center.  Therefore, you must rely on your ability to determine the sharpness at the edges of the ground glass screen.  Why not use a wide angle lens stopped down to produce this long depth of field?  First, stopping down reduces the amount of light entering the camera, requiring a longer exposure.  Any camera movement or movement in front of the camera will cause a blur.  Second, you may not be able to stop the lens down to a small enough aperture to produce the necessary long depth of field.