Discovery Park of America






Interpretation of Focus

It is likely that you are reading this text on some form of screen.  Find an object that has writing on it, maybe a bottle or a paperback book.  Hold that object in front of your screen about half way between it and your eyes.  Move it a little to the side so that you can see part of your screen.  Read a bit of text on the bottle or book, then read a bit of text on your screen.  Switch back and forth another time or two and concentrate on any physical sensation you may feel inside your eyes.  Also note that you can't read text on the book and your screen at the same time.  This is because your eyes, or any other lens, cannot 'focus' simultaneously on two or more objects at significantly different distances.  The sensation you feel is due to muscles in your eyes changing the shape of your cornea and the lens behind it.  Since camera lenses are made of glass, their shape cannot be changed.  So, focus is varied by altering the distance between two or more optical elements within the lens assembly.

hummingbird in and out of focus

Hummingbird.  In focus (left); out of focus (right).



In most modern cameras, focus is set automatically by the camera depending on where you aim the camera before taking a picture.  This is called 'autofocus'.  Cameras with advanced features also let the user set focus manually.  This is called 'manual focus'.  It may not be obvious why manual focus is considered an advanced feature.  Suppose, however, that you are about to photograph a scene consisting of one or more persons and objects at various distances from the camera.  Suppose further that the person or persons are not going to be in the center of the picturethey are going to be off to the side.  You probably don't want the faces to be out of focus (blurry), while the object(s) in the center of the picture are in focus (sharp).  Some cameras have autofocus with face detection, and sometimes it even works.  Not all photos have faces in them, however, and for various other reasons it can be desirable to set focus manually.  There are other ways that autofocus can be employed to handle a wider range of situations.  Some of these involve utilizing alternate 'focus points' inside the viewfinder or on the LCD screen.  Focus point selection is either controlled on the camera or sometimes by eye movement.  Another method of controlling autofocus is autofocus lock.  This is done by aiming the focus point at the object or person who is intended to be in focus, causing the camera to focus on that object or person, locking focus (how this is done varies by camera model), and then recomposing before shooting.  Occasionally, manual focus is still easier.


Focus Distance

Most removable lenses have a scale on them labeled in feet or meters (perhaps both).  This scale is the focus distance scale and usually ends with the symbol for infinity ().  When you focus your lens on an object at a certain distance, that distance should be indicated on the focus distance scale.


Depth of Field

From the above discussion of focus distance, we may conclude that if we set the focus control on our lens at 30 feet, then only objects at 30 feet from with camera will be in focus.  That is true, but it may also be true that objects a little closer than 30 feet and a little farther away than 30 feet may appear to be in focus.  Depth of field refers to the range of distances for which objects are acceptably in focus.  Just how 'acceptable focus' is defined relates to a concept known as the circle of confusion.  The amount of depth of field in a photo can be controlled by changing the aperture (f-stop).



There are other photographic terms in this glossary related to focus.  You can also find numerous articles on the Internet that discuss focus in much greater detail, either from a technical standpoint or as it relates to photography as an art form.  One such article is this one on the hyperfocal distance.