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Depth-of-Field Demystified
Frank J. Dispensa, M.Photog.Cr., API, APM
In our profession, most photographers learn from each other.   This often leads to incorrect information becoming widespread and taken for granted.  Depth-of-field is one principle that is generally misunderstood.


Let's now investigate the following diagram to logically explain my declaration. 
In the following diagram sometimes the 40 mm lens displays more depth-of-field (dof) then the 150 mm lens and sometimes the opposite is true.



Above, both camera lenses are set to the same f/stop.  The 40 mm lens to subject distance displays a 3/4 crop of the bride as seen in "View A".  Notice the 150mm camera lens is placed at the same distance from the subject, showing a head and shoulders crop "View B".  Because "View A" displays a smaller subject size than "View B"  the 40 mm lens camera displays more dof than the 150 mm camera lens.  
Now let's move the 150 mm lens camera back so that this camera "View C" displays  exactly the same 3/4 crop as "View A".  Now that both camera lenses display the same size subject they have exactly the same dof
One more step please.  If we now move the 150 mm camera lens further back to show an even smaller subject size than "View A", the 150 mm camera lens "View D" will display more dof than the wide angel 40 mm lens camera  
The Bottom Line:
Any two lenses set to the same f/stop that display the same image size have exactly the same dof regardless of focal length. 
A change in focal length only affects the size relationship between the foreground and background subjects.  We refer to this principal as perspective.  We will discuss perspective in a future seminar.
A Special Thank You To My Mentor Frank E. Cricchio!
Frank Edward Cricchio
M. Photog., Cr., Hon. M. Photog.; CPP, F-ASP; Hon. F-ASP
Hon.M.Photog., Mexico; Hon.M.Photog., Japan;
Hon. Gold Circle Photog., U.K.
Hon. Professor Sam Houston University
Hon. Fellow-ASP
"When a teacher is ready to learn, he will surround himself with students." ~ Frank J. Dispensa
             "Photography: the science is a journey, the art is a gift." ~ Frank J. Dispensa 

Copyright 2008 Frank Dispensa Photography.  All Rights Reserved