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From: Bob Buckles buckles@home.com
Newsgroups: rec.photo.misc
Subject: Determining the Depth of Field
Date: Fri, 27 Mar 1998

The subject of this column is based partly on an article that appeared last Summer in one of the popular camera magazines and depth of focus lens charts that appear on the web. Many times we find ourselves in the field and don't have the information at our finger tips. Here's an easy way to determine the depth of focus in the field.

When photographing landscapes, we are often faced with a decision as to what will be "in-focus" and what will have to be sacrificed to "out-of -focus." There's a simple formula that can be used in the field to ensure a distant object (infinity) will be sharp and at the same time, let you know how much in front of infinity will also be in sharp focus. After you select the focal length of the lens you will be using, you can determine your point of focus (at F/11) with the following formula: FD = FL divided by 10 squared, where FD is your focus distance and FL is the focal length of the lens you are using to photograph the scene. Example: Using a 50mm lens, 50/10 X 50/10 or 5 X 5 = 25 feet. Set the distance scale on the lens at 25 feet. With the lens aperture at F/11, we can be assured infinity will be in-focus. To determine how much foreground will be in-focus simply divide 25 by 2 = 12.5 feet. In other words, with a 50mm lens at F/11, focused at 25 feet, everything from 12.5 feet to infinity will be in focus. Most auto-focus lens today have very few markings to help you set the distances manually. I suggest you view the scene, select a spot about 25 feet away and auto-focus on this spot. Then switch off the camera's auto focus feature and take the picture.

Should the scene require more foreground in focus, increase the depth of field by stopping the lens down to a smaller aperture. For F/16, multiply 25 (the distance we determined in the above example) by 2/3. The new focusing distance is 17 feet. With a 50mm lens focused at 17 feet and F/16, now everything from 8.5 feet to infinity is in focus. More depth of field is required? Stop down another stop to F/22 and multiply 17 again by 2/3. The new distance is now 11 feet. Set the 50mm lens at 11 feet and everything from 5.5 feet to infinity will be in focus.

If all of this seems a little too complicated, the table below should help. I suggest you cut out this table, laminate it, and carry it in your camera bag. An inexpensive way to laminate is to buy clear, self sticking shelf paper and put it on both sides of the object to be laminated, leaving a 1/8 " border. This works great for protecting important papers and booklets such as your camera manual.

                  
                   F/11  F/16    F/22    F/32
  28mm              8'     6'      4'      3'
  35mm             11'     8'      5'      4'
  50mm             25'    17'     11'      8'
  70mm             49'    33'     22'     15'
 100mm            100'    67'     45'     30' 
 135mm            182'   121'     81'     54'
 200mm            400'   267'    178'    119'
 500mm           2500'  1667'   1111'    741'