Simulate natural vision's selected focus

Conventional wisdom in photography states that to achieve optimum image sharpness it is necessary to stop  the lens down as far as possible (highest f-stop number).  In reality, f 11 or f 16 typically achieves the best results much to the chagrin of the Group f 64 purists, but that's a whole other topic.  The theory is that the higher the f-stop, the greater the depth of field and therefore the greater perception of sharpness.

In most image-making situations this is true. For example, far off subjects like landscapes and group portraits tend to render a more pleasing representation of what the human eye sees if the image is in sharp focus throughout the frame.  In many cases, however, the human eye sees the world with selective focus.  It helps us "focus our attention" on the most important elements of what we are looking at.  By using a very narrow depth of field technique in certain image-making situations, the photographer can often better represent what the human eye naturally sees.  And in some cases, the technique can provide some interesting surprises.

In this exercise, it is best to choose a focal length that best matches with the human eye's field of view.  We typically refer to these lenses of a "normal" focal length lenses as opposed to wide angle or telephoto.  For a 35mm sized image,  a 50 to 55mm lens best matches this field of view.  A good way to test this is with a wide angle to telephoto zoom lens (15mm to 85mm works well) mounted on the camera, then look through the viewfinder and adjust the zoom so it matches up with the perspective of what your eyes see when not looking through the viewfinder.  Typically the zoom should come to rest around the 45 mm to 70mm range.  The image will crop out the peripheral view we experience when viewing a scene through our own eyes.  Discounting that phenomenon, the normal lens will give us the best simulation of our own actual vision.

Our own eyes have a natural depth of field mechanism similar to the camera lenses' f-stop adjusted depth of field.  Not everything in our view is in focus.  The focus plane shifts as we shift the primary subject point.  The closer (and smaller) the subject is, the narrower the depth of field (or acceptable sharpness).  Try looking at a specific item sitting on your desk like your phone or a pen.  Focus on it and then note that items in front, to the side and in back of the item are blurred.  We can exaggerate this phenomenon to even a greater degree in photography by choosing a  very narrow depth of field setting, or low numeric f-stop.  By opening up the lens to a maximum level -- often f 1.2 or 1.4 on most normal lenses -- we can create an effect that simulates the reality of human sight.

At first, viewing a finished image with very narrow depth of field can be a little disconcerting to the viewer.  Initially, the scene actually looks very unnatural.  But the effect is very dramatic and the more you look a the image, the more it looks three-dimensional.

 In this image of a strawberry pierced by a fork (above), the primary focus is on the top portion of the berry and the sharpness falls away.  With a lack of a  background, the berry seems to "pop" out at the viewer. That effect can be even more dramatic as seen in the example below.

Here the Windows 7 decal on the laptop becomes even more prominent because of the separation the extreme narrow depth of field creates. 

Here is another example of how you can make a scene more dramatic by zeroing in a specific item to give it more emphasis.  The key autographs on the ball are sharp and standout while the insignia on the cap, while recognizable, provides a backdrop for the main subject of the image.

And here is a fun example of the technique.  This image of a Elvis look-a-like wiener dog, puts the emphasis on the head, creating a more stunning effect than if the whole dog was in sharp focus.

Play around with the technique to see what circumstances it works best for your intended use.  Remember, the technique is different from using selected focus with a telephoto lens.  This method is intended to simulate the true natural selected focus phenomenon of human vision.  So mount a normal focal length lens on your camera, open it up all the way, and see what you can focus on.