http://dofmaster.com/dofjs.html is one great tool for understanding that shooting “wide open” doesn’t automatically generate good depth of field (DOF).

Another, more versatile tool is http://fcalc.net  – it requires Adobe AIR and can be run either “online” ( http://fcalc.net/online/ ) or as a local application. ( previously http://tangentsoft.net/  )

DOF has to be balanced; three factors decreases the depth of field:

1. Wider aperture (lesser f-number)
2. Longer focal length.
3. Shorter distance to subject.

Smaller sensor (film/frame) apparently also makes DOF changes – see http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size for more info on that.

Two stops on the f-number (i.e. multiply by 2) and the depth of field will quadruple (multiply by 2*2).
Double the distance and that also quadruples the DOF. Approximately, and at close distance – where it matters to know about it.

Generally, using wider than f4 at a one meter distance (~ three feet) equals a DOF of a few centimeters (2.54cm = 1″).

I hope you do know that one stop equals multiplying the f-number by 1.414 (squareroot of two). Two stops, multiply by 2…
The same goes for half  or double the time on shutter speed.
(Half time = half the light = one stop less)

More:
http://www.naturephotographers.net/articles0905/rb0905-1.html and http://www.flickr.com/photos/wintrhawk/3246501784/comment72157613303088370/

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