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Photography Question 
Kix  Pix

member since: 3/21/2004
 

A Narrow Depth of Field ... At f/22?


Peter or others: I think I know this answer but want to double-check. Why is it that f/22 - which normally gives a large DOF - results in a blurred background when used on macro for smaller objects? Does this make any sense?

5/7/2004 5:28:25 PM

 
Jon Close
BetterPhoto Member

member since: 5/18/2000
  Depth of field is determined by a mathematical function of lens focal length, aperture, focused distance, and diameter of acceptable circle of confusion (which itself is dependent on the image size on film/sensor and final viewing size). Generally, DoF shrinks as the focused distance decreases. So, at very close macro focus, the DoF is very shallow, even if the aperture is very small.

The formula is
DoF = D(far) - D(near)
D(far) = d*f^2/[f^2 - A*C*(d-f)]
D(near) = d*f^2/[f^2 + A*C*(d-f)]
where
d = focused distance
f = focal length
A = aperture (eg. 5.6 for f/5.6)
C = diameter of the maximum circle of confusion, typically 0.025mm for 35mm film use.

5/7/2004 8:55:07 PM

 
Jon Close
BetterPhoto Member

member since: 5/18/2000
  oops,
C = diameter of the MAXIMUM circle of confusion.

5/7/2004 8:56:55 PM

 
Peter K. Burian
BetterPhoto Member
PeterKBurian.com
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  Jon is one of the most knowledgeable people on this Q&A forum and his reply is perfect!!

But the simple answer is: Any small aperture (such as f/16 or f/22) provides more depth of field - a greater range of apparent sharp focus, in front of your subject, and behind it. (Of course, even at f/22, the depth of field may be a few centimeters in extreme close focusing: not much.)

Note: Most lenses do not provide optimal sharpness at f/22. (Due to diffraction, bending of light rays as they enter a miniscule opening/aperture.) Personally, I would use f/16 instead, and often do in macro nature photography.

P.S. At any small aperture, shutter speeds will be long - increasing the odds of unsharp images from camera shake or subject motion. Use a tripod and shoot when the breeze is not moving your subject. Or load an ISO 800 film to be sure of fast shutter speeds.

5/9/2004 8:02:06 AM

 
Chris 

member since: 1/22/2004
  Note that in the formulae above, for all but macro photography, d>>f.

Therefore (d-f) = d for all practical purposes.

The formulae therefore in all cases (except macro) simplify further to:

D(near) = d*f^2/[f^2 + A*C*d]
D(far) = d*f^2/[f^2 - A*C*d]

Chris

5/11/2004 1:50:45 AM

 

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